Charm - Why a Delta Hedge Ages Overnight

By EC Assets Research Team, Derivatives Strategy · Published 12 August 2026 · Updated 12 August 2026

Charm: Charm, also called delta decay, measures how an option's delta drifts purely with the passage of time while everything else stays fixed. It is why a hedge that was flat on Friday is no longer flat on Monday, and why dealer hedging becomes mechanical and predictable in the final days before a large expiry.

What Charm Measures

Charm is the rate at which delta changes with time:

Charm = ∂Delta/∂t = ∂²V/∂S∂t

It is often called delta decay or delta bleed, and it captures something that is easy to state and easy to forget: an option's delta is not a property of the underlying price alone. Hold spot, volatility and rates perfectly constant, let a day pass, and the delta has changed anyway.

The reason is that delta is, loosely speaking, the market's assessment of how likely the option is to finish in the money, adjusted for how much that matters. Time passing removes opportunity for the underlying to travel. An out-of-the-money option therefore watches its delta drift toward zero as expiry approaches, while an in-the-money option watches its delta drift toward one. The option is becoming more certain of its own fate.

Where Charm Is Largest

Charm is most pronounced for options that are near the money and close to expiry, which is precisely where the largest open interest tends to sit in index markets. Far from the money, delta is already near zero and has little room to decay. Deep in the money, delta is already near one. In the middle, and with days rather than months remaining, delta can move substantially from one session to the next with no help from the market at all.

The sign follows the moneyness. Below the strike, a call's delta bleeds downward; above it, upward. This creates a systematic, direction-dependent hedging requirement that grows as expiry approaches.

The Weekend Problem

Charm is measured in calendar time, but hedging happens in trading time. Between Friday's close and Monday's open, three days of delta decay accrue while no market exists in which to adjust.

Desks handle this by pre-hedging: on Friday afternoon they set a delta that anticipates where the book's delta will have drifted by Monday, rather than the one that is correct at the closing bell. The practice is routine, and it means Friday afternoon flows in index markets carry a component that reflects nothing about the coming week's fundamentals. The same logic applies, in larger form, ahead of long holiday weekends.

Charm Into a Large Expiry

In the final week before a major monthly expiry, charm and vanna together make dealer hedging unusually mechanical. The delta of a large book of near-the-money options is decaying fast and in a knowable direction, so the required hedging transactions are, to a considerable extent, predictable in advance.

This has two consequences. Positions clustered around heavily traded strikes reinforce the pinning behaviour associated with positive dealer gamma, since the hedging flow needed to stay neutral pushes spot back toward those strikes. And once the expiry passes, the entire block of decaying delta disappears from the book at once, removing a flow that had been steadying the market for days.

Worked Example

A desk holds a large position in index calls struck slightly above the current level and goes home on Friday with a flat delta.

No news arrives over the weekend and the index opens unchanged on Monday. The calls, however, are now three days closer to expiry and remain out of the money, so their delta has bled lower. The desk's option delta has fallen, its hedge is unchanged, and the book is therefore no longer neutral. To restore the hedge it must transact on Monday morning, in a direction determined entirely by the calendar.

A desk that pre-hedged on Friday has already made most of that adjustment, which is why the flow appears on Friday afternoon rather than Monday morning, and why it is invisible to anyone reading Monday's move as a reaction to weekend news.

[!key] Charm is delta decay: the drift in an option's delta caused purely by time passing. It is largest for near-the-money options close to expiry, pushes out-of-the-money deltas toward zero and in-the-money deltas toward one, and accrues over weekends when no market is open to hedge it.

[!warning] A delta hedge is not self-maintaining. Set it and leave it, and charm alone will have made it wrong within a day, with no move in the underlying required. On a large near-dated book this is a material exposure rather than a rounding error, and it is the most commonly overlooked of the second-order Greeks.

Why It Matters for Institutional Investors

• Rebalancing is not optional. Any strategy that relies on a maintained delta hedge, including covered-call programmes and structured product hedging, incurs charm-driven transactions regardless of market direction. A back-test assuming a static hedge understates both cost and tracking error.
• Reading Friday and expiry-week flows. Index flows late on a Friday and in the days before a monthly expiry contain a mechanical component from pre-hedging. Treating that flow as a signal about the coming week is a straightforward misreading.
• Position ageing. Options positions change character as they approach expiry even in a still market. An exposure sized on today's delta is not the exposure held next week, which matters for any mandate with fixed risk limits.

References

1. Taleb, N. N. (1997). Dynamic Hedging: Managing Vanilla and Exotic Options. Wiley.
2. Hull, J. C. (2022). Options, Futures, and Other Derivatives (11th ed.). Pearson.
3. Natenberg, S. (2015). Option Volatility and Pricing (2nd ed.). McGraw-Hill.
4. Ni, S. X., Pearson, N. D., & Poteshman, A. M. (2005). Stock price clustering on option expiration dates. Journal of Financial Economics, 78(1).
5. Bossu, S. (2014). Advanced Equity Derivatives: Volatility and Correlation. Wiley.