Position Size Calculator
Work out how many lots or units to trade so a stop-loss loses only a fixed percentage of your capital.
Quick answer: The position size calculator converts a fixed-fractional risk rule into a concrete quantity. It takes your capital and the percentage you are willing to lose on the trade, works out the rupee risk budget, then divides that budget by the per-unit loss at your stop (stop distance in points multiplied by the point value of one lot). The result is the largest whole number of lots whose worst-case loss stays inside the budget.
How to use it
Enter your capital, the percent you will risk, and the entry and stop prices. Point value per lot is the rupee change in one lot for a one-point move in price (65 for a standard Nifty lot, revised from 75 by NSE effective 28 Oct 2025 per circular NSE/FAOP/70616). The output shows the rupee risk budget, the loss per lot at your stop, the whole number of lots that fits, and the actual rupee at risk. It ignores brokerage, STT and slippage, which make the real loss slightly larger.
Formula
Lots = floor( (Capital × Risk% ÷ 100) ÷ ( |Entry − Stop| × Point value per lot ) )
Where the numerator is the rupee risk budget and the denominator is the loss on one lot if the stop is hit. floor() rounds down so the budget is never exceeded.
Limitations of the Position Size Calculator
The Position Size Calculator is a teaching aid, not a live risk system. It does not model the following:
- Brokerage, STT, exchange transaction charges, GST and stamp duty, which make the realised loss larger than the modelled price risk
- Slippage and partial fills — the stop may execute worse than its stated level, especially on a gap
- Correlated open positions, so account-level risk (heat) can exceed the single-trade budget
- Changes in NSE lot size over time — you must enter the point value that applied on the trade date
Frequently asked questions
Why must a backtest model the position-sizing rule, not fixed lots?
Because the same set of entries and exits can produce a smooth compounding curve or a blown-up account depending only on how positions are sized. A backtest that trades a constant lot size measures raw signal quality; to estimate real equity, drawdown and risk of ruin, it must apply the actual sizing rule.
How does rounding down to whole lots affect a backtest?
F&O trades in fixed lot sizes, so a realistic backtest must floor the computed quantity to whole lots. A fractional-lot backtest quietly overstates capacity and understates the risk that indivisibility forces on a small account, especially early in the equity curve.
Why does the point value need to be point-in-time in a backtest?
NSE revises lot sizes periodically, so a backtest that hard-codes one point value will mis-size every trade before or after a revision. Reconstructing the lot size as it actually was on each historical date keeps the simulated position risk honest.
Does this figure include costs when I backtest?
No. The output is raw price risk. A trustworthy backtest subtracts brokerage, STT, exchange fees and slippage from each trade separately, because those frictions turn a marginally positive strategy negative and shrink the sized position's realised edge.
How does the sizing rule change the backtested drawdown?
Directly. Fixed-fractional sizing compounds gains and de-risks in drawdowns, changing the shape and depth of the equity curve versus fixed sizing on the identical trades. Comparing sizing schemes on the same signal is a core robustness check.
Runs entirely in your browser — no data leaves your device. Illustrative and educational only; real-world charges and market conditions apply in practice.