A calendar spread (also called a time spread or horizontal spread) has two legs at the same strike but different expiries. You sell the option that expires soon and buy the same option that expires later. Because the later option has more time value, it costs more, so the spread is opened for a net payment, a debit.
In India, index options have weekly and monthly expiries, which makes calendars easy to build: sell this week's NIFTY option, buy next month's. The expiry guide explains how the expiry cycle works.
Calendar spread value = far option price − near option price
Why it works: options decay at different speeds
An option's time value does not melt evenly. It falls slowly when expiry is far away and very fast in the last few days. This is theta decay. A NIFTY option with two days left may lose a third of its value in a day, while the same strike with a month left loses only a few percent.
The calendar seller collects that fast decay on the near option while holding the slower-decaying far option. If NIFTY sits near the strike, the near option dies worthless and the far option still has most of its value. The spread widens, and that widening is the profit.
A worked example on NIFTY
NIFTY is at 25,000. This week's 25,000 call, with two days left, costs ₹120. Next month's 25,000 call costs ₹380. You sell the weekly and buy the monthly: net debit ₹260. That debit is about the most you can lose, as long as you close both legs together when the near option expires.
When the weekly expires, its value is just its intrinsic value. The monthly still has time value, so its price has to be estimated. Assuming implied volatility stays about the same:
| NIFTY at near expiry | Weekly call (sold) | Monthly call (est.) | Spread value | Profit per unit |
|---|---|---|---|---|
| 24,500 | 0 | 100 | 100 | −160 |
| 24,800 | 0 | 230 | 230 | −30 |
| 25,000 | 0 | 330 | 330 | +70 |
| 25,200 | 200 | 460 | 260 | 0 |
| 25,500 | 500 | 640 | 140 | −120 |
The payoff looks like a tent: highest when NIFTY is at the strike, falling away on both sides. Far from the strike, both options become either nearly worthless (far below) or nearly pure intrinsic value (far above), and the spread shrinks toward zero, so the loss approaches the debit. With 75 units a lot as an example (check the current lot size on NSE), the best case in the table is about ₹5,250 per lot and the worst about ₹12,000.
The volatility side: a calendar is long vega
Vega is how much an option's price changes when implied volatility (IV) changes by one point. Longer-dated options have more vega than near ones. Since the calendar owns the longer option, the position as a whole gains when IV rises and loses when IV falls. The vega guide has the details.
In the example, if IV dropped sharply after an event, the monthly call might be worth ₹270 instead of ₹330 with NIFTY at 25,000, and the expected ₹70 profit would become a ₹10 profit. A calendar opened when IV is unusually high and then falls can lose even if NIFTY behaves perfectly.
The live box below shows where implied volatility stands compared with its own past year:
A low IV rank means options are cheap compared with their usual level, which is when the long-vega side of a calendar has more room to help. A high IV rank means the far option you are buying is expensive.
Reading calendars from the option chain
To build a calendar you need the same strike's prices in two expiries. The live NIFTY chain below shows the current expiry; on the option chain page you can switch expiries and compare:
Look at the ATM row and compare its premium with the same strike in a later expiry. If the gap between the two is small, the near option is relatively rich and the calendar is cheaper to open. Also check the bid-ask spreads: the far expiry is usually less traded, and a wide spread there eats into a small expected profit.
Managing a calendar while it is open
A calendar is usually held for only a few days, from opening until the near option expires. During that time, three things decide the result, and it helps to check them every day:
- Distance from the strike. The spread earns most when NIFTY stays within roughly one expected move of the strike. If the price drifts far away, the tent no longer covers it, and many traders close the spread rather than hope for a return.
- Implied volatility of the far option. Because the long leg carries most of the vega, a fall of a few IV points can erase a day or two of decay gains. Watching India VIX gives a quick sense of the direction.
- Time to the near expiry. The spread's value usually grows fastest in the last day or two, when the short option decays hardest. That is also when gamma is highest, so a late move does the most damage.
A simple rule many traders use is to take profit once the spread has gained a set share of the debit, such as a quarter or a third, and to exit if it loses a similar share. It keeps a time-limited trade from turning into a hopeful one.
If NIFTY moves away from the strike but you still expect it to settle, one common repair is to add a second calendar at the new price, forming a double calendar. It widens the profitable zone but doubles the debit at risk, so it is a deliberate new trade, not a free fix.
Variations
- Put calendar. The same structure with puts. At the same strike, call and put calendars behave very similarly.
- Diagonal spread. The two legs have different strikes as well as different expiries, for example sell this week's 25,200 call and buy next month's 25,000 call. It adds a directional lean.
- Double calendar. One calendar above the price and one below, which widens the profitable zone at the cost of a larger debit.
- Rolling. After the weekly expires, some traders sell the next weekly against the same monthly option, collecting decay several times from one long leg.
Risks and common mistakes
- A big move either way. The tent is narrow. A trending week takes NIFTY away from the strike and the spread loses.
- A fall in implied volatility. Opening before an event and holding through the IV crush hurts the long leg more than it helps the short leg.
- Leaving the long leg naked. After the near option expires, the far option alone is a plain bought option that keeps decaying. Decide in advance whether to close it, roll a new short against it, or hold it as a view.
- Margin assumptions. Margin rules for spreads across expiries can differ from same-expiry spreads, and the benefit may change near the short leg's expiry. Check with your broker.
- Trusting the estimate. The far leg's value at the near expiry is a model, not a fact. Use the strategy builder to see the curve, and treat it as approximate.