Theta decay explained for option sellers

What theta is

Theta is how much an option's price falls each day if nothing else changes. A put with a theta of 0.05 loses about five cents of value a day. The seller collects that decay; the buyer pays it.

Decay is not even. It is slow when expiry is months away and speeds up in the final weeks, especially for options near the money.

Why theta is not free money

Theta is the price of the risk the seller carries. The option loses time value each quiet day because on some days the stock moves enough to make it worth much more. Over many trades a seller profits only if the decay collected beats those moves, which is the variance risk premium covered in the implied volatility guide.

In our paper book that premium was steady for months and then near zero for a month, and returns followed it. The decay kept happening every day either way.

Fast decay versus net result

Short-dated options decay fastest, which makes them look like the best theta trade. Our replays did not bear that out: puts sold 7 to 14 days out returned minus 0.34% a trade on collateral against plus 0.97% for 30 to 45 days on the same weeks. Faster decay came with less cushion against a move and a bigger share of the credit lost to spreads and fees.

The win and loss shape explains the rest. On our paper book the average winner was about plus 1.4% of collateral and the average loser about minus 2.8%, a two-to-one ratio, so the strategy needed about 86% winners to stay ahead, and it got 85.6%.

Reading theta on your own positions

Theta tells you what a quiet day pays. Delta and gamma tell you what a loud day costs. Look at both before judging a position. The guide on DTE covers choosing the decay window, and the one on closing at 50% covers when to stop collecting it.