Charm — also called delta decay — measures how much an option's delta changes purely from the passage of time. It answers a question theta cannot: how much must a dealer adjust their hedge overnight if the underlying does not move at all?
Theta and charm are not the same thing
The distinction trips up most people, and it is worth being precise:
- Theta is the time decay of an option's price. It is what the contract loses overnight.
- Charm is the time decay of an option's delta. It is how much hedging must change overnight.
Theta is the seller's income. Charm is the dealer's homework.
How it works
An out-of-the-money option's delta drifts toward zero as expiration approaches, because there is less time left for the underlying to reach the strike. A 0.20 delta call with three weeks to run might be 0.12 delta with one week left — same price, same volatility, just less time.
For a dealer hedging that short call, the required stock hedge shrinks along with the delta. They must sell the excess. Multiply across a chain of expiring contracts and the flow becomes significant.
In-the-world terms: time alone forces transactions, even on a flat tape.
Why Fridays look the way they do
Charm accelerates sharply into expiration, and the expirations carrying the most open interest land on Friday. Through a Friday session, out-of-the-money deltas collapse toward zero and dealers unwind the hedges behind them. That unwind is directional — and it is the mechanical explanation for the drift into the close that traders notice and often misattribute to sentiment.
Monthly OPEX amplifies it, because far more open interest is expiring and vanna is usually unwinding at the same time.
Using it
Charm is most useful as an explanation, not a signal. If you find yourself asking why a stock drifted steadily higher on a quiet Friday afternoon with no news and no volume, charm and vanna are usually the answer. Knowing that keeps you from reading meaning into flow that carries none.
For traders holding short-dated positions into expiration, charm is also a practical warning: your delta is changing even when nothing happens. A position that felt balanced on Wednesday can be meaningfully directional by Friday afternoon without the underlying moving a cent.