0DTE options: how same-day decay actually works
What the math says about same-day contracts, hour by hour
Published July 20, 2026
A 0DTE option is a contract with zero days to expiration: it expires at the end of today's session. Once a niche, these same-day contracts have grown into the busiest corner of the index options market, and on many days they make up roughly half of all S&P 500 (SPX) option volume.
If you have searched for a 0DTE strategy or a 0DTE options calculator, the useful place to start is not a setup, it is the math. Same-day options decay on a different clock than the weekly and monthly contracts most tools were built for. Measure that clock in days and the numbers come out wrong. This page shows what actually happens to a 0DTE contract's value across a single session, with every figure computed by the same Black-Scholes engine the calculator runs.
Open the free calculator → Set days to expiry to 0 and the board switches to an hour-by-hour view of the session automatically.What a 0DTE option is, and why volume moved here
Every listed option has an expiration date. A 0DTE option is simply one whose expiration is today. Index products like SPX now list expirations for every trading day of the week, and SPY and QQQ do the same, so on any given day there is always a set of contracts with only hours left to live.
The draw is cost and immediacy: a contract with hours of life left is cheap, and the bet resolves the same afternoon. That is why 0DTE volume has exploded. The behavior of these contracts, though, is where the surprises live, starting with how their value falls apart over a few hours.
Why same-day contracts behave differently
Three things separate a 0DTE contract from the 30-day option a normal calculator assumes.
Theta is measured in hours, not days
Time decay, or theta, is usually quoted per day, because a normal option has weeks of life to give back. A 0DTE option has one session, so the meaningful unit is the hour. Value bleeds out continuously through the day, and by the final hour there is almost nothing left to lose.
Gamma is large near the strike
Gamma measures how fast an option's delta changes as the stock moves. With expiration hours away, an at-the-money contract's delta swings hard between "acts like the stock" and "acts like nothing" on small moves in the underlying. A single dollar in the stock can flip the outcome.
A daily grid is the wrong shape
The projection table on most calculators, including the default view here, spreads its columns across days. For a contract whose entire life is a few hours, a daily grid has nothing to show: the first column is today and the next is expiry. You cannot see the path in between, which is the only thing that matters for a same-day trade.
A worked example, hour by hour
Take an SPX-style put with the stock at $743.29 and a $743 strike, so it is 29 cents out of the money. Suppose it is quoted at $2.59 near the open. Enter that price into the calculator and the solver returns an implied volatility of about 15%. The site prices intraday options on a trading-day basis by default, treating the 6.5-hour session as one of the 252 trading days in a year; the methodology page covers both conventions.
Hold the stock still at $743.29 and let only time pass. Here is what the model says the same put is worth as the session runs out:
| Time to expiry | Put value (IV 15%) |
|---|---|
| 6.5 hours (open) | $2.59 |
| 5 hours | $2.27 |
| 3 hours | $1.73 |
| 1 hour | $0.95 |
| 30 minutes | $0.64 |
| Expiry (4:00pm ET) | $0.00 |
Values computed with the Black-Scholes model at a 4.5% risk-free rate, trading-day basis. It is the same math the calculator runs.
The shape is the point. With the stock unchanged, the put loses only about 12% in the first ninety minutes, but it is down to a quarter of its opening value with thirty minutes to go, and it finishes at zero because it expired out of the money. That pattern, slow at first and then a cliff, is time decay measured in hours. A daily calculator cannot draw it.
Load this example → Opens the calculator in hour-by-hour mode with the put above, at 15% IV over a full session, so you can watch the decay path and change the inputs.Why 0DTE strategies are hard
This page does not recommend trades. But it can be honest about what the math implies for anyone drawn to same-day contracts.
- The decay is fast and uneven. As the table shows, most of a 0DTE option's value can evaporate in the final hours. Being on the wrong side of that clock is expensive, and being on the right side, as a seller, carries the mirror-image risk of a sudden move against a thin cushion.
- Spreads eat a cheap premium. When a contract costs $0.64, a bid-ask spread of a few cents is a large share of the trade. Costs that are a rounding error on a $5 option are material here.
- Gamma cuts both ways. The same large gamma that can turn a small move into a fast gain can turn a small adverse move into a fast loss, and near expiry it does so in minutes. There is no slow bleed to react to.
None of that says whether any trade is good or bad. It says these contracts move faster and less forgivingly than longer-dated options, which is exactly why measuring them on the right clock matters.
The honest limits of any 0DTE model
The calculator assumes volatility is spread evenly across the session. Real intraday volatility is not uniform: it tends to be highest near the open and again near the close, and quieter in the middle of the day. So the smooth curve above is an approximation of a lumpier reality.
The model is least reliable in the final hour. Near expiry, pin risk (the tendency of the underlying to gravitate toward a heavily traded strike) and the extreme gamma of at-the-money contracts make the true price jumpier than any single volatility number can capture. Treat the last column before expiry as the roughest estimate on the board, not a precise quote. The tool marks it for exactly that reason.
Price your own 0DTE contract → Enter days to expiry of 0 and the board counts down the session hour by hour. Enter the option's market price and it solves the IV for you.Frequently asked questions
How fast do 0DTE options decay?
Faster than any other option, and unevenly. Because a 0DTE contract has only one session of life, its time value bleeds out over hours rather than weeks, starting gradually and accelerating into a cliff in the final hour. In the worked example on this page, an out-of-the-money put worth $2.59 at the open, with the stock held still, is worth about $0.95 with an hour left and expires at $0.00. Most of the damage happens late in the day.
Do 0DTE options decay overnight?
There is no overnight for a 0DTE option, because it expires the same day it is traded. The overnight and weekend time decay that matters for weekly and monthly options does not apply. A 0DTE contract simply decays through the single session and settles at the close.
What time do 0DTE options expire?
For most US index and equity options, the last trading happens at 4:00pm ET, the market close. Some cash-settled index options follow specific settlement procedures and a few products expire at other times, so always confirm the exact expiration for the contract you are looking at. The calculator's hourly mode counts down to the 4:00pm ET close by default.
Can you use a normal options calculator for 0DTE?
Only if it can measure time in hours. A standard projection that steps across days has no column between today and expiry, so it cannot show a same-day contract's path. OptionWit handles this automatically: enter zero days to expiry and the board switches to an hour-by-hour view of the session, pricing each moment on a trading-day basis.