Theta decay is the erosion of an option's extrinsic value as expiration approaches. It is the seller's income and the buyer's rent — the cost of holding a position that needs time to be right.
What is actually decaying
An option's price has two parts:
- Intrinsic value — what it is worth if exercised right now. A $100 call with the stock at $105 has $5 of intrinsic value.
- Extrinsic value — everything else. The premium paid for the possibility the stock moves further before expiration.
Only extrinsic value decays. At expiration it is zero by definition, and the contract is worth its intrinsic value or nothing. Theta measures the daily rate of that erosion.
The curve is not a line
This is the part people get wrong.
Theta accelerates as expiration approaches. A 90-day option loses extrinsic value slowly; the same contract with 7 days left bleeds it fast. The curve steepens sharply inside the final two weeks, and in the last few days it falls off a cliff.
Two consequences follow directly:
Sellers favour short-dated contracts. The 30–45 day window is a common preference precisely because you capture the steep part of the curve while retaining time to manage the position.
Buyers of short-dated options are fighting the clock. A weekly option needs the move to happen almost immediately. Being right about direction and wrong about timing loses money, and it loses it fast.
Theta is not free money
Selling premium collects theta. It also accepts the risk the premium was pricing.
You are short optionality: your gain is capped at the premium, and your loss is not. Theta pays you a small amount reliably in exchange for occasionally paying out a large amount. That trade can be excellent — the volatility risk premium is real and persistent — but the shape of it is many small wins punctuated by the loss that matters.
Anyone describing theta as "income" without describing that shape is not describing the trade.
Theta and charm
They are related and distinct. Theta is the decay of the option's price. Charm is the decay of its delta. Theta tells you what the contract loses overnight; charm tells you how much your directional exposure changed while you slept.
Why the platform is called Theta Vantage
Theta decay is the engine behind income strategies — the wheel, covered calls, cash-secured puts, credit spreads. Selling it deliberately, on names you understand, at strikes chosen with real data rather than instinct, is the difference between running a strategy and collecting pennies in front of something heavy. The vantage point is the point.