Charm (Delta Decay)

What Is Charm?

Charm — also called delta decay — measures the rate at which an option’s delta changes with respect to the passage of time. It answers the question: if nothing else changes and one day passes, how much will delta shift? An option with a charm of -0.05 will see its delta decrease by 0.05 per day, all else equal.

Charm is a second-order Greek derived from the interaction of delta and theta. For out-of-the-money options, charm is typically negative for calls and positive for puts, pulling delta toward zero as expiration approaches. For in-the-money options the dynamic reverses — delta is drawn toward 1.0 (calls) or -1.0 (puts) as time runs out.

The magnitude of charm is not constant. It accelerates as expiration approaches, mirroring the behavior of theta. Just as time decay is not linear, neither is delta decay. The final days before expiration see the fastest charm values, which is why an option’s directional behavior can shift dramatically in a matter of hours for short-dated contracts.

Why It Matters for Options Traders

Charm is most consequential for traders managing positions over multiple days or through overnight risk. A delta-hedged position that was neutral at the open may become directionally exposed by the next morning simply due to charm — even if the underlying price did not move. Dealers and market makers who maintain delta-neutral books must account for charm when rolling hedges overnight.

For 0DTE traders, charm operates in compressed real-time. What normally unfolds over days telescopes into hours or minutes. An at-the-money 0DTE option near the lunch session may carry a very different delta by the final hour of trading, even without a significant price move. This is the mechanical drag that works against holding long 0DTE positions through intraday time decay.

Charm also explains why dealer hedging flows can persist in a direction even on quiet days. If dealers are long short-dated puts (e.g., from clients buying protection), the charm on those puts pulls put deltas toward zero as time passes, causing dealers to gradually unwind their long underlying hedges — a slow but predictable bid erosion without any price catalyst.

Key Characteristics

• Second-order Greek: Charm is derived from delta and theta; it is not directly visible in most retail platforms but underlies observable delta drift
• Strongest near expiration: Charm accelerates alongside theta as expiration approaches, particularly in the final 1-5 days
• Direction by moneyness: OTM options have charm pulling delta toward zero; ITM options have charm pushing delta toward 1.0 (calls) or -1.0 (puts)
• Overnight hedge drift: Positions that are delta-neutral at close can be off-neutral at the next open due to charm acting over the non-trading hours
• 0DTE amplification: Same-day expiration compresses charm’s effect into hours, making delta behavior extremely sensitive to time passage
• Dealer flow implication: Aggregate charm across open interest creates predictable directional drift in hedging flows on low-volatility days