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). 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 — what this calculator does not model
- Excludes brokerage, STT, GST, stamp duty and exchange fees, so the true loss at the stop is slightly larger than the budget shown.
- Assumes the stop fills exactly at your stop price; a gap or slippage past the stop can lose more than one risk budget.
- Treats point value per lot as constant, so it does not fit options, where the rupee value of a one-point move changes with delta.
- Sizes one trade in isolation — it does not cap total open risk across correlated positions (portfolio heat).
Frequently asked questions
What is point value per lot?
It is the rupee value of a one-point move in the instrument for a single lot. For a Nifty contract with a lot size of 65 (as of NSE's 28 October 2025 revision), one index point is worth 65 rupees per lot, so you enter 65.
It says zero lots. What does that mean?
Your risk budget is smaller than the loss on a single lot at that stop distance. Either widen the stop, increase the risk percentage, or accept that this trade does not fit your capital at that stop.
Should I size based on my available margin instead?
No. Margin decides whether a trade is feasible, not how big it should be. Sizing on margin invites oversized positions in low-margin instruments; size on stop-based risk and treat margin only as a secondary feasibility check.
Should position size reflect how confident I am in a trade?
Generally no. Detaching size from conviction keeps the worst-case loss constant and prevents emotional over-sizing. If conviction is to matter at all, encode it explicitly in the strategy logic rather than by informally adding lots.
How do lot-size revisions affect an automated sizing engine?
On NSE, lot sizes are set by the exchange and revised periodically, so a sizing engine must read the current lot size and point value from a reference table rather than hard-coding them. A stale value quietly mis-sizes every position after a revision — a classic silent sizing bug.
Where does position sizing sit in a trading system?
In a well-factored system it is its own module: the strategy proposes a signal, the position-sizing engine converts the risk budget and stop distance into a quantity, and the risk engine independently vets that quantity against portfolio-level limits before any order is sent.
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