An option's time value does not bleed away at a steady, even pace. It decays along a curve that steepens as expiration approaches — and the steepest stretch is the final two weeks. This is not a pattern someone noticed on a chart, and it is not a prediction. It falls directly out of the Black-Scholes model. Below is the math, and what it means if you sell covered puts or covered calls.
What theta actually measures
Theta is the time-decay term. It measures how much value an option loses from the passage of a single day, holding the price of the underlying and everything else constant. An option's price is the sum of two parts: intrinsic value (how far in-the-money it is right now) and extrinsic value — the time value — which is everything you pay for the possibility that the option moves further into the money before it expires. Theta acts only on that time value.
At expiration, time value is zero by definition: there is no more time left to pay for. So whatever time value an option carries today must decay to nothing by the end. The only question is when, along the way, that decay actually happens. That is the whole subject of this page.
Time value decays on a curve, not a straight line
If time value fell by the same dollar amount every day, a 90-day option would shed one-ninetieth of its time value daily and the graph would be a straight diagonal line to zero. That is the intuition most people start with, and it is wrong.
For an at-the-money option, the Black-Scholes model implies — approximately — that time value is proportional not to the time remaining, but to the square root of the time remaining. That modeled relationship, value roughly proportional to √T, is what bends the straight line into a curve and loads the heaviest decay toward the end of the option's life. It is an approximation, not an identity: real options track it loosely, not exactly.
The Black-Scholes reason, shown
One caveat before the formula, because it matters: what follows is a deliberately simplified approximation — a way to see the shape of decay, not an exact law and not a guarantee about any one option. It describes an at-the-money option under the model's standard assumptions (constant volatility, no dividends, European-style exercise), and it captures the behavior that dominates near the money. Options that are deep in- or out-of-the-money, or that straddle an earnings event, depart from it. Use it to build intuition, not to price a specific contract.
With that caveat in place, a standard approximation gives the time value of an at-the-money option as:
where S is the price of the underlying, σ is its annualized volatility, and T is the time to expiry in years. Near the money and over a short window, the terms in front of √T change slowly, so to a first approximation the option's time value tracks √T. That approximation is the whole source of the curve.
Theta is simply how fast that value changes as T shrinks — the slope of the curve. Under this approximation, the rate of change of √T is 1 ÷ (2√T): the daily decay runs inversely to the square root of the time left. As T gets small, 1 ÷ √T grows, so the daily decay grows with it. Concretely, a day of decay at 4 days to expiry removes on the order of √(40÷4) ≈ 3.2× as much time value as a day of decay at 40 days out, for the same option. The clock speeds up precisely because there is less time left to run.
What it looks like in the final two weeks
The √T relationship lets us sketch how an at-the-money option's time value melts as expiry nears. Taking the time value roughly three months out (90 days) as a baseline of 100%:
| Days to expiry | Approx. time value remaining |
|---|---|
| 90 | 100% |
| 60 | 82% |
| 45 | 71% |
| 30 | 58% |
| 21 | 48% |
| 14 | 39% |
| 7 | 28% |
| 3 | 18% |
| 1 | 11% |
Illustrative only — the √T approximation for an at-the-money option under constant volatility, normalized to the 90-day value. Individual options deviate with volatility shifts, dividends, and moneyness. Not a forecast, and not a guarantee of decay.
Read the table this way: by two weeks out, an at-the-money option has already shed roughly 60% of the time value it carried three months earlier — but the remaining ~40% burns off over just those final fourteen days, and the daily burn is fastest at the very end. The last two weeks are where the largest share of decay is compressed into the fewest days.
Why this matters if you sell covered puts and calls
When you sell an option, theta works in your favor. Each day that passes, all else equal, transfers time value from the buyer to the seller. The final two weeks is where that transfer is fastest per day — the densest stretch of decay to harvest.
That is the entire reason Fischer's analysis is scoped to short-dated options, 14 days to expiry or fewer: it is the window where the time-decay you are selling is richest per day of risk carried. Longer expiries hold more total premium, but they surrender it slowly; the near-dated tail is where decay per day is concentrated.
Which position you use to harvest that decay — selling puts or a covered call — shares this same theta math but carries very different risk, the subject of those two guides.
Accelerating theta is not free money, and anyone who frames it that way has something to sell. The same forces that accelerate decay accelerate risk. Faster decay and faster risk are two sides of one coin.
What accelerates alongside theta
- Gamma. As expiry nears, an option's sensitivity to the underlying's price rises sharply. The same short-dated option that decays quickly in your favor also swings quickly against you if the underlying moves. You are not paid the accelerated decay for free — you are paid it for carrying accelerated price sensitivity.
- Assignment. Short-dated options that drift in-the-money carry real assignment risk. For a covered seller that means being put the stock or having it called away — a defined, plannable outcome, but one you have to plan for rather than be surprised by.
- Volatility and moneyness. The √T curve above assumes an at-the-money option with steady volatility. Deep in- or out-of-the-money options decay on different profiles, and a jump in implied volatility can add time value faster than theta removes it. Theta is one force among several, not the only one.
This is why the useful question is not “how much premium can I collect,” but “what is the model-derived probability that this position profits, after weighing decay against gamma, assignment, and volatility.” Decay is a tailwind; it is not the whole forecast.
How Fischer approaches it
Fischer runs Black-Scholes pricing across a curated universe of liquid US equities and ETFs every trading morning, and ranks covered-put and covered-call positions by model-derived probability of profit and theta efficiency — not by premium alone. The scan is deliberately confined to the 0–14-day window, where decay per day is highest, and to strikes near the money, where the model is most informative. The output is analysis, not advice: the math comes first, and the decision stays yours.