Theta: Time Decay & The Trade Life Cycle

Every option premium has two parts. The first part is intrinsic value: the amount the option would be worth if you used it right now. A NIFTY 24,500 call with NIFTY at 24,700 has ₹200 of intrinsic value. The second part is time value: the extra price people pay for the chance that the market moves in their favour before expiry. Theta melts only the second part.

Think of the ice block. The hard core of the block is intrinsic value and it does not melt with time. The outer layer is time value, and it melts every day. On expiry day the outer layer is gone completely. What remains is only the core, or nothing at all if the option is out of the money.

The picture below follows one at-the-money NIFTY call from 30 days to 2 days before expiry. NIFTY does not move. The premium goes from ₹393 to ₹102. Look at the loss per day at each stage. It gets bigger as the end comes closer.

Many beginners think decay is a straight line: ₹300 of time value over 30 days means ₹10 a day. It is not. With 90 days left, the market has a long runway to make a big move, so a day passing costs very little. With 2 days left, one more day is a large share of what remains.

A rough rule from the standard pricing model: time value of an at-the-money option grows with the square root of the time left. That is why the premium in the picture fell only ₹115 in the first 15 days (about ₹8 a day) but ₹102 in the last 2 days (about ₹51 a day). Square-root behaviour means the last stretch is the steepest.

The steepness is greatest for at-the-money options, where the whole premium is time value. Deep in-the-money options carry very little time value, so they melt slowly. Far out-of-the-money options have small premiums to begin with, so they have small rupee decay but can lose most of their price in percentage terms.

Take the buyer first. Suppose you buy the at-the-money call for ₹190 with 7 days left, because you expect NIFTY to rise. Theta charges you about ₹18 a day. NIFTY must rise enough, fast enough, to beat that charge. If NIFTY moves up quickly, delta can pay you far more than theta takes, and time decay hardly matters. If NIFTY drifts sideways for a week, you can lose most of the premium without being wrong about the direction in the long run.

Now the seller. The seller collects ₹190 today and hopes to keep it. Each quiet day, theta moves about ₹18 from the option price into the seller's favour. This is why some traders call selling "collecting rent". But the rent is small compared to the danger. If NIFTY gaps up by 300 points on news, the seller can lose several times the premium collected, and the loss on an unhedged short option has no fixed limit. Sellers of options must also keep margin with the broker, and that margin can be raised on volatile days.

So neither side is safe. The buyer's loss is limited to the premium but happens often when the market is slow. The seller's gains are limited to the premium but the loss can be much larger and can come suddenly. Professionals who sell often choose contracts a few weeks from expiry, and many close early to avoid the final-week swings. These are common habits, not rules, and they do not remove risk.