Premium Decay Calculator

An option premium melts a little every day, even if the stock never moves. This premium decay calculator tracks the option you already hold: it splits your premium into intrinsic and time value, computes true Black-Scholes theta (not a shortcut), and projects what the premium looks like after 1 day and 7 days, floored at intrinsic value. It answers “how fast is my position bleeding,” while our option premium calculator answers a different question: “what is this option fairly worth from scratch.”

Quick Answer

How to use the premium decay calculator

Enter the option's details, click Calculate decay, and read the split, the daily theta, and the two projections. The fields come pre-filled with the worked example below, and results stay hidden until you calculate.

1. Pick call or put: the interest-rate term flips sign for puts, so moneyness behaves differently on each side.
2. Enter the premium you are tracking: the last traded price per share. The tool immediately splits it into intrinsic value and time value.
3. Fill in stock, strike, days to expiration, IV, and the risk-free rate: the tooltip on each label explains where the number comes from. The Fed's H.15 release publishes Treasury yields you can use for the rate input.
4. Read the badge first, then the numbers: "Severe theta pressure" means the clock is your main opponent; then check whether the 7-day projection still leaves a workable premium.

The theta formula behind the numbers

Theta is the Black-Scholes partial derivative of the option price with respect to time: the value that evaporates per day when the stock, volatility, and rates all sit still. Theta is one of the five option Greeks, and our option Greeks calculator computes delta, gamma, vega, theta and rho side by side. For a call:

Theta = -[S x phi(d1) x sigma] / [2 x sqrt(T)] - r x K x e^(-rT) x Phi(d2)

For a put, the second term flips sign. The first term is the volatility-driven melt (shared by calls and puts); the second is the interest-on-strike adjustment. The tool computes d1 and d2 exactly (for the worked example: d1 = 0.0908, d2 = 0.0048), then divides the annualized figure by 365 for the daily number brokers display. Some desks use 252 trading days, which makes the daily figure slightly larger; this tool uses the 365-calendar-day convention. That matches the convention most theoretical models use to account for weekend decay, as Schwab's options Greeks guide notes when it describes theta as including weekends.

Worked example: an at-the-money call, checked against this exact tool

A call on a $100 stock with a $100 strike, 30 days to expiration, 30% implied volatility, a 5% risk-free rate, and a $5.00 premium. Enter these inputs and click Calculate decay:

The 7-day figure is roughly a 9% haircut on the time value for a single flat week. Compare that to the return you actually need: a 10% annual return works out to barely 0.03% a day, so a 1.28% daily bleed means the position must earn about 40 times its required pace just to stand still.

Why decay accelerates near expiration

The square root of remaining time sits in the denominator of the theta formula, so the same pile of time value is divided by fewer and fewer days. Hold the example option's parameters fixed and watch the daily loss grow as the clock runs down:

With one week left, the identical option bleeds more than three times faster per day than it did at 90 days. That is why the last two weeks of an option's life are called the theta zone, and why long premium buyers avoid holding into it while short premium sellers harvest it. CME Group's options primer makes the same point: time decay is not linear, and it increases as expiration approaches.

Premium decay vs option premium: which question are you asking?

These two tools share a Black-Scholes core but answer opposite questions. The option premium calculator prices a fair value from parameters: given stock, strike, time, volatility, and rates, what SHOULD this option cost? This premium decay calculator starts from the premium you already hold and tracks what happens next: what is intrinsic, what is time value, how fast does it melt, and where does it land in 1 and 7 days.

Use the premium calculator when you are evaluating whether an option is overpriced before you buy. Use this one after you own it, to decide whether to hold, roll, or exit as theta eats the position. The split matters because only time value decays: deep in-the-money options are mostly intrinsic, so their theta is small; at-the-money options are pure time value, so their theta is largest. The tool floors all projections at intrinsic value because decay can never take the premium below it.

What competing calculators miss

The most common theta calculators in this SERP fall into three buckets, and each has a real gap. The naive ones divide the premium by the days left (a popular Indian options site does exactly this), which assumes decay is linear and ignores volatility and rates entirely: on the worked example that shortcut says $0.17 a day, nearly three times the real -$0.0638. The spreadsheet templates do run Black-Scholes but live in a downloaded Excel file with no intrinsic split and no bleed framing. The chain-theta tools take a theta you type in yourself and project it linearly, which is useful only if you already know theta, the one number most retail traders cannot find.

This calculator closes all three gaps: genuine Black-Scholes theta for calls and puts, the intrinsic/time-value split up front, the bleed rate as a percentage of time value (the number that actually tells you whether the clock is affordable), and 1/7-day projections floored at intrinsic value. The projections are linear in theta, which is an honest simplification worth knowing: over a week, theta itself shifts as days pass, so treat the 7-day figure as a direction and a scale, not a quote.

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