Why does the ATM strike change during the day?

Because spot moves while strikes are fixed. When the underlying crosses the midpoint between two strikes, the nearest strike changes, and so does the ATM implied volatility read from it. This can make an ATM volatility series jump slightly even if the smile itself did not move.

Is the ATM call delta exactly 0.5?

Not quite. In Black-Scholes a spot-ATM call has a delta of N(d1), and d1 is positive because of the σ²/2 and carry terms, so the delta is a little above 0.5. The 30-day example with 20% volatility gives about 0.51.

At the money (ATM)

An option is at the money (ATM) when its strike equals, or is the closest listed strike to, the current underlying price. Several definitions coexist, the strike nearest spot, the strike at the forward price and the delta-neutral straddle strike, and they can point to different strikes on the same chain.

Senzoukria · Glossary · Updated September 2026

At a glance

Spot ATM: Strike closest to the current underlying price
Forward ATM: Strike equal to the forward F = S·e^((r−q)τ)
Delta-neutral straddle: Strike where call and put deltas cancel, K = F·e^(σ²τ/2)

Three definitions

Spot ATM is the plain-language meaning: the strike closest to where the underlying trades now. Forward ATM uses the forward price, which differs from spot by carry, and is the natural reference for pricing because the model's expected terminal price is the forward. Delta-neutral straddle ATM, common in currency options, is the strike at which a straddle has zero delta; in Black-Scholes it sits slightly above the forward, at F·e^(σ²τ/2).

Worked example

Assume SPY at 500, a 4% rate, a 1.3% dividend yield, 30 days to expiry and 20% implied volatility. The forward is 500 × e^((0.04 − 0.013) × 30/365) = 501.11, and the delta-neutral straddle strike is 501.11 × e^(0.2² × 0.0822/2) = 501.94. On a chain with one-dollar strikes, the three definitions select 500, 501 and 502. For a 30-day option the gap is small; for a one-year option or a high-carry underlying it widens.

What is special about ATM options

Extrinsic value, vega and, for short maturities, gamma are near their maximum at the money.
An ATM call has a delta close to 0.5, slightly above it in Black-Scholes because of the σ²/2 drift term.
ATM implied volatility is the reference level of a smile, and the term structure is usually drawn from one ATM point per expiry.
An ATM straddle price is a quick gauge of the move the market prices to expiry, about 0.8 times the one-standard-deviation move.

In Senzoukria

Senzoukria uses the spot definition. In the GEX module, the ATM IV card and each point of the IV term structure take the implied volatility of the listed strike closest to spot; when the provider publishes no ATM IV for the front expiry, the card falls back to the first expiry that carries enough implied volatility and marks the value as computed. In Option Flow, a print is labelled ATM when its strike is within ±0.5% of the underlying price.

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Frequently asked questions

Why does the ATM strike change during the day?: Because spot moves while strikes are fixed. When the underlying crosses the midpoint between two strikes, the nearest strike changes, and so does the ATM implied volatility read from it. This can make an ATM volatility series jump slightly even if the smile itself did not move.
Is the ATM call delta exactly 0.5?: Not quite. In Black-Scholes a spot-ATM call has a delta of N(d1), and d1 is positive because of the σ²/2 and carry terms, so the delta is a little above 0.5. The 30-day example with 20% volatility gives about 0.51.

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