Calendar Spread
Prerequisite strategies: you must have traded the covered call with real money, so that you have granted an option and been assigned, and the long straddle, so that you have paid for volatility and watched it evaporate. Clear the Level 2 gate first. Next: the diagonal spread, then the poor man's covered call.
Why this structure exists
Every structure before this one is a bet on a price. A vertical asks where the share finishes; a straddle asks how far it travels. A calendar asks neither. It asks whether the option market has priced the next four weeks correctly relative to the next four months — a question about the shape of the volatility term structure, not about BP.
The engine is arithmetic on time. Extrinsic value drains roughly with the square root of the time remaining, so a 30-day at-the-money option gives up a much larger share of itself per day than a 121-day one at the same strike. Sell the near one, own the far one, and you keep the difference: £1.39 a day at entry on the position below, rising to £3.50 a day in the front month's final week.
The safety comes from the same fact. A December call can never be worth less than a September call at the same strike, because it does everything the September one does and then continues for another 91 days. So the spread's value is floored at zero whatever BP does, and the worst case is the £127.29 you paid. No cash and no shares are doing that work — the construction is.
Why not just write the covered call from the tier below, which also sells the front month and also profits from a share going nowhere? Because it needs £5,300 of BP shares standing behind it and this needs £127.29 — you are renting the collateral rather than buying it, at 2.4% of the price. The bill for that leverage is precision: a covered call is happy anywhere below the strike, while a calendar has a profit window 57.6p wide and loses if BP leaves it in either direction.
Construction
| Leg | Buy / Sell | Quantity | Strike rule | Expiry rule | Target delta | Price |
|---|---|---|---|---|---|---|
| Front option | SELL (credit) | 1 contract = 1,000 shares (ICE UK); 100 (US) | Where you expect the share at front expiry, within 2.5% of spot | 21–45 days; the monthly, never a weekly | 0.40–0.50 | 11.275p = £112.75 |
| Back option | BUY (debit) | 1 contract, same strike, same size | Identical — one penny apart and it is a diagonal | 2–4× the front; 60–150 days | 0.45–0.55 | 24.004p = £240.04 |
| NET | Net debit | 1 calendar | 540 / 540, BP spot 530p | 18 Sep / 18 Dec 2026 | +0.050 | 12.729p = £127.29 |
Same underlying, same strike, same size, back expiry later, both legs calls or both puts: break the strike rule and you own a diagonal, a different trade with a directional bias bolted on. Four inequalities before the order goes in:
Formulas: max loss = debit × contract size + opening commission, and it is a true maximum because a longer-dated option at the same strike is never worth less than a shorter-dated one, so the spread cannot settle below zero. Max profit = the modelled value of the surviving back-month option at the strike on front-expiry day − debit − both commissions — a model output, not an identity, because it depends on where December's implied volatility is that morning. Breakevens are the two prices at which that value equals the debit. Note the payoff ratio: 0.92 to 1. This structure does not pay more than it risks; it pays more often.
This is a value chart, not an intrinsic-value chart. Every other payoff diagram in this library draws a straight expiry line, because at expiry an option is worth its intrinsic value and nothing else. Here the solid line is drawn on the day the front leg dies, when the December leg still has 91 days and £252 of extrinsic value in it, so it is a modelled price, not a settlement: move December's implied volatility and the whole hump moves with it. Pinned at 540p it is worth £119.08 at 24% volatility, £161.56 at 28% and only £76.58 at 20%. The dashed line is the same position today, and it is almost flat — none of the profit exists yet.
Entry criteria
| Gate | Rule | Reason |
|---|---|---|
| IV rank / IV percentile | IVR below 30 on the back month. 30–50 only if the term-structure gate passes comfortably; above 50, sell an iron butterfly instead | Net vega is +£6.06 a point, so you are buying volatility and want it cheap: four points off both months costs £25.49 with the share standing still |
| Term structure | Front IV ÷ back IV ≥ 1.05, hard floor 1.00. Here 26 ÷ 24 = 1.083 | The only edge in the trade. At 1.083 the debit is £127.29 and the return on risk 91.5%; at 0.846 the same calendar costs £174.67 and returns 52.4% |
| Days to expiry | Front 21–45 days, back 2–4× the front; managed at 7 days on the front leg | Inside 21 days the front leg's gamma grows faster than its theta pays you |
| Strike selection | One strike, both legs, within 2.5% of spot; front-leg delta 0.40–0.50 | Maximum profit sits exactly at the strike, so the strike is the forecast. Calls above spot, puts below |
| Liquidity | Spread ≤ 10% of mid on each leg in both expiries; open interest ≥ 100 in both | The binding constraint. 10% on both legs costs £70.56 round-trip — 55.4% of the debit, 12.6× the commission |
| Underlying | A range-bound liquid FTSE 100 name, or a US name where the ICE chain will not price two expiries | ICE UK series are 1,000 shares, physically delivered; a trending share is the wrong underlying whatever the volatility says |
| Event calendar | No results, ex-dividend date or index review before the front expiry, and no event between the expiries | An event before front expiry moves the share out of the window; one between the expiries inflates the back month you are buying |
Do not enter if: the back month's IV rank is above 50, because you are long vega and would be buying volatility at the top of its range — the commonest way this trade is lost; the front month's implied volatility is below the back month's, a ratio under 1.00 meaning you sell the cheap month to buy the dear one; a scheduled binary event falls before the front expiry, however rich that makes the front month look; either leg in either expiry fails the liquidity screen, which on ICE UK single stocks is most of them; you are in a cash account; or you hold any directional view at all, in which case you want a diagonal.
Debit or credit: the same view, two structures
A calendar and an iron butterfly express one identical opinion — BP finishes September near 540p — from opposite sides of the premium and, crucially, from opposite sides of volatility. IV rank picks between them, and it is not a close call in either direction.
| Calendar spread (debit) — this page | Iron butterfly (credit) | |
|---|---|---|
| Legs on BP at 530p | Sell Sep 540 call 11.275p, buy Dec 540 call 24.004p | Sell Sep 540 call 11.275p and Sep 540 put 21.460p, buy Sep 600 call 0.824p and Sep 480 put 1.620p |
| Cash at entry | Pay £127.29 | Receive £302.91 |
| Max profit / max loss | £119.08 / £130.09 | £297.31 / £302.69 |
| Profit window | 512.22p–569.85p — 57.6p wide | 509.71p–570.29p — 60.6p wide |
| Buying power used | £127.29 (the debit) | £297.09 (width − credit) |
| Net vega | +£6.06 a point — long volatility | −£7.81 a point — short volatility |
| Net theta at entry | +£1.39 a day | +£3.36 a day |
| Use it when | IV rank below 30 | IV rank above 50 |
| Day-one taxable gain | £112.75 (Sep 540 call granted) | £327.35 (both short legs granted) — 108% of the net credit |
Two positions with almost the same profit window, opposite cash flows and opposite volatility exposure. That is what makes the choice mechanical: if implied volatility is at the bottom of its range you want the structure that gets paid when it recovers; if it is at the top, the one that gets paid when it falls. Choosing on preference — "I like collecting premium" — is how a reader ends up short vega at an IV rank of 12. Note the last row: the credit structure books a day-one chargeable gain larger than the cash it received, because HMRC taxes granted legs and ignores bought ones until they close.
Greeks at entry and how they evolve
| Greek (net, per contract) | Entry: 30 / 121 DTE, 530p | 15 DTE front, unchanged | 7 DTE front, unchanged | +1 SD (569.5p) at 15 DTE | −1 SD (490.5p) at 15 DTE |
|---|---|---|---|---|---|
| Delta | +0.050 (50 shares) | +0.089 | +0.148 | −0.177 | +0.206 |
| Gamma | −0.00447 | −0.00777 | −0.01245 | −0.00299 | +0.00185 |
| Theta | +£1.39/day | +£2.26/day | +£3.50/day | +£1.11/day | −£0.23/day |
| Vega | +£6.06/pt | +£7.15/pt | +£8.19/pt | +£8.15/pt | +£7.42/pt |
Black–Scholes at 26% implied volatility in the front month and 24% in the back, 4% rates and a 4.5% dividend yield, per 1,000-share contract — the model and BP chain used on the long call and bull call spread pages, with the flat volatility replaced by a term structure, because a calendar cannot be priced from one number.
Vega decides this trade, and the sign is what readers get wrong. The position is net long vega, +£6.06 a point. That surprises anyone who sees a short option and assumes short volatility, so take the legs apart: the September call you granted is short £5.90 a point; the December call you own is long £11.96, because vega scales with the square root of time and December has four times as much of it. Long beats short. Add four points to both months with BP unchanged at 25 days and the position gains £25.38; take four off both and it loses £25.49.
Now the trap "long vega" hides: it is only true of a parallel shift, and volatility rarely moves in parallel. In a selloff the front month spikes and the back barely moves. Put eight points on September and two on December — an ordinary backwardation event — and the position is £19.77 worse off than if implied volatility had not moved at all, and £12.35 under water. It has lost money on rising volatility while long vega. What you own is the spread between two months' volatility, which is why the entry gate is a ratio rather than a level.
The character flips in the front leg's last week. Net gamma at the strike goes from −0.0046 at 30 days to −0.0146 at 7, −0.0253 at 3 and −0.0482 at 1 — ten times the entry figure. At 3 days out at 540p, theta pays £7.11 a day but a 10p move costs £11 to £13 and a 20p move £38 to £43, so one ordinary session takes back most of a week's decay. That is the gamma reason not to hold to expiry: the last £68.08 of the maximum — the gap between £51.00 at 7 days at the strike and £119.08 at the pin — is payable only if BP finishes on one exact price on one specific Friday. One more cell deserves attention: 40p below the share price, theta turns negative. What every reader believes about calendars — that time is on your side — stops being true once the share has left the window.
BP p.l.c. modelled at 530p, IV rank 18, September volatility 26% against December's 24%
BP is modelled at 530p on 19 August 2026. The ICE Futures Europe BP option is quoted in pence per share, one contract confers rights over 1,000 shares, it is physically delivered and the tick is 0.25p, so one penny of option price is £10 of contract value. September expires on Friday 18 September 2026 and December on Friday 18 December 2026 — 30 and 121 days away.
The trade, placed as a single spread order: sell 1 × BP September 2026 540 call at 11.275p, buy 1 × BP December 2026 540 call at 24.004p.
Branch A — BP 530p on Tuesday 8 September, 10 days left. Nothing happened, which is the entire thesis. The spread marks 16.634p.
Branches B and C — BP runs away, 3 September, 15 days left. This is what the structure exists to lose on, and it loses in both directions for one reason: the further BP travels from 540p, the more alike the two calls become, so the spread between them collapses toward zero.
Branch D — BP 535p on Friday 11 September, 7 days left. The time stop, and the decision that separates a calendar from a one-off trade.
Branch E — you ignored the time stop and BP settled at 542p on 18 September. The branch with the stamp duty in it.
Do not settle the assignment by exercising the December call. That is right on a vertical, where both legs share an expiry; here it saves 10p of SDRT and destroys £242.37 of December time value. Buy the shares and keep the long leg.
The honest reality check on the UK series. At a bid-ask of 10% of mid on each leg — optimistic for an ICE UK chain two expiries out — the round trip costs £70.56, 55.4% of the debit; even at 5% it is £35.28. A calendar's debit is small relative to the premiums traded, so crossing the spread hurts in a way it does not on a vertical. The FTSE 100 index series is tighter, European and cash-settled — no assignment at all — but at £10 per index point the same structure is roughly a £1,550 debit, needing an account of about £77,800 at a 2% risk limit. For most UK readers the workable route is a US chain at 100 shares a contract, where the gain is still computed in sterling on each disposal date. A $1.25 calendar costs $125, or £92.24 at GBP/USD 1.3552; close it for $162 with the rate at 1.4000 and the proceeds are £115.71. The dollar profit is 29.6%, but the chargeable gain is £23.48 rather than the £27.30 an unchanged rate would have given — £3.83 of currency drag, before the conversion spread, which lands on top and twice.
This is a single, hand-picked, favourable scenario. It is not a typical or expected outcome. Every premium is modelled from Black–Scholes at the stated inputs rather than taken from a live chain, and real ICE UK quotes are materially wider. Past performance, including any illustrative example shown here, is not a reliable indicator of future results.
Management and adjustment
| Trigger | Diagnosis | Action | Do NOT do this |
|---|---|---|---|
| BP reaches ±1 SD from the strike, or closes outside 499.6p–584.2p | The share is at or beyond the edge of the profit window and net gamma is against you; at the band the −25% stop has fired | Close both legs as one order. At 569.5p with 15 days left the position is only −£3.79 — an almost free exit | Wait for it to come back, or roll the strike toward the price. Short gamma means the next move is bigger than the last |
| IV rank rises above 50 after entry | The vega you bought has been paid to you early | Take it. At +£6.06 a point a four-point expansion is +£24.24 with no help from the share at all | Hold on for the pin as well. You have been paid for the reason you entered |
| Implied volatility falls 4 points | The position is £25.49 down with BP unchanged | If the term-structure ratio still passes, hold to the plan; if the front fell further than the back, close — the edge has inverted | Buy a second calendar to average down. That doubles a long-vega position into a falling-vega market |
| Ex-dividend date falls inside the front cycle | With the short call in the money and its extrinsic below the dividend, early assignment is rational for the holder | Assume assignment. Close or roll the front leg the business day before the ex-date | Leave it and hope. You wake up short 1,000 BP shares |
| Assigned early on the short call | You are short 1,000 shares against a long December call — a synthetic long put, on margin | Buy the shares back the same morning: £47.10 at 542p including £27.10 of SDRT, and the December leg survives | Exercise the December call to deliver. £242.37 of time value to save 10p of stamp duty |
| Front leg reaches 7 days, BP inside the window | The position is working and gamma is about to treble | Roll the front leg out one month, or close. Both are correct; holding is not | Hold for the pin. The remaining £68.08 is a bet on one price on one Friday |
| An earnings date appears between the expiries | The back month now carries event premium you did not price | Nothing, if it was there at entry. If it moved in, close on the first liquid morning | Congratulate yourself on cheap vega. It is not cheap; it is about to be crushed |
ROLL WHEN the front leg has 7 days or fewer, BP is still inside the profit window, and the December leg has more than 60 days left. ROLL TO the next monthly at the same strike, as a single order, and only for a net credit. In Branch D the roll pays £89.66 and cuts the cost basis from £130.09 to £40.43 — the real reason calendars are run as a series. Roll again on 9 October with BP at 538p, granting the November 540 call at 17.739p against a 6.754p buy-back for a further £107.05, and the December call is held at a net credit of £66.62: before further dealing costs or SDRT on delivery, it can no longer lose money. That is the doorway to the poor man's covered call, which is this idea industrialised.
DO NOT ROLL the strike to follow the share, and never roll the front leg for a net debit. Rolling the strike converts a term-structure trade into a directional one at the worst moment, after you have been proved wrong about direction; rolling for a debit raises the maximum loss above the number you wrote down before entry, the one figure on this page that is not allowed to move.
THE CORRECT ACTION IS TO CLOSE, NOT ADJUST, when BP is outside the profit window, when the stop has fired, when the December leg is inside 60 days, when the front-to-back volatility ratio has fallen below 1.00, or when the roll cannot be done for a credit. A calendar has exactly one adjustment — rolling the front leg forward at the same strike — and it is available only while the trade is still working. Once the share has left the window there is nothing to defend: what was going to make money was time passing at a particular price, and the price has gone.
Exit rules
If all four rules are silent, do nothing, and check the front-to-back volatility ratio tomorrow rather than the price.
Margin and broker reality
A cash account cannot hold this trade, and that is where most UK first attempts die. A calendar contains a granted option and a cash account has no mechanism to carry one: Interactive Brokers permits only limited purchase and sale of options in a Cash account, so the order is rejected in the preview rather than at the exchange. You need a Margin account, for which IBKR's published minimum is USD 2,000 or equivalent. Hargreaves Lansdown, AJ Bell and Trading 212 offer no options at all, so this is an Interactive Brokers or Saxo trade.
What you do not need is uncovered-option permission, because the December call at the same strike covers the September one for the whole of the September option's life: the initial requirement is the net debit of £127.29, maintenance is nil, and buying power cannot fall further whatever BP does. Two cautions specific to this structure. First, brokers stop treating the position as a spread the moment the front leg is assigned or the back leg is closed, so a partial fill can turn a £127.29 requirement into a naked short call requirement overnight — enter, exit and roll as a single order. Second, the bid-ask is the real margin here: £70.56 round-trip at 10% of mid on each leg against £5.60 of commission, 12.6 times what the broker charges and 55.4% of the debit. The FTSE 100 index series is tighter, and European and cash-settled at the Exchange Delivery Settlement Price, so it cannot be assigned and carries no SDRT — but at £10 per index point one calendar is roughly a £1,550 debit, and the expiring series stops trading shortly after 10:15 London on the third Friday, so you cannot manage the front leg during the afternoon.
no scheduled binary event before the front-month expiry, whatever the IV ratio says. Check the results calendar before the volatility.Portfolio fit
One calendar contributes a net delta of +0.050 — 50 BP shares, about £265 of share-equivalent exposure — which is as close to directionally neutral as anything in this tier. What it contributes is +£6.06 of vega a point and +£1.39 of theta a day on £130.09 of risk. That vega figure is why the position belongs in a book at all: an iron condor book is short vega and a strangle book very short vega, and a volatility expansion hurts all of them together. Calendars are the only Level 2 structure that is meaningfully long vega while still defined-risk.
At the 2% rule a £130.09 maximum loss needs at least £6,505 of account. Six of them on £25,000 risk £780.53 — 3.1% of capital — and use the same £780 of buying power, which is trivial. The binding constraint is therefore not margin and not delta but vega concentration: six calendars is +£36.37 of vega a point, so an eight-point collapse in implied volatility across the market costs £291 with every underlying standing exactly still. The honest cap is three or four concurrent, staggered across different front-month expiries so they do not all reach their gamma week together.
What to trade instead
Simpler, from the tier below: the covered call. It sells the same front-month option for the same nothing-happens, but the collateral is 1,000 shares you own rather than an option you rent, so it needs about £5,300 instead of £127.29, has no upside stop-out and cannot be assigned into a short position. Take it whenever you would be content to own the shares anyway.
Sideways, at this tier: the iron butterfly is the same view sold rather than bought, for IV rank above 50; the iron condor is its wider cousin. Both are short vega, which is the point of the choice.
More precise, from later in this tier: the diagonal spread is this structure with the strikes separated, keeping the time edge and adding a directional lean; the poor man's covered call runs the same idea permanently, with a deep LEAP as the back leg and a new front month sold every cycle. Both need more capital and a firmer view on term structure.
Risk statement
Listed options are complex instruments and most retail positions lose money. This is educational material about mechanics, margin and UK tax treatment, not a recommendation to trade BP or anything else, and it takes no account of your circumstances. Every premium here is modelled rather than quoted, and a calendar is more sensitive to modelling assumptions than any structure before it, because its maximum profit depends on an implied volatility three months in the future. If your trading becomes frequent enough to raise the investor-versus-trader question, that is one for a qualified adviser.