Payoff at the near expiry
Profit or loss the moment the short leg expires, across a range of underlying prices. The short leg is worth its intrinsic value; the long leg is repriced with Black-Scholes over its remaining days at the IV you set. The peak sits at the strike; past the two break-evens you are in the loss zone, capped at the debit.
| Underlying at near expiry | P&L | On debit |
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Time decay is the engine, volatility is the steering
A calendar spread earns because the short, near-dated leg loses time value faster than the long, far-dated leg you hold — park the price at the strike and the short one evaporates while the long one keeps most of its worth. But direction is only half the story. Because you are net long the longer option, you are net long volatility: a rise in the far-leg's implied volatility lifts the trade even if price never moves, and a post-event volatility crush can sink a spread that landed perfectly on the strike. That is why the far-leg IV at expiry input matters as much as the strike — drop it to model a crush and watch the max profit shrink or flip red. Calendars shine in quiet stretches between catalysts and ahead of an event that lands after the near expiry; they lose to a big directional move or a volatility collapse, and either way the loss is capped at the debit you paid. Model the pure volatility drop with the IV crush calculator, judge whether the back-month IV is cheap enough to be worth owning with the IV rank calculator, watch each leg's greeks in the greeks calculator, and compare against a same-expiry vertical spread or a strike-and-time diagonal.
The math
Write the strike K, the near days d₁ and far days d₂, and an implied volatility for each leg. The tool prices every option with the Black-Scholes model at zero rate: the net debit is the far-leg value minus the near-leg value, BS(K,σ_far,d₂) − BS(K,σ_near,d₁), and it is what the spread costs you up front and your maximum loss.
At the near expiry the short leg is worth only its intrinsic value — max(S−K,0) for a call calendar, max(K−S,0) for a put — while the long leg is repriced by Black-Scholes over its remaining d₂ − d₁ days at the far-leg IV at expiry you choose. The position value is long − short, and the profit is that minus the debit. Max profit occurs at S = K, where the short leg is worthless and the long leg holds the most time value.
The two break-evens — one below the strike, one above — are the prices where profit crosses zero, found numerically from the payoff. Because you are net long vega, raising the far-leg IV at expiry lifts the whole curve and lowering it (a crush) drops it. Every figure scales by your contract count. This is a model, not a quote — real fills, skew and a moving IV surface will shift the outcome.
FAQ
What is a calendar spread in crypto options? A calendar spread — also called a horizontal or time spread — is an options position where you sell a near-dated option and buy a longer-dated option at the same strike and of the same type, both calls or both puts. You pay a net debit because the longer option is worth more, and the whole trade is a bet that the underlying finishes near the strike when the near option expires. The reason it works is time decay: the short, near-dated option loses its time value much faster than the long, far-dated one, so if price sits still the short leg melts to nothing while the long leg keeps most of its value, and the difference is your profit. It is fundamentally a play on time and volatility rather than direction — you want low realised movement between now and the near expiry, and ideally a rise in implied volatility on the back month, which lifts the long leg you still hold. In crypto it is used to harvest premium around quiet stretches between catalysts, or to position for an implied-volatility expansion into a known event that lands after the near expiry. The position is cheap relative to owning the underlying and its loss is capped at the debit paid, but it is not free money: a big move in either direction, or an IV collapse on the back month, turns the debit into a loss.
Where does a calendar spread make and lose money? Maximum profit happens when the underlying is sitting right at the strike at the near expiry. There the short option expires worthless, you keep every cent of its premium, and the long option you still hold is at-the-money with the most time value it can have — the gap between the two is the biggest it gets, and after subtracting the debit you paid, that gap is your peak profit. Move away from the strike in either direction and the profit shrinks: the short option gains intrinsic value that eats into your gains, while the long option's time value bleeds as it goes further in or out of the money. Past the two break-even prices — one below the strike and one above — the position turns into a loss, and the loss is capped at the net debit you paid, which happens when price runs so far that both legs are deep in or out of the money and their values converge. The second, hidden driver is implied volatility on the back month: because you are net long a longer-dated option, a rise in its IV inflates the long leg and boosts your profit even if price does not move, while an IV crush after an event does the reverse and can produce a loss on a spread that looked perfectly placed. This calculator prices the long leg with Black-Scholes at the near expiry, so both the strike-distance effect and the IV effect show up in the payoff, and it marks the exact strike-peak, both break-evens and the capped max loss.
How does implied volatility change a calendar spread? Implied volatility is the make-or-break variable for a calendar spread, arguably more important than where price lands. You are net long vega — you own more time in the far option than you are short in the near one — so higher implied volatility on the back month makes the position more valuable, and lower implied volatility hurts it. That is why the classic setup is to open a calendar when back-month IV is cheap and you expect it to rise, often into a scheduled catalyst that falls after the near expiry: the near leg decays away, and if the event pumps implied volatility, the long leg you are still holding gets repriced upward. The danger is the mirror image. If you put the spread on when IV is already elevated and it then collapses — the typical post-event crush — the long leg loses value even though it still has weeks of life left, and the trade can lose money despite price parking exactly at your strike. The far-leg IV at expiry input on this calculator is there precisely so you can test that: set it below your entry IV to model a crush and watch the max profit shrink or flip negative, or set it above to model an expansion and see the payoff lift. Never price a calendar assuming volatility stays frozen — the volatility path is usually why these trades win or lose.
How is this different from a vertical spread or a diagonal? The three spreads differ in what varies between the two legs. A vertical spread uses the same expiry but two different strikes — it is a directional bet with a fixed, defined payoff at a single expiry, and time decay and volatility mostly cancel between the legs. A calendar spread flips that: same strike, two different expiries — it is a bet on time and volatility rather than direction, and it can only be evaluated by pricing the surviving long leg after the short one expires, because its value then depends on remaining time and implied volatility. A diagonal spread, such as a poor man's covered call, varies both at once — different strikes and different expiries — blending a directional lean with the time-decay harvest of a calendar. Because a calendar's outcome hinges on the long leg's leftover time value, you cannot read its profit off a simple payoff diagram the way you can a vertical; you need a model. That is what this tool does — it values the far leg with Black-Scholes at the moment the near leg expires, using an implied volatility you control, so the max profit at the strike, the two break-evens and the capped loss are computed rather than guessed. Use the vertical spread calculator for pure directional defined-risk trades, the diagonal or poor man's covered call calculator when you want strike and time to differ, and this one when you are trading time decay and a volatility view at a single strike.