Diagonal Spread
This page is the library's home for the mechanics that every two-expiry structure shares: why the short leg must expire first, how the two volatilities interact, and how a chain of short calls is taxed. The poor man's covered call is the deep, long-dated version and the calendar spread is the same-strike version; both link back here. BP is used as a model underlying at 530p, an illustrative level (BP closed at 519.6p on 17 August 2026; price data: Yahoo Finance); this is not a view on BP.
The March 460 call and the October 560 call
Four conditions make the pair a spread that a broker margins as one position, rather than a long call plus an uncovered short one:
Held as a recognised spread, the requirement is normally the debit already paid (spread margin). The debit is also the break-even floor for the short strikes: the back leg's strike plus its cost is 544.25p, so any short call at 550p or above can be met by exercising the back leg without a loss on the pair before costs. The first short call pays 12.00p, which covers 12.00p of the back leg's 14.25p of time value; the remaining 2.25p is what later short calls have to earn back (American against European value explains the 84.13p against 81.06p gap on the back leg).
Diagonal, poor man's covered call and calendar compared
Term structure: what each expiry's volatility does to the debit
A diagonal buys one expiry's implied volatility and sells another's. How those two volatilities sit against each other on the day of entry, the term structure, changes the price of the pair before the share moves at all (term structure explained). The chart shows four shapes a BP chain could plausibly take on 17 August; none is market data.
The pair is cheapest to buy when the front month is expensive relative to the back: the inverted shape costs £690.00 against £722.50 on the flat one and pays +£108.00 at an unchanged price on 16 October, if the volatilities stay where they were. The upward shape does the opposite: a cheap front call and a dear back one, £755.00 and +£25.70. The results bump barely matters here, because BP's 30 October results (a modelled move of 5.55% on the day, the implied-volatility page's illustration) sit inside the March leg but after the October one: spread over 214 days, they lift the March volatility only to 26.91%. The bump matters far more to the calendar, whose long leg is the November expiry itself.
The example was priced with IV at 26%, an IV rank of 25 on the library's model range for BP (20% to 44% over twelve months; IV rank). At 38%, rank 75, both calls cost more: the debit rises to £762.50, and the unchanged-price result on 16 October to +£122.60, because the front call's larger premium decays faster than the back call's. That is a description of the model at two volatilities, not a signal: a higher IV also means the model expects larger moves, which the diagonal's cap on the upside does not share in.
Payoff on 16 October and at the March expiry
Unchanged at 530p the diagonal is +£49.90 after the opening commission, or +£48.50 once the March call is sold (the October call lapses, so it needs no closing trade). The peak is at the short strike, +£313.60. Against buying the March call alone, the October credit cushions a fall (−£531.50 against −£650.10 at 440p) and caps a rally (+£289.80 against +£771.20 at 620p). The breakeven on 16 October is 523.6p, 1.2% below entry, or 524.1p after the opening and closing commissions. The model probability that BP is above 523.6p on 16 October is 54.8% (risk-neutral, lognormal, IV 26%), and one standard deviation either side of 530p spans 477.0p to 588.9p by then.
Open this example in the strategy builder (the builder prices European options only, so the typed March fill shows there as a slightly higher implied volatility).
Price and volatility on 16 October
On the day the October call expires, two numbers decide the result: BP's price and the implied volatility of the March call that remains. The fan and the grid below hold everything else fixed and move only those two.
At 530p the result runs from +£8.40 to +£103.00, a £94.60 spread driven only by the March call's volatility; that is nearly twice the +£49.90 the position makes at an unchanged volatility. At 620p the spread is £15.60, because a call 160p in the money is mostly intrinsic value. At 440p it is £128.40: after a fall, the March call is near the money and its value is mostly volatility, so a crash that also lifts implied volatility loses less than the price axis alone suggests (−£466.70 at 32% against −£595.10 at 20%). In practice a sharp fall usually lifts front-month volatility more than back-month (the inverted shape above), which this grid does not model.
Greeks across the short leg's life
The October call's gamma does not grow into a problem here the way a single-expiry short option's does: at 530p, 30p below the strike, net gamma is −38.3 per 10p at entry and −38.7 a week before expiry. What changes over the month is vega: it rises from £2.53 to +£7.48 as the October call's own vega melts away, leaving the position increasingly a long-volatility bet on the March call. Delta falls from 473 to 211 after an instant rise to 588.9p, and rises to 546 after a fall to 477.0p. A parallel drop in IV to 20% on both legs at entry costs −£11.20: the smaller loss on the short call only partly offsets the larger one on the long (position Greeks).
Five short calls over one March call
The campaign follows an assumed path, chosen to include a rally into BP's November ex-date and a drift lower afterwards: 548p on 25 September, 600p on 11 November, 585p on 27 November, 570p on 23 December and 575p on 17 February 2027. None of it is a forecast. The worked plan's conventions: roll each short call about three weeks before its expiry (the library's 21-day convention) into the next month at the 10p strike nearest a model delta of 0.30, never below the 544.25p floor; close a short call that is in the money the day before an ex-date; and close everything the day before the last ex-date inside the back leg's life.
The five calls took in £527.50 of premium and cost £360.00 to buy back, with £14.00 of commission: a net £153.50. The November call is the costly one: BP rallied through its strike, and closing it on the eve of the ex-date cost 20.50p against 10.25p received. The March call, bought at 84.25p, was sold at 115.00p: exactly its intrinsic value, because on the eve of an ex-date a deep in-the-money American call is worth what exercising it would give. Its European value that day, 110.05p, would have understated it by £49.50. Selling below intrinsic value would have handed that difference to the buyer. The campaign made +£458.20 on the £725.30 committed, against +£304.70 for the March call bought alone (£843.90 committed) and +£487.40 for 1,000 shares (£5,326.50 with SDRT, including the November dividend).
Each convention has a price. Rolling at three weeks, rather than holding to expiry, pays time value back when buying the old call (9.25p for the October call with 21 days left). Rolling up to a 0.30-delta strike after a rally keeps the upside open but leaves a higher strike to chase if the rally continues, as the November call shows. The February call cost only 0.25p to close, but closing it was not optional: selling the March call with the February call still open would have left a short call uncovered for two days. The generic mechanics of each roll, including the choice between rolling for a credit and rolling for time, are on the rolling page.
11 November: the November 580 call on the eve of BP's ex-date
On Wednesday 11 November, on the assumed path, BP is at 600p and the November 580 call is 20p in the money with nine days left. Left unexercised, the call opens Thursday on a share 6.39p lighter and is worth 18.25p on the model; exercised by 18:30 on Wednesday, it is worth its 20p of intrinsic value, which on Thursday is 13.61p of share value above the strike plus the 6.39p dividend. The difference, £17.50 a contract, is the holder's reward for exercising. A private holder paying 0.5% SDRT on the 580p strike, £29.00, would be −£11.50 worse off and would not exercise; an options intermediary with SDRT relief would. So this is the grey zone the assignment page describes: whether the writer is assigned depends on who holds the calls, and at ICE the holders include intermediaries. The worked plan closes the call that afternoon at 20.50p.
Route 2 is the one the phrase "covered by the long leg" suggests, and it is the most expensive: exercising the March call throws away its 5.51p of time value (it is worth 139.12p on Thursday against 133.61p of intrinsic value) and pays SDRT that an intermediary would not. The March call itself raises the same question on Wednesday evening: exercising it for the dividend would gain £8.80 over holding it into Thursday, less than the £23.00 of SDRT a private holder would pay, so the plan holds it.
If BP had fallen: 480p on 16 October
Take the other side of the path. BP falls to 480p by 16 October. The October call lapses, and its grant stands as a £118.60 gain. The March call is worth 40.43p, and the position is −£321.00 on paper after both opening commissions. Three choices face a holder that afternoon:
The floor is the line between a spread that is paid for its short calls and one that has sold its recovery. The worked plan stops writing calls below it; whether to hold the March call alone or close is then a view on BP, which this page does not take.
UK tax: five grants, one net figure
Each short call is granted and then bought back. A buy-back is not a separate disposal: its cost is added to the incidental costs of the grant it closes, and the grant's gain falls or becomes a loss, with relief at once (TCGA 1992 s148; HMRC's CG55536 points to CG55545, which sets it out). So the five grants give five computations, each a net figure dated in the tax year the call was written, and together £153.50, not the £527.50 of premiums received. The March call is a bought option: one more computation when it is sold, +£304.70. All six fall in 2026/27, a net +£458.20: £82.48 of tax at 18% or £109.97 at 24%, assuming the £3,000 annual exempt amount is used by other gains. Taxing the premiums as received would have put £126.60 on the grants alone at 24%, against £36.84 on their net result.
The trap particular to a rolling diagonal is the buy-back that lands after 5 April. A short call written in March 2027 and bought back in April belongs, net, to 2026/27; if the 2026/27 return has already been filed on the gross premium, it has to be amended. A back leg that closes after 5 April is the opposite case: its loss arises in 2027/28 and cannot be carried back to the grants (across 5 April). If the March call were sold at a loss and the same series bought again within 30 days, the sale would be matched with the new purchase rather than the original cost (CG55535; matching rules). The options go in the "other property, assets and gains" part of the SA108 (which boxes), and none of this can sit in an ISA (wrappers). The rules with HMRC's examples: written options and counting computations.
Costs in pounds
Each roll is two trades: £2.80 of commission (IBKR UK tiered, £1.40 a contract, checked 26 September 2026), plus half the quoted spread on each leg. The campaign paid £14.00 of commission on the five short calls and £2.80 on the March call, all included in the figures above. With an illustrative 0.5p-wide quote on the monthly calls and 2.5p on the March call, half-spreads add £2.50 for each short-call trade and £12.50 each way on the March call: £25.00 for the ten short-call trades and £25.00 for the back leg, £50.00 in all, on top of the +£458.20 (cost conventions). No SDRT arises unless shares are delivered, as in routes 2 and 3 above.
The put diagonal on BP
Turned over, the structure buys a later-dated put and sells an earlier-dated put at a lower strike: bearish to neutral, or a way to be paid for waiting below the market while owning protection. On the same chain: buy the March 2027 530 put (model 42.16p, bought at 42.25p; delta −472) and sell the October 2026 490 put (model 6.46p, sold at 6.50p; delta −199), for £360.30 with commission. Both are American puts on the binomial tree.
The shape mirrors the call version, peaking at the short strike, but two UK details differ. Deep in the money, a short put tends to be exercised early once the interest a holder earns on the strike outweighs the put's remaining time value: at 440p on 1 October the October 490 put is worth 50.00p, exactly its intrinsic value, with 0.00p of time value left, so exercise against the writer is likely (early put assignment). And the assigned put writer is the buyer of the shares, so pays the 0.5% SDRT on the strike, £24.50. The ex-dividend dates raise put values, because the share drops on each one, and they make early exercise of the long put less attractive, which is why its American value, 42.16p, is only a little above the European 41.61p.
The double diagonal
A call diagonal above the market and a put diagonal below it make a double diagonal: sell the October 490 put (6.50p) and the October 570 call (9.25p), buy the December 480 put (11.50p) and the December 580 call (14.25p), each long leg one strike further out and two months later. The debit is £105.60 with four commissions. The December legs span BP's 30 October results and 12 November ex-date.
Because each long leg sits one strike outside the short leg it covers, the debit is not the most the position can lose. Far below 480p or far above 580p on 16 October, the short option in the money is worth up to 10p more than the long one beside it, so the loss approaches £205.60: the debit plus £100.00 for the 10p gap on one side. A broker adds that gap to the requirement (the third condition above; for the puts, the long strike would need to be at or above the short one). The table shows it starting: −£113.20 at 440p and −£108.60 at 620p.
The result has two peaks, at the short strikes, and a shallow dip between them, because at 530p both short options expire worthless while both long options are out of the money. It is the diagonal cousin of the iron condor and of the double calendar: a range view that is long volatility in the later month.
Account, permissions and the BP contract
BP has no ICE mini option (it is not one of the 22 names), so every BP diagonal is 1,000 shares a leg (contract sizes). ICE lists standard serial months to one year on every name and to two years on its Target Group; a March 2027 back leg, 214 days out, needs no Target Group listing. Interactive Brokers classes a diagonal whose short leg expires first as Level 3 and one whose long leg expires first as Level 4, and spreads need a margin account (account types and permissions). A reader wanting the same shape at a tenth of the size would need one of the mini names, such as Shell (8SQ); the poor man's covered call works a mini example on Tesco.