Option prices react to the underlying price (delta), to time passing (theta) and to the market's nervousness. That third force is measured by implied volatility (IV), and the Greek that tells you how much IV moves the price is vega.
Change in option price ≈ vega × change in IV (in percentage points)
If a BANKNIFTY call has a vega of 31, it gains about ₹31 when IV rises from 15% to 16%, and loses about ₹31 when IV drops from 15% to 14%, with the index unchanged. Vega is the same sign for calls and puts: both become more valuable when IV rises. (Vega is not a real Greek letter; the name stuck anyway.)
What vega depends on
- Time to expiry. Vega grows with the square root of the time left. An option with a month to go has roughly twice the vega of one with a week to go. On expiry day, vega is very small: there is too little time left for a change in IV to matter much.
- Moneyness. Vega is highest at the money and falls off for strikes far in or out of the money. In rupees, an ATM option has the most vega; as a share of its price, a far out-of-the-money option can be the most sensitive.
- Price of the underlying. Vega is quoted in rupees, so a bigger underlying (BANKNIFTY at 56,000 against NIFTY at 25,000) has bigger vega numbers for the same IV change.
A worked example: weekly vs monthly BANKNIFTY
Take BANKNIFTY at 56,000 and IV at 15%. Compare the at-the-money call with 7 days left and with 30 days left (illustrative numbers from the usual pricing model):
| ATM call | 7 days left | 30 days left |
|---|---|---|
| Price at 15% IV | about ₹465 | about ₹963 |
| Vega (per IV point) | about ₹31 | about ₹64 |
| Price if IV rises to 18% | about ₹558 | about ₹1,155 |
| Price if IV falls to 12% | about ₹372 | about ₹771 |
A three-point IV change moves the monthly call by about ₹190 and the weekly call by about ₹93, with the index not moving at all. As a share of the price, both move about 20%, but in rupees per lot the monthly option carries twice the exposure. Lot sizes change from time to time; check the current figure on NSE before turning these into rupees per lot.
A live example: where volatility stands now
India VIX, NSE's measure of NIFTY's expected volatility, gives a quick sense of whether IV is high or low right now:
When the IV rank is low, there is more room for IV to rise than to fall, which matters to anyone short vega. When it is high, the reverse applies. Neither is a guarantee: IV can stay low or high for long stretches. The BANKNIFTY straddle reacts to these IV changes directly:
When the straddle rises sharply while BANKNIFTY barely moves, that is usually vega: the market is paying more for options, often before an event or after a sudden fall in another market.
Long vega and short vega
For a whole position, add each leg's vega times its quantity (bought legs positive, sold legs negative).
- Long vega positions gain when IV rises: bought calls, bought puts, long straddles and strangles, and calendar spreads (which buy the later expiry with more vega and sell the nearer one with less).
- Short vega positions gain when IV falls: sold options, short straddles and strangles, iron condors and iron butterflies.
- Near-zero vega positions, such as narrow vertical spreads, care less about IV and more about direction.
A short straddle seller collecting premium before an event is making two bets at once: that the move will be small (short gamma) and that IV will fall afterwards (short vega). The second bet often pays; the first is where large losses come from.
When vega matters most
- Around scheduled events. IV usually builds before results, the Union Budget, RBI policy and major global data, then drops sharply after. That drop is the IV crush, and it is a vega loss for buyers.
- In sudden sell-offs. Index IV tends to jump when markets fall fast. Put buyers gain from both delta and vega; short option positions lose on both.
- For longer-dated options. Positional traders in monthly options often find that vega, not theta, drives most of their day-to-day profit and loss.
Vega, delta and theta together
In real trading the Greeks act at the same time, and vega often decides the result. Suppose a trader buys the weekly BANKNIFTY call from the table above at ₹465. Over the next day, BANKNIFTY rises 200 points, which adds roughly ₹105 through delta. A day of decay takes away about ₹35. If IV stays at 15%, the call ends near ₹535, a healthy gain.
Now suppose the rise came on the morning after an RBI policy announcement, and IV fell from 15% to 12% as uncertainty cleared. Vega takes away about 3 × ₹31 = ₹93. The call ends near ₹442, below what was paid, even though the index moved the right way. The same move with IV rising to 17% would have left the call near ₹597. Three traders, one price move, three different outcomes: that is why buyers check where IV stands before they buy.
Vega and strategy choice
Because vega is larger for longer expiries, traders use it to shape positions. Someone who expects IV to rise but has no view on direction may prefer a longer-dated straddle to a weekly one: more vega for each rupee of daily decay. Someone who expects IV to fall after an event may prefer to sell the expiry that contains the event, because that is where the extra IV sits. A calendar spread is a direct bet on the difference between near and far IV.
None of these choices removes risk. A long-vega position still pays theta every day, and a short-vega position still carries gamma risk on a big move.
Common mistakes
- Buying options right before an event and holding through it. The move may come, but the IV drop afterwards can cancel it.
- Assuming IV will "return to normal" soon. High IV can stay high in a nervous market for weeks.
- Ignoring vega on expiry day trades. It is small then, so IV changes matter less; for monthly options a week before expiry they matter a lot.
- Comparing vega across symbols without scaling. Vega is in rupees, so compare it with the option price or the position size, not as a raw number.
For the options whose IV is highest right now, see the IV leaders page.