Charm — the Hidden Greek of 0DTE
Charm measures how an option's delta changes as time passes, with the underlying held constant. For 0DTE, charm pushes OTM deltas toward zero and ITM deltas toward ±1 over the trading day — a force that doesn't show up on most retail platforms but moves position deltas materially.
Charm is a second-order greek — the rate at which delta changes as time passes, with everything else held constant. Most retail platforms don't display it.
For 0DTE specifically, charm is the force that pushes OTM deltas toward zero and ITM deltas toward ±1 over the trading day, independent of any underlying movement. It's why a 0DTE short OTM position can lose delta exposure even when the underlying doesn't move at all — and why an ITM 0DTE position gains "automatic" delta as the clock runs out.
This article covers what charm is, the OTM/ATM/ITM drift pattern, why charm is favorable for short OTM premium-sellers and dangerous for short ATM positions, the magnitude of charm on 0DTE, and what to do about it in practice.
The definition
A second-order greek measuring the rate at which an option's delta changes as time passes, holding everything else constant. Mathematically, charm = ∂Delta/∂T (or equivalently the cross-partial ∂²Price/∂T∂S). Units: delta change per day of time passing. Also called delta decay or DdeltaDtime. Most retail platforms don't display it.
If a call has delta 0.30 and charm -0.04, then one day of time passing (with the underlying flat) would reduce the call's delta to approximately 0.26. The sign convention: charm is typically negative for OTM positions (delta drifts toward zero) and positive for ITM positions (delta drifts toward 1 or -1).
The magnitude varies with strike, time, and IV. For longer-dated options, daily charm is small enough that traders can ignore it. For 0DTE, charm becomes large enough that ignoring it means misreading the position's actual delta a few hours later.
How charm acts on different strikes
The drift pattern follows a clean logic. As time passes:
- OTM calls: delta drifts toward 0. The option is increasingly unlikely to finish ITM, so its delta — which is roughly the probability of finishing ITM — shrinks toward zero.
- ITM calls: delta drifts toward 1. The option is increasingly certain to be exercised, so its delta climbs toward the maximum of 1.
- OTM puts: delta drifts toward 0 (from a starting point of slightly-negative).
- ITM puts: delta drifts toward -1.
- ATM options (call or put): delta sits near ±0.5 and is unstable. As expiration approaches, ATM delta can drift either direction depending on minute underlying movements. Covered below.
The intuition mirrors theta. Both greeks describe what time does to the option, with everything else held constant. Theta tells you how the option's price changes; charm tells you how its delta changes. Both grow in magnitude as expiration approaches.
Why this matters for short premium
A short premium-seller's position delta is the opposite sign of the short option's delta. If they're short an OTM put with delta -0.16, the position has delta +0.16 (long the underlying exposure — shorting a put is bullish). As time passes and charm pulls the put's delta toward zero, the position's delta also drifts toward zero — becoming less directional.
This is favorable for the seller in the absence of underlying moves: the position becomes less directionally exposed automatically as time passes. A concrete sketch (assuming SPX stays flat all day):
- 10am: short 16-delta SPX put. Position delta = +0.16.
- 1pm: charm has drifted the put's delta toward zero. Position delta around +0.08.
- 3pm: position delta near +0.03 — essentially flat.
- 3:50pm: near zero. The position is no longer meaningfully directional.
The position has become progressively less directional over the day — even with no underlying movement — because the short OTM option's delta was steadily shrinking.
This is part of why short premium "feels good" in the middle of a quiet day — the position appears to be working as deltas shrink. The catch is that this favorable drift depends on the short staying OTM. The moment the underlying threatens the short strike, gamma takes over and the favorable charm story flips.
Why charm is dangerous for short ATM
The flip side shows up for positions near ATM at expiration. As time runs out, ATM delta becomes increasingly unstable. The same minute underlying movement that produced a small delta change in the morning produces a large delta change in the final hour — and charm amplifies that by pulling delta sharply toward 0 or ±1 depending on which side of the strike the underlying lands.
A position that was delta-neutral at noon can become meaningfully directional by 3pm without the underlying having moved much:
- 5000 short straddle at 10am — delta-neutral by construction.
- By 2pm, SPX is at 5002. The short call's delta might be around 0.55, the short put's around -0.45. Position delta = -0.10.
- By 3pm, with SPX still around 5002: short call's delta has drifted to ~0.65 (charm pulling it toward 1 since the call is now ITM), short put's to ~-0.30. Position delta = -0.35.
- By 3:50pm, with SPX still at 5002: short call delta has drifted to ~0.85, short put to ~-0.15. Position delta = -0.70.
Same underlying level (5002) at 10am vs 3:50pm, but the position is substantially more short-biased by the close. That's charm acting on top of gamma.
Combined with gamma at ATM (covered in Pin Risk and the Gamma Trap), charm makes short ATM positions in the final hours behave in ways that look chaotic relative to a static-delta mental model. The position's actual delta is moving from two sources simultaneously: underlying movement (gamma) and time passing (charm).
Charm's magnitude on 0DTE
Charm scales roughly with 1/T — same pattern as gamma. It's largest near expiration. For a typical SPX option at moderate IV:
- A 30-day OTM option has daily charm small enough to be irrelevant — delta might drift a fraction of a percent per day.
- A 7-day OTM option's daily charm is noticeably larger but still modest in absolute terms.
- A 0DTE OTM option's hourly charm can move delta by several percent depending on strike and remaining time.
The exact numbers depend on strike, IV, and time remaining. The qualitative pattern — charm grows fast as expiration approaches — is what matters for 0DTE position management. By the final hour, charm on near-ATM strikes is large enough that the position's delta can change meaningfully every few minutes from time alone.
What to do about it
Practical guidance for charm in a 0DTE context:
Don't trust a single delta reading. A delta you saw earlier in the session may not be the delta you have now. Re-check before sizing or hedging decisions, especially as the afternoon progresses.
For short OTM premium positions, expect favorable drift. The position's delta will shrink over the day as long as the underlying stays away from your shorts. That's charm working in your favor.
For short ATM positions, recognize the instability. Charm makes ATM delta increasingly unpredictable as expiration approaches. This is another reason to close short ATM positions before the final hour — the delta you're managing is moving from both gamma (underlying) and charm (time) simultaneously.
For long-option positions, charm works against you. Your long OTM call's delta drifts toward zero as time passes — meaning your directional exposure is shrinking even as theta drains the premium. Long-OTM 0DTE trades are losing both ways during the day.
Pre-committed exit times help. A 3:30pm exit avoids the most charm-active period for any position type. Charm is largest in the final hour, when both gamma and charm are working at maximum magnitude.
Why platforms don't show it
Most retail options platforms display delta, gamma, theta, vega — but not charm. The reason isn't that charm doesn't matter. It's that for typical multi-day or longer-dated positions, charm is small enough to be ignored.
For 0DTE specifically, charm is large enough to materially affect position management — but the platform convention hasn't caught up. Traders who care about charm have to either compute it themselves (the formula is in any standard derivatives reference) or work backwards from observed delta drift over the day.
The practical implication: if you're trading 0DTE, you're operating on a greek the platform isn't showing you. Reading the position's actual delta a few times per session and tracking how it changes — even with the underlying flat — gives you a working sense of how charm is acting on your specific position.
Key takeaways
- Charm is the rate at which an option's delta changes as time passes, with everything else held constant. The greek most retail platforms don't display.
- The drift pattern: OTM deltas drift toward 0, ITM deltas drift toward ±1, ATM deltas are unstable near expiration. Charm is gravity pulling delta toward its expiration value.
- For short OTM premium positions, charm is a silent helper — the position becomes less directional as time passes, automatically.
- For short ATM positions near expiration, charm makes delta increasingly unstable, combining with gamma to produce sharp directional swings from small underlying moves.
- Charm is large enough to matter on 0DTE even though platforms don't show it. Re-check position deltas through the session; charm changes them even when the underlying doesn't move.