Position Sizing Calculator for Indian F&O

Position size is the risk budget divided by the risk per unit. The risk budget is your account size multiplied by a fixed risk fraction, usually 0.5 to 1 percent. The risk per unit is the distance in rupees between your entry and your stop. Quantity equals (capital times risk percent) divided by the absolute value of entry minus stop. On a 5,00,000 account risking 1 percent, that is 5,000 of risk; with a stop 15 rupees from entry, the raw quantity is 5,000 divided by 15, about 333 units. Conviction, the target and the account size never enter this formula; only the risk budget and the stop distance do.

From the January 2026 derivatives series, NSE revised the index lot sizes. Nifty 50 moved from 75 to 65, Bank Nifty from 35 to 30, Nifty Financial Services (FinNifty) from 65 to 60, and Nifty Midcap Select from 140 to 120. Nifty Next 50 stayed at 25 and the BSE Sensex contract stayed at 20. Weekly contracts reflected the change from the first January 2026 expiry and monthly contracts from the January month-end expiry. Any sizing tool still using 25 for Nifty or 15 for Bank Nifty is on lot sizes that were superseded twice over.

Because you cannot buy a fraction of a unit or a fraction of a lot. The formula gives a raw quantity that is almost never a whole number. In equity you round down to the nearest share; in F&O you round down to the nearest whole lot, which is a far coarser step. Rounding down usually leaves your actual rupee risk below budget, which is safe but under-deploys capital. The danger is the F&O trader who rounds up to one lot when the budget did not cover even one: a Bank Nifty lot of 30 with a 40-rupee stop is 1,200 of risk per lot, so on a 50,000 account that single indivisible lot is already 2.4 percent, more than double a 1 percent budget. The lot is the real minimum position, not the formula output.

R is your initial risk on the trade, the rupee amount between entry and stop times the quantity. Every outcome is then measured in R. If risk is 5,000, a target that pays 10,000 is a 2R trade and the reward-to-risk is 2 to 1. Sizing in fixed-fractional terms makes R roughly constant across trades regardless of price, so a 250-rupee instrument and a 2,000-rupee instrument both risk the same rupees when sized correctly. That constancy is the point: it lets you reason about a strategy in R rather than in the noise of individual share prices, and it keeps one loss from being three times another purely because the second stock was more expensive.

No. A stop is a trigger, not a fill. It caps risk only if the market trades continuously through your level. Across an overnight gap, a stop at 492 can fill at 470 if the stock opens there on news, and your realised loss is set by the open, not the trigger. Worse, an instrument locked at its lower circuit has no buyers, so a sell stop cannot execute at all until the circuit lifts, by which point the price may be far below. Position sizing assumes the stop holds; gap risk and circuit risk are the reasons a sensible desk also caps single-name exposure and total portfolio heat, so that one adverse gap cannot end the account even when the stop fails to protect the planned amount.

Portfolio heat is the sum of the open risk across every live position, expressed as a percent of the account: what you lose if every stop is hit at once. A common institutional ceiling is 6 percent total heat, roughly six simultaneous trades each risking 1 percent. The reason for a cap separate from per-trade risk is correlation: five long positions in the same sector are not five independent 1 percent bets, they behave like one larger bet that all stop together on a sector move. This calculator adds the trade you are sizing to the open risk you enter and warns when the total breaches the cap, because the per-trade number alone hides the real exposure.

They matter most for the smallest and the most frequent positions, and they were made heavier by the 2026 Budget. Securities transaction tax on options rose to 0.15 percent of the sell-side premium and on futures to 0.05 percent of the sell-side turnover, both effective 1 April 2026. Add exchange transaction charges, an 18 percent GST on brokerage plus those charges plus the SEBI turnover fee, stamp duty on the buy side, and every round trip has a fixed and a proportional cost. On a tiny position the round trip can be a large fraction of the rupee risk, which means the trade has to clear a breakeven move before it earns anything. Cost drag is the reason under-sizing to feel safe can quietly turn a positive-expectancy method negative.

No. Increasing risk after losses is a martingale, and a fixed-fractional model does the opposite by design. Because size is a percent of the current, now-smaller account, the rupee risk falls automatically after a drawdown and rises only as the account recovers. That is anti-martingale, and it is the property that makes ruin mathematically hard to reach: each loss shrinks the next bet rather than enlarging it. Doubling up to get even is how a survivable drawdown becomes a terminal one, because a fixed percent applied to a shrinking base cannot blow up, but a fixed rupee amount, or a rising one, applied to a shrinking base can.

Fixed-fractional convention is 0.5 to 1 percent per trade, with 2 percent as an outer bound reserved for the highest-conviction setups and never a default. The reason is drawdown arithmetic, not caution for its own sake: at 1 percent risk a run of ten consecutive losses costs roughly 10 percent of capital and is recoverable; at 5 percent the same run costs about 40 percent and needs a 67 percent gain merely to break even. The SEBI FY25 finding that over 91 percent of individual F&O traders were net loss-making, with aggregate net losses of about 1,05,603 crore rupees, is in large part a sizing failure: positions too large for the account relative to the stop, so that ordinary losing streaks became unrecoverable.