0DTE Options: Why Same-Day Expiration Gamma Hits Different

Before this article: Long Gamma vs Short Gamma Regime: Why the Market Trends or Mean-Reverts.

What 0DTE means

0DTE stands for "zero days to expiration": an option that expires the same day it's traded. Today they represent a huge share of daily options volume on indices like the S&P 500 and the Nasdaq-100.

Why Gamma spikes near expiration

The standard Gamma formula (Black-Scholes model) is:

$$ \Gamma = \frac{\phi(d_1)}{S \cdot \sigma \cdot \sqrt{T}} $$

Where:

• $\Gamma$ = the option's Gamma.
• $\phi(d_1)$ = the normal probability density evaluated at $d_1$ (it's highest when the option is ATM, current price ≈ strike).
• $S$ = the underlying's price.
• $\sigma$ = implied volatility.
• $T$ = time to expiration, in years.

Key idea: $T$ sits in the denominator, inside a square root. As $T$ approaches zero (expiration nears), the denominator shrinks fast and Gamma grows quickly, especially for ATM options, where $\phi(d_1)$ is already at its peak.

Comparison by time to expiration

flowchart LR
    A["Large T<br/>(30+ days)"] --> B["Low Gamma"]
    C["Small T<br/>(1DTE)"] --> D["High Gamma"]
    E["T → 0<br/>(0DTE)"] --> F["Peak Gamma"]

Numeric example

Example: an ATM option on the underlying with 30 days to expiration might have a reference Gamma of 0.01. That same option, on its expiration day (0DTE), can have a Gamma several times larger, an illustrative order-of-magnitude figure, consistent with the formula: $T$ goes from a large fraction of a year to a tiny fraction, and the denominator shrinks drastically.

What this means for trading

Because 0DTE Gamma is so high, dealer hedging at those strikes is much more reactive: it rebalances on small moves, many times per hour. This is what drives the most reactive intraday levels on the map (Long-Wall and Short-Fuel), covered in a dedicated article later in this guide.

Note: more Gamma means more reactive: price can react more forcefully to relatively small amounts of options flow.

Next step

You've now seen why 0DTE Gamma is different. The next article ties together everything you've learned (Delta, Gamma, per-strike aggregation) into the metric you'll actually read on the chart: Net GEX.

Continue with: Net GEX (Net Gamma Exposure): What It Measures and How to Read the Chart