Options Probability Calculator: Expected Move and Limits

An options probability calculator estimates a defined price event from assumptions about volatility and time. Expiring in the money, touching a price before expiry and making a net profit are different events. This tool's probability percentages need numerical repair, so do not use them to choose a trade. The expected-move arithmetic below can still be checked separately. Target Price and Option Premium do not change the probability cards displayed by this form.

Name the event before reading a percentage

• In the money, or ITM: a call finishes above its strike, or a put below its strike, and has exercise value.
• Out of the money, or OTM: the option finishes on the other side of its strike and has no exercise value.
• Touch: the underlying reaches a specified price at some point during the remaining time.
• Profit: the position's payoff or closing proceeds cover the premium and relevant costs.

A price can touch a strike and later finish elsewhere. An option can have exercise value that is smaller than the premium paid. These distinctions matter even before considering which probability model is used. The options breakeven page explains the premium-recovery boundary and also flags its own current calculator fault.

A model percentage is conditional on the model and inputs. An observed win rate is a count from completed trades under a defined method. This form has no completed-trade history and cannot measure an observed win rate for your strategy.

What to enter in the form

1. Current Spot Price: enter the underlying price yourself. The default contains invalid text, so replace it with a numeric price for your scenario.
2. Target Price: labels a price you want to examine, but its separate calculated result is not included among this form's displayed cards.
3. Days to Expiry: use the calendar-day horizon you intend to model; the code converts it using a 365-day year.
4. Implied Volatility (%): enter an annualised percentage, meaning a yearly rate. This represents the price variation implied by option prices.
5. Option Strike: this price drives the visible ITM and OTM cards.
6. Option Premium: used internally for a separate buyer-breakeven probability that the configured output list does not display.
7. Option Type: choose Call or Put so the strike comparison uses the appropriate direction.

There is no visible interest-rate control, dividend input or historical-volatility series. A fixed interest-rate assumption is embedded in the implementation. The implied volatility guide explains the input's meaning; entering an index-wide volatility measure does not establish the volatility of the specific option being considered.

Read the range without endorsing the probability cards

Expected Move uses spot × annual volatility × square root of time in years. Upper Range (1 SD) adds that move to spot, while Lower Range (1 SD) subtracts it. SD means standard deviation, a model's scale for spread. Std Dev Away divides the strike's absolute distance from spot by Expected Move.

These are arithmetic scenario bands, rather than guaranteed boundaries or a prediction of the closing price. The probability calculation uses a separate model step. A range calculation that checks out does not prove that the probability calculation is correct.

The shared numerical function used to convert model values into probability percentages disagrees with a standard normal-distribution reference. Probability ITM, Probability OTM and Probability of Touch should therefore be treated as unverified until that function is repaired and checked. The displayed Probability Assessment and Seller Advice labels are derived from those percentages and cannot establish suitability.

Worked expected-move example

For an invented scenario, enter spot ₹20,000, strike ₹22,000, target ₹24,000, 365 days, implied volatility 20%, premium ₹200 and Call. This deliberately uses a full-year horizon to keep the square-root step transparent. It is not a current contract or a forecast.

Time in years is 365 ÷ 365 = 1. Volatility as a decimal is 20 ÷ 100 = 0.20. Expected Move is ₹20,000 × 0.20 × 1 = ₹4,000. The range cards show ₹24,000 above and ₹16,000 below, from adding and subtracting ₹4,000.

The strike distance is (₹22,000 − ₹20,000) ÷ ₹4,000 = 0.5 expected-move units. The local calculator reproduces Expected Move 4,000, Upper Range 24,000, Lower Range 16,000 and Std Dev Away 0.5. No probability percentage from this example is being presented as accurate.

Change only Target Price and watch which displayed cards respond. The visible strike-based cards do not become target probabilities. Likewise, changing Option Premium does not turn the displayed ITM estimate into the buyer's probability of making money.

A higher probability does not settle the trade decision

A probability by itself omits the size of the possible gains and losses. It also omits execution prices, spreads, costs and changes in volatility. Reviewing a short option requires understanding its payoff and capital requirements as well as any model estimate. This page gives no order or trade recommendation.

For a related explanation, read how to read option Greeks, the measures of option-price sensitivity. For charges, the F&O trading cost calculator addresses a separate part of the outcome. Neither a Greek nor a cost estimate repairs the probability calculation here.

Record assumptions so a scenario can be checked again

A useful scenario note identifies the underlying, option contract, price observation, volatility source and intended horizon. Record whether the strike or a different target defines the event you are examining. Keeping those details beside a result prevents a percentage from becoming detached from the question it was supposed to answer.

After the probability calculation is repaired, changing an assumption should still be treated as a new scenario rather than fresh evidence about an unchanged trade. A higher volatility input describes a wider assumed distribution. It does not independently establish that a specific directional move will happen. A longer horizon also changes the scenario; it does not turn an estimate of the final price into the chance of touching a level along the way.

Keep the time convention explicit when comparing tools. A calendar-day input cannot be exchanged for a count of trading sessions while leaving the annualisation convention unexamined. Likewise, a percentage volatility input should not receive a decimal fraction intended for another interface.

For now, use this page's worked range only as a transparent arithmetic exercise. Keep the probability cards out of a recorded strategy win-rate claim, and retain the original trade observations separately. A model could only be assessed against a clearly defined event and a corresponding set of outcomes, neither of which this form collects.

Probability questions

Sources

• Fidelity: probability-calculator concepts and assumptions.
• Options Industry Council: Black-Scholes model.
• NIST: standard normal probability table.
• References and local outputs checked 15 October 2026.