1. What you will learn
By the end of this lesson you will be able to compute a position size from a stated risk budget rather than from what a broker will permit, include friction in that calculation, apply the capital and liquidity limits that override it, explain why the realised loss can still exceed the budget, and aggregate risk across positions that are not independent. This is education about sizing arithmetic and is not advice about how much risk anyone should take.
2. The idea explained
Sizing runs in one direction only, and reversing it is the commonest structural error in trading. Decide first how much money you are prepared to lose on the idea. Then work out how many units that allows. Never start from how many units the available margin permits and then discover what the loss would be, because that procedure lets a broker's leverage decide your risk.
The arithmetic is straightforward. The risk budget is an amount in rupees. The per-unit loss is the distance from the intended entry to the intended exit, plus an estimate of the friction per unit, because brokerage and charges reduce the account exactly as adverse price movement does. Divide the budget by that per-unit figure and round down, because a partial unit cannot be traded and rounding up breaches the budget by design.
Three limits then override the result. Capital: the position must be affordable, and a size that requires more money than the account holds is not a size. Liquidity: a quantity larger than the instrument trades in a reasonable time is a quantity you cannot exit, and a position that cannot be exited has no meaningful stop. Concentration: a single position, however correctly sized, may still be too large a share of the account.
Now the honest part, which the arithmetic cannot deliver. A stop is a plan, not a guarantee. It can be defeated by an overnight gap, by a halt, by a price band, by a market so thin that the exit moves the price, or by an error in the estimate of friction. The budget is therefore the intended loss, not the maximum one, and a risk framework that treats it as a maximum will be surprised.
Finally, aggregation. Several positions each sized to the same budget are not several independent risks if they respond to the same thing. Positions in one sector, or in instruments that move together, are one bet in several names, and the aggregate exposure to a single event is what matters. Stress the portfolio by assuming that every correlated position gaps beyond its stop simultaneously, because that is the scenario that ends accounts.
One caution about the numbers in any lesson of this kind. A risk budget in an example is arbitrary and illustrative. Nothing here suggests what proportion of capital any individual should risk, which depends on circumstances no course can know, and a SEBI-registered investment adviser is the person to consult about an individual's own money.
3. The market, the regulator and the rulebook
Indian equity and derivative markets are regulated by the Securities and Exchange Board of India, the statutory securities regulator constituted by an Act of Parliament, which supervises exchanges, brokers and clearing members and sets the framework for order handling, margins and client protection.
The clearing corporations publish the margin framework, which decides how much capital a position ties up and which changes with volatility. The exchanges publish price bands and halt rules, which determine whether an exit is possible at all, and lot sizes for derivative contracts.
Friction comes from the broker's tariff, exchange transaction charges and statutory levies, all of which change. Assume a per-unit figure, label it as an assumption, and name where the current components are published with a date. Never state a margin percentage, leverage multiple, brokerage rate or tax rate as a fixed fact.
Worked example
4. Worked example
All figures are invented, the risk budget is arbitrary, and everything is on paper. A paper risk budget of one thousand rupees is chosen for the exercise.
The intended entry and the intended exit are five rupees apart, so the planned loss is five rupees a share. Friction is estimated at fifty paise a share across the round trip. The per-unit risk is therefore five rupees fifty paise.
Divide: one thousand divided by five and a half is about one hundred and eighty-one point eight. Round down to one hundred and eighty-one shares. Rounding up to one hundred and eighty-two would have put the planned loss at one thousand and one rupees, which is outside the budget by construction.
Now the overrides. If the share trades at four hundred rupees, one hundred and eighty-one shares cost seventy-two thousand four hundred rupees, and the position is only possible if the account can fund that or the margin framework permits it. If the instrument typically trades only a few hundred shares in a session, a position of one hundred and eighty-one cannot be exited quickly and the stop is decorative. Either limit reduces the size below the arithmetic answer.
Now why the loss can exceed one thousand rupees. Suppose the next session opens three rupees below the intended exit. The realised loss is eight rupees a share rather than five, plus friction, which on one hundred and eighty-one shares is about one thousand five hundred and forty rupees rather than one thousand. Nothing was done wrong; the plan met the market.
Finally, aggregation. Suppose five positions are each sized to a one-thousand-rupee budget and all five are in instruments that move together. The framework says five thousand rupees is at risk. If a single event gaps all five by the same proportion as above, the realised loss is about seven thousand seven hundred rupees. That figure, not the five thousand, is what the account must be able to absorb.
5. Common mistakes and how to fix them
The first mistake is sizing from available leverage. Start from the rupee budget and derive the quantity; never the reverse.
The second is omitting friction. Add an estimated per-unit cost to the stop distance before dividing.
The third is rounding up. Round down, always, because the budget is a ceiling.
The fourth is ignoring the capital and liquidity overrides. Check affordability and typical traded quantity, and reduce the size when either binds.
The fifth is treating the budget as a maximum loss. It is an intended loss; gaps, halts, thin markets and cost errors can exceed it, and correlated positions can exceed it several times over. Most active traders lose money, leverage magnifies losses as much as gains, no position is safe or guaranteed, past performance does not indicate future results, and a SEBI-registered investment adviser is the person to consult about an individual's own money.
Key takeaways
6. Board summary
Decide the rupee risk budget first, then derive the quantity; never size from available margin. Per-unit risk is the entry-to-exit distance plus estimated friction per unit. Quantity is the budget divided by per-unit risk, rounded down, then reduced by capital, liquidity and concentration limits. The budget is the intended loss, not the maximum, because gaps, halts and thin markets defeat stops. Correlated positions are one bet, so stress them all gapping beyond their stops at once.
Check your understanding
7. Practice and self-check
One. A paper budget is two thousand rupees, the stop distance is eight rupees and friction is fifty paise a share. Per-unit risk? Eight rupees fifty paise.
Two. Quantity before overrides? Two thousand divided by eight and a half is about two hundred and thirty-five point three, so two hundred and thirty-five shares.
Three. Why round down? Because rounding up breaches the budget.
Four. The share trades at three hundred rupees. What does the position cost? Seventy thousand five hundred rupees.
Five. The account holds fifty thousand. What happens to the size? It is reduced to what the account can fund, regardless of the arithmetic.
Six. The next session opens four rupees below the intended exit. Realised loss a share, including friction? About twelve rupees fifty paise.
Seven. On two hundred and thirty-five shares, what is that? About two thousand nine hundred and forty rupees.
Eight. Four such positions in correlated instruments all gap that way. Approximate total? About eleven thousand seven hundred and fifty rupees.
Nine. What had the framework said was at risk? Eight thousand rupees.