What is a calendar spread?

A calendar spread sells a near-dated option and buys a longer-dated one at the same strike, both calls or both puts. You pay a small net debit and profit from the near leg decaying faster than the far leg while the stock sits near the strike. It is a bet on time and volatility, not direction.

How is a calendar spread built?

Two options on the same stock and the same strike, but different expirations:

• Sell the near-dated option (the short leg). It decays fastest and is the engine of the trade.
• Buy the longer-dated option (the long leg) at the same strike. It costs more, so the position opens for a net debit — the most you can lose.

Calls or puts both work; at the same strike the payoff shape is nearly identical. The position is market-neutral around the strike and benefits from the stock standing still.

Why is it a time and volatility trade?

Two forces drive the profit. First, time decay: the near option loses value faster than the far one, so the gap between them widens in your favour as the near expiry approaches — as long as the stock stays near the strike. Second, volatility: the position is long vega, so rising implied volatility on the back-month option helps and falling volatility hurts. That second force is why the profit at expiry is an estimate, not a fixed number like a vertical spread.

Max profit, max loss and breakevens

• Max loss = the net debit you paid × 100. Both legs are options you hold, so you cannot lose more than the cost to open.
• Max profit ≈ reached if the stock finishes at the strike at the near expiry — the leftover value of the long leg there, minus the debit. It is an estimate because it depends on the long leg's implied volatility at that point.
• Breakevens = one below and one above the strike. Outside that band the position loses; the further the stock runs from the strike, the more of the debit you give back.

Calendar vs diagonal spread

A plain calendar uses the same strike for both legs, so it is direction-neutral. A diagonal spread uses different strikes as well as different expirations — a calendar tilted in a direction, adding a directional view on top of the time-decay edge. The poor man's covered call is the best-known diagonal: a long-dated in-the-money call standing in for stock, with shorter calls sold against it. (The calculator here models the same-strike calendar.)

A worked example

With the stock at $100, you sell the 30-day $100 call for $2.50 and buy the 60-day $100 call for $3.70 — a net debit of $1.20, or $120, which is your maximum loss. If the stock is still near $100 at the 30-day expiry, the call you sold expires near-worthless while your 60-day call still holds roughly $2.40 of value — so the spread is worth about $2.40, nearly double the $1.20 you paid. If the stock instead races to $130 or sinks to $80, both calls move together and you give back most of the debit.

When a calendar spread makes sense

Use a calendar when you expect a stock to sit near a price through the near expiration, ideally when near-term implied volatility is rich relative to the back month. It is a cheap, defined-risk way to sell time decay. Check whether volatility is high or low first with the IV Rank Calculator, and judge how far the stock might travel with the Expected Move Calculator.