Theta Decay Calculator

Theta decay is one of the most important concepts every options trader must master. In the world of options pricing, theta measures how much an option's value decreases with the passage of a single calendar day, all else being equal. Because options are time-limited contracts, their premium includes a component that compensates the seller for taking on risk over a defined period — and that component erodes steadily as expiration approaches. This erosion is called time decay or theta decay.

Every option you buy or sell carries an embedded time value. When you purchase a call or put, you are paying a premium that reflects both the option's intrinsic value (how far in-the-money it currently sits) and its time value (the possibility that it could move further in your favor before expiry). Theta quantifies precisely how fast that time value is being consumed each day. A theta of -0.05 on a call option means the option loses approximately $0.05 in value for each day that passes — regardless of whether the underlying stock price moves at all.

This has profound practical implications. Options buyers are fighting against theta every single day they hold a position. If the underlying asset does not move sufficiently in the right direction to compensate for ongoing time decay, the position will lose money even though no adverse price movement occurred. Conversely, options sellers — those who write covered calls, sell cash-secured puts, or construct credit spreads — actually profit from theta decay. Time working against the buyer is time working in the seller's favor.

The theta decay calculator on this page uses the Black-Scholes model to compute precise daily theta values for both call and put options. Enter your stock price, strike price, days to expiry, implied volatility, risk-free rate, and number of contracts to see exactly how much premium erodes each day, each week, and each month. The calculator also displays the theta acceleration table, showing how decay speeds up dramatically as the option approaches expiration.

Understanding theta is essential for structuring positions wisely — whether you are buying options and need to know how long you can hold them before time value destruction overwhelms any favorable move, or selling options and want to maximize the rate of premium collection relative to the risk you are taking on.

This calculator derives theta using the standard Black-Scholes framework. The model treats the underlying asset's price as following a log-normal random walk with constant volatility, allowing a closed-form analytical solution for option sensitivities. The theta formula decomposes into two terms: a volatility-driven term and an interest-rate-driven term. For a call option the interest rate term is subtracted (accelerating decay), while for a put it is added back (slightly reducing the decay rate for puts on low-rate environments).

The inputs used are: S = current stock price, K = strike price, T = time to expiry in years (days ÷ 365), r = risk-free rate as a decimal, σ = implied volatility as a decimal, and N'(x) = the standard normal probability density function evaluated at x.

Once theta per share is computed, the calculator scales it to a per-contract basis by multiplying by 100 (standard U.S. equity options lot size), then multiplies by the number of contracts for total daily dollar decay. Weekly decay = total daily theta × 7; monthly decay = total daily theta × 30.

One of the most critical features of theta decay is that it does not erode linearly over time. The rate of time decay accelerates as the expiration date draws closer, particularly for at-the-money options. This non-linear behavior arises mathematically because theta contains a 1/(2√T) term — as T shrinks toward zero, that term grows rapidly, driving theta to very large negative values.

In practice, options traders observe that the final 30 days before expiration are when time decay becomes most aggressive for at-the-money options. The final 7–14 days are especially punishing for buyers: a significant portion of remaining time value can vanish within a single week. This is why experienced traders who buy options typically avoid holding them through the last month unless they are deep in-the-money or have a specific near-term catalyst thesis.

The theta acceleration section of this calculator illustrates this effect concretely. It shows your position's total daily theta at three fixed reference points: 30 days to expiry, 15 days to expiry, and 7 days to expiry — computed using the current implied volatility and other parameters but substituting those fixed time values. This lets you see exactly how much the daily burn rate increases as the clock winds down, helping you plan position sizing, profit-taking targets, and exit timelines.

For options sellers running theta strategies such as covered calls, cash-secured puts, iron condors, or credit spreads, this acceleration is a feature rather than a bug. By selling options in the 30–45 day window and closing them at 50% profit (around 15–21 days remaining), many systematic sellers aim to capture the steepest portion of the theta decay curve while minimizing the gamma risk that also increases near expiry.

While both calls and puts experience theta decay, the exact magnitude differs due to the interest-rate component in the Black-Scholes theta formula. For call options, the formula subtracts the term r·K·e^(−rT)·N(d₂), which increases the absolute value of theta relative to a zero-interest-rate world. For put options, this same term is added rather than subtracted — partially offsetting the volatility-driven decay and resulting in slightly smaller absolute theta values compared to equivalent calls, particularly when interest rates are meaningful.

This difference becomes more pronounced when the risk-free rate is elevated. In a 5% rate environment, at-the-money call options carry a somewhat higher daily theta than equivalent puts. In near-zero rate environments the difference largely disappears. For most practical option trading analysis, calls and puts with identical strikes and expirations will have very similar — but not identical — theta values.

Another key distinction is that deep in-the-money options carry very little time value regardless of type; most of their premium is intrinsic. As a result, their theta is small. At-the-money options carry the maximum time value and therefore the highest absolute theta. Out-of-the-money options have moderate theta, lower than at-the-money but not zero — they still contain purely speculative time value that decays with each passing day.

When analyzing a multi-leg options strategy such as a straddle, strangle, or condor, the position-level theta is simply the sum of individual leg thetas. This calculator focuses on single-leg positions, but understanding single-leg theta is the foundation for managing any complex options portfolio.

The theta decay calculator outputs several complementary metrics that together give a complete picture of your position's time value exposure. Here is how to read each output:

Daily Theta Decay (total position): This is the dollar amount your entire position loses to time each calendar day, assuming no change in stock price or implied volatility. It is expressed as a negative number for buyers (a daily cost) and represents positive daily income for sellers.

Theta per Share: The raw Black-Scholes theta for a single share's worth of option exposure. Expressed in dollars per share per day, this is the standard way theta appears on brokerage platforms and in academic literature.

Theta per Contract: Theta per share multiplied by 100 (the standard lot multiplier for U.S. equity options). This is the daily dollar decay for a single contract in your position.

Weekly and Monthly Decay: Daily theta multiplied by 7 and 30 respectively. These figures help contextualize the position cost over typical holding periods. A weekly decay of $380 on a $3,000 premium position represents roughly 12.7% of premium consumed per week — a significant hurdle rate for any bullish or bearish directional bet.

Daily Decay as % of Premium: Divides total daily theta by total position premium to express decay as a percentage. This normalized metric makes it easy to compare time decay intensity across positions of different sizes or premium levels. A value above 2–3% per day indicates extreme time decay exposure typical of very short-dated options.

Theta Acceleration table: Shows fixed reference theta values at 30, 15, and 7 days to expiry using your current inputs. Review this before entering any position to understand the trajectory of your daily carry cost.

Knowledge of theta decay underpins some of the most popular systematic options income strategies. Understanding which strategies benefit from theta and which are hurt by it is essential before entering any options trade.

Covered Call Writing: A stock owner sells an out-of-the-money call against their shares. The daily theta works in the seller's favor, generating premium income that lowers the effective cost basis of the stock position. This strategy benefits most when the stock stays flat or rises modestly — exactly the conditions where theta decay on the short call proceeds undisturbed.

Cash-Secured Puts: An investor sells a put option while holding enough cash to buy the shares if assigned. Like covered calls, this benefits directly from theta decay. Many income investors rotate between covered calls and cash-secured puts to continuously harvest time value.

Iron Condors and Credit Spreads: These defined-risk strategies sell both a call spread and a put spread simultaneously, collecting net premium that decays toward zero if the stock stays within a range. The combined position theta is positive for the seller — the entire position profits from the passage of time.

Long Options (Directional Bets): When buying calls or puts, theta is the enemy. Buyers should use this calculator to confirm they have a strong enough directional thesis to overcome daily time value destruction. As a rough benchmark, many traders require an expected move (delta × expected price change) that is at least 2–3 times the projected theta cost over the intended holding period.

Calendar Spreads: These strategies buy a longer-dated option and sell a shorter-dated option at the same strike, exploiting the fact that near-term options decay faster. The position is net-positive theta (from the faster-decaying short leg) while retaining longer-term optionality through the long leg.