Option Greeks Explained Simply: Delta, Gamma, Theta and Vega

Why Greeks exist

An option's premium changes for several reasons at once: the underlying moves, a day passes, and the market's expectation of volatility rises or falls. The Greeks split these effects apart so you can see which one is driving your profit or loss.

They come from option pricing models such as Black-Scholes and are estimates, not guarantees. But even approximate Greeks make option behaviour far less mysterious.

Delta: sensitivity to price

Delta estimates how much the premium changes for a 1-point move in the underlying. A call with a delta of 0.50 gains about ₹0.50 when the index rises 1 point. Puts have negative delta: a put with -0.40 gains about ₹0.40 when the index falls 1 point.

ATM options have a delta near 0.5 (or -0.5). Deep ITM options approach 1, and far OTM options approach 0. Delta is also a rough guide to how far the option is from expiring in the money, which is why many traders read a 0.20 delta option as an unlikely winner.

For a position, multiply by quantity. One lot of 75 with a delta of 0.5 behaves roughly like 37-38 units of the index for small moves.

Gamma: how fast delta changes

Gamma measures how much delta changes for a 1-point move. High gamma means delta can swing quickly, so the option's behaviour changes fast as price moves.

Gamma is highest for ATM options close to expiry. That is why expiry-day options can go from nearly worthless to valuable, or the reverse, within minutes. For option sellers, high gamma means risk can grow very quickly.

Theta: the cost of time

Theta estimates how much premium the option loses per day if nothing else changes. A theta of -8 means the option loses about ₹8 per unit per day from time decay alone.

Theta hurts option buyers and helps option sellers. It accelerates as expiry approaches, especially for ATM options. A buyer needs the price to move enough to outrun theta; a seller hopes the price stays calm so theta does the work.

Vega: sensitivity to volatility

Vega estimates how much the premium changes when implied volatility (IV) changes by 1 percentage point. If vega is 12 and IV rises from 14% to 16%, the premium rises about ₹24, even if the price has not moved.

IV often rises before events such as results, budgets or policy announcements and falls sharply afterwards. This "volatility crush" is why buyers can lose money after an event even when they guessed the direction correctly.

Putting the Greeks together

Example: you buy an ATM NIFTY call with delta 0.5, gamma 0.002, theta -10 and vega 11. The next day NIFTY rises 40 points, one day passes, and IV is unchanged. Approximate change: delta gain 40 x 0.5 = +20, gamma adds a little more (about +1.6), theta costs -10. Net about +12 per unit.

If IV had also dropped 2 points, vega would cost about -22, turning the trade into a small loss despite the rise. Breaking a result down like this tells you whether you were wrong about direction, time or volatility.

Implied volatility in a little more depth

Implied volatility is the volatility number that, plugged into a pricing model, gives the current market premium. It reflects what traders collectively expect, not what has actually happened. Historical volatility measures what did happen over a recent period.

When IV is well above recent historical volatility, options are relatively expensive and buyers need a bigger move to profit; sellers are paid more for the risk. When IV is unusually low, options are relatively cheap. Comparing IV with its own past range (IV percentile) gives a rough sense of whether premiums are high or low today.

How traders use the Greeks

Choosing strikes: delta helps pick an option that matches how strongly you expect the price to move.

Managing risk: summing delta across positions shows your overall directional exposure; summing theta shows how much time decay earns or costs you each day.

Around events: checking vega and IV before results or budgets helps avoid paying for volatility that will disappear afterwards.

The Greeks also change over time and with price, so they need to be checked again as the position evolves.

A worked example: same move, different strikes

Take two calls on the same index with seven days left. An ATM call costs ₹180 with delta 0.50 and theta -12. A far OTM call costs ₹30 with delta 0.12 and theta -4.

If the index rises 100 points over two days with IV unchanged, the ATM call gains roughly 100 x 0.5 = 50 plus a little from gamma, minus about 24 of theta, ending near ₹210. The OTM call gains about 12 plus a bit more from gamma, minus 8 of theta, ending near ₹36-38. In percentage terms the cheap option moved more, but it needed the move to arrive quickly; had the index risen only 30 points, the OTM call would likely have lost value while the ATM call held up better.

See Greeks on iTradeChart

Switch the option chain from OI to Greeks to see IV, delta and theta for every call and put. The IV card compares implied volatility with recent historical volatility so you can tell when options look expensive or cheap relative to recent movement.

The options strategy builder shows the payoff of combinations at expiry, and paper trading lets you watch how a real premium changes as the Greeks predict.