Gamma Risk on Expiry Day: A Bank Nifty Example That Explains Everything

In short: On expiry day, an at-the-money option's Gamma is enormous, so its Delta swings from near 0.50 toward 0 or 1 on a small move in the index. A short ATM Bank Nifty option that looks like a calm ₹40/point liability at the open can become a ₹65 or ₹80/point liability within an hour as Gamma pushes its Delta up — the loss accelerates rather than growing in a straight line. That acceleration, not the size of the move, is what ruins expiry-day option sellers.

Why expiry-day Gamma is the whole story

Gamma is the rate at which Delta changes for a move in the underlying, and it is inversely related to the square root of time left. As time to expiry collapses toward zero, the Gamma of the at-the-money strike goes vertical. Away from expiry, an ATM Bank Nifty option's Delta might move from 0.50 to 0.55 on a 100-point move; on the morning of expiry the same 100 points can drive it from 0.50 to 0.75 or beyond. This is not a rounding effect — it is the defining feature of expiry-day risk. Every rupee of Theta a seller collects on expiry day is compensation for carrying this concentrated Gamma, and on the final day the Gamma bill can arrive faster than the Theta ever accrues.

The setup: a short ATM straddle on expiry morning

Suppose Bank Nifty opens expiry day at 52,000. A seller writes the 52,000 CE and 52,000 PE — the classic expiry-day short straddle — collecting, say, ₹180 for the call and ₹170 for the put, ₹350 combined. Lot size is 35 for Bank Nifty, but the mechanics are identical to Nifty's 75, so we will track the per-share Greeks and multiply at the end. At the open each leg has Delta near ±0.50, and combined the position is roughly Delta-neutral. The trap is that neutrality is an illusion that lasts only as long as the index sits still. The straddle's combined Gamma is at its peak for the entire life of these contracts, precisely because it is expiry morning and both legs are at-the-money.

Hour one: the move begins

Say Bank Nifty grinds up 150 points to 52,150 by 10:30 am. The 52,000 CE, starting at Delta 0.50 with a Gamma of about 0.004 per point, sees its Delta climb toward roughly 0.50 + (0.004 × 150) = 1.10 — capped at 1.0, so call it ~0.72 once you account for the curve flattening. The 52,000 PE's Delta falls from −0.50 toward about −0.28. Net position Delta for the short straddle (short call, short put) is now short roughly 0.72 − 0.28 = 0.44 per share on the wrong side. The seller, who was neutral 40 minutes ago, is now effectively short about 0.44 units of Bank Nifty per share into a rising market — the position un-hedged itself, which is exactly what short Gamma does.

Hour two: the loss accelerates, not grows

Bank Nifty adds another 150 points to 52,300 by 11:30 am. Because Gamma kept feeding the call's Delta, the CE is now around 0.85 Delta and the PE around −0.15. The second 150-point leg of the move cost far more than the first: the position was losing roughly ₹0.44/point at the start of the hour and is losing closer to ₹0.70/point by the end. This is the heart of Gamma risk — the losses are convex. A linear thinker budgets for a 300-point move as twice a 150-point move; in reality, on expiry day, the second half of the move hurts far more than the first because Delta itself grew while the move was happening.

The rupee tally

Track the call alone. Sold at ₹180 with Bank Nifty at 52,000; with the index at 52,300 the intrinsic value is already ₹300, and with a few hours left there is still time value on top — say the CE now trades at ₹330. That is a ₹150 loss per share on the call. The put, sold at ₹170, has decayed to perhaps ₹40, a ₹130 gain. Net: down ₹20 per share, or ₹20 × 35 = ₹700 per lot — and that is after only a 300-point move with hours left to run. If Bank Nifty extends to 52,600, the call's Delta is near 1.0, the put is nearly worthless, and the loss on the pair widens fast while the put can no longer offset anything. The put's cushion is exhausted; the call's loss is now nearly one-for-one with the index.

Why the Theta never saves you in time

Sellers comfort themselves that Theta is highest on expiry day, and it is — the straddle might decay ₹80–₹100 over the full session if the index sits still. But Theta accrues smoothly across hours, while Gamma-driven losses arrive in minutes when the index trends. A 400-point Bank Nifty move — routine on expiry day — can hand a short ATM straddle a loss several times larger than the entire day's Theta before lunch. You cannot out-collect a convex loss with a linear income. This is why the expiry-day short straddle is a strategy that wins small on most days and loses catastrophically on the few days the index trends hard.

How professionals actually handle it

Desks that sell expiry-day premium do three things retail sellers skip. First, they size tiny — a fraction of the capital a directional trade would use — because they know one trending expiry can erase a month of collections. Second, they adjust the tested side early: the moment the call's Delta crosses a threshold (say 0.65), they buy it back or add a hedge rather than hoping for a reversal. Third, many avoid naked ATM straddles entirely and instead sell iron flies or condors with defined-risk wings, so the Gamma is capped by long options further out. The common thread is respect for convexity — they never let the position's Delta run unbounded while the clock and the Gamma work against them together.

The one-line lesson

Direction did not blow up the expiry-day seller — acceleration did. A 300-point Bank Nifty move is unremarkable, but on expiry day the second-order Greek turned it into a runaway loss because Delta grew while the move was underway. Understand that Gamma is highest exactly where and when sellers are most tempted — at-the-money, on expiry day — and you understand why 'easy' expiry premium is the most expensive lesson in Indian options trading.

Frequently asked questions

Sources & references

• NSE India — Derivatives
• Zerodha Varsity — Gamma

Published 6 July 2026. Educational content only — not investment advice.