Every guide to position sizing gives you the same formula. Take your account, decide a risk percentage, divide by your stop distance, and out comes the number of shares to buy. It's a good formula. It also quietly assumes something that isn't true for you: that you can buy any number of shares you like. If you trade options, you can't, and that changes everything.
What position sizing is
Position sizing is the decision of how big each trade should be. Not which trade to take, but how much to put on once you've decided. It is, quietly, the most important risk decision you make, because it determines how much a single loss costs you. Two traders can take the exact same trade with the exact same stop and one loses ₹2,000 while the other loses ₹20,000. The difference is size, not skill.
The standard method works backwards from risk. First you decide the most you'll lose on this trade, say 1 to 2% of your account. Then you measure the distance to your stop-loss. Divide the first by the second and you get your size. Risk decides size: you don't pick a size and hope, you pick a loss you can survive and let it set the size.
Where the formula breaks for options
That formula was written for stocks, where you can buy 137 shares, or 42, or 1,000. Options don't work that way. They trade in fixed lots set by the exchange: 75 for Nifty, 35 for Bank Nifty, and specific sizes for each stock. You buy one lot, two lots, three. You cannot buy 1.4 lots.
This sounds like a small detail. It is not. It means the formula often gives you an answer you can't use. Suppose your math says your risk budget allows a position of "half a lot." There's no such thing. Your real choices are zero lots or one full lot, and one full lot might carry double the risk you intended.
On a small account, this is the central problem of options position sizing: one lot is frequently already too big. A single Bank Nifty lot, at a typical premium with a sensible stop, can put far more than 2% of a small account at risk in one trade. The honest answer the generic articles never give is sometimes "this trade doesn't fit your account, so don't take it."
How to actually size an options trade
Because you can't adjust size in fine increments, you size options trades backwards, in this order:
1. Set your rupee risk. The most you'll lose on this trade, as a number. Not a percentage you'll convert later, an actual rupee figure you're at peace with losing. Say ₹2,000.
2. Work out the per-lot risk. Take the premium and your stop. If you buy a lot at ₹120 with a stop at ₹90, you're risking ₹30 per unit. Multiply by the lot size. For a 35-unit Bank Nifty lot, that's ₹30 × 35 = ₹1,050 of risk for one lot.
3. See how many lots fit, if any. Your ₹2,000 budget divided by ₹1,050 per lot is 1.9. You round down, never up. So: one lot. If the per-lot risk had been ₹2,500, the answer would be zero lots, and the correct action is to skip the trade or find one with a tighter stop.
This is the discipline the formula hides. Rounding down, and being willing to land on zero, is the whole game. The trader who rounds 1.9 up to 2 "because it's basically 2" has just broken their own risk rule on the very first step.
Why premium volatility makes this harder
There's a second options-specific trap. Your stop distance is measured in premium, and premium is far more volatile than the underlying. A calm 40-point move in Bank Nifty can swing an option's premium 20 or 30%. So a stop that looks comfortable can be hit by ordinary noise, turning a planned one-lot trade into a quick loss that feels like bad luck but was really a sizing-and-stop mismatch.
The fix is to set the stop on something stable (the underlying level or your thesis) and then size to that, rather than picking a premium stop that's really just a number that felt okay. Stop placement and position size are one decision, not two. More on placing the stop here.
The number you set when you're calm
Position sizing is decided before the trade, for a reason. The moment you're in a position, your judgment about size is compromised, and it gets worse after a result. Size up after a win and you're euphoria trading. Size up after a loss to recover faster and you're revenge trading. Both are sizing decisions made by the wrong version of you. The number set in advance, and the discipline to round down, is what protects you from both.
This is one of the three numbers in any real risk-management system, alongside the stop and the daily loss limit. Get sizing right and most account-destroying days simply can't happen, because no single trade is ever big enough to cause one.
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