What is Option Charm?
Option Charm measures how an option's Delta changes as time passes, with the other pricing inputs held constant under the selected model.
Charm is sometimes called Delta decay. It is a Second-Order Option Greek because it measures the change in a first-order Greek rather than the option price directly.
Charm is different from Theta
Option Theta measures how the theoretical option value changes as time passes.
Charm measures how Delta changes as time passes.
A position can therefore have modest Theta but meaningful Charm, or vice versa. They answer different questions about the evolution of an option position.
Why a Delta hedge can drift without a price move
Suppose a portfolio is delta-neutral at today's close.
If the underlying price and implied volatility were unchanged overnight, the portfolio's Delta could still be different the next day because time to expiration is shorter. Charm describes that local time-driven change in Delta.
The effect can matter for portfolios that are frequently rehedged, especially near expiration when option sensitivities can change quickly.
A simple intuition
As expiration approaches, an option's payoff becomes increasingly tied to whether the underlying is above or below the strike.
For many options, Delta therefore evolves as the remaining time shrinks. Deep in-the-money and deep out-of-the-money options tend toward their expiration-state Delta behavior, while near-the-money options can remain highly sensitive to small changes in the underlying.
Charm provides one local measure of that time evolution.
Charm is not a guaranteed daily Delta change
Charm is model-based and local.
It does not mean an option's Delta will change by exactly the quoted amount over the next calendar day. During that period, the underlying can move, implied volatility can change, interest rates and dividend assumptions can change, and the volatility surface can reshape.
Reporting conventions also matter. A system may express the time increment differently, so users should confirm whether the displayed value is scaled per day, per year, or another convention.
Where Charm can matter
Charm is particularly relevant when:
- a position is actively delta-hedged;
- expiration is near;
- the portfolio contains many short-dated options;
- traders care about the Delta expected to exist after a known passage of time; or
- a multi-leg structure has offsetting current Delta but different time evolution across legs.
The net Charm of a portfolio matters more than the Charm of an isolated contract when assessing hedge drift.
Relation to Vanna and Vomma
Option Vanna describes how Delta changes as implied volatility changes. Charm describes how Delta changes as time passes. Option Vomma describes how Vega changes as implied volatility changes.
These higher-order sensitivities emphasize the same lesson: an option Greek is a local snapshot, not a permanent characteristic of the position.
Sources and further reading
- Options Industry Council: Options Glossary, Charm
- Options Industry Council: May Office Hours FAQs
- Options Industry Council: Understanding Options Greeks
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