Why prop firm sizing is different
On a personal account, position size comes from one number: the share of the account you are willing to lose on a trade. On a prop firm evaluation there are two hard limits on top of that, and breaking either one usually ends the account, however well the rest of the month went:
- The maximum drawdown. The balance may never fall below a floor, typically set a percentage below the starting balance.
- The daily loss limit. Losses within one trading day may not exceed a set amount, even if the account is well above its floor.
So the question is not “what percentage should I risk?” but “how much room do I have left, and how many losing trades does that room need to absorb?” This calculator works backwards from both limits and uses whichever is tighter.
How the calculator works
drawdown room = current balance − drawdown floor
daily room = daily limit − today’s loss so far
risk per trade = smaller of (daily room ÷ losses to survive today) and (drawdown room ÷ losses to survive overall)
position size = risk per trade ÷ (stop distance × value per point)
The size is rounded down to whole contracts, lots or shares, because rounding up is how an account breaks a limit by a few dollars. The results then show how many full stop-outs at that size each limit can absorb.
The “losses in a row” inputs are the part most sizing tools skip. Losing streaks are normal: at a 50% win rate, a run of seven or eight losses somewhere in a few hundred trades is what you should expect. Choosing the streak you want to survive before the evaluation starts is what stops one bad morning from ending it.
Static and trailing drawdown
A static drawdown sets the floor once, at the start: a 10% limit on a $100,000 account puts the floor at $90,000, and it stays there however much you make.
A trailing drawdown moves the floor up behind your highest balance. With a 6% ($6,000) trailing limit, reaching $103,000 lifts the floor to $97,000. A profitable run therefore shrinks your room rather than growing it, until the floor stops moving.
That is the counter-intuitive part: on a trailing account, doing well early can leave you with less room to lose than on day one. Many firms stop trailing once the floor reaches the starting balance or once you pass, and some trail your open (unrealised) equity rather than closed balance, which is stricter still. This calculator trails closed balance with no lock, so enter the highest balance your firm actually measures from.
A worked example
A $100,000 evaluation, now at $101,200 after a good week. Today has started with a $300 loss. The trader wants to survive 3 losses in a row today and 8 overall, and trades Nasdaq futures with a 25-point stop.
| 10% static drawdown | 6% trailing, high $103,000 | |
|---|---|---|
| Drawdown floor | $90,000 | $97,000 |
| Drawdown room | $11,200 | $4,200 |
| Daily room (5% limit, −$300 today) | $4,700 | $4,200 (capped by drawdown) |
| Risk allowed per trade | $1,400 (drawdown ÷ 8) | $525 (drawdown ÷ 8) |
| NQ at $20 a point ($500 per contract) | 2 contracts | 1 contract |
| MNQ at $2 a point ($50 per contract) | 28 contracts | 10 contracts |
Same account, same trader, same trade. The trailing drawdown cuts the size by more than half, because the good week raised the floor. Notice too that on the trailing account the daily room is capped by the drawdown room: the daily limit can never let you lose more than the account has left.
In both cases the overall drawdown, not the daily limit, decides the size. That changes late in a bad day: after a $4,300 loss on the static account, the daily room falls to $700, and at 3 losses to survive that allows $233 a trade, less than one NQ contract.
Rules that differ between firms
Before relying on any number here, check how your own firm defines each limit. These are the details that most often catch traders out:
- Balance or equity. Some firms measure limits on closed balance, others on equity including open trades. An open trade deep in loss can breach an equity-based limit before it is closed.
- The daily limit’s starting point. It may be a percentage of the starting balance, of the previous day’s closing balance, or of the day’s highest equity. This calculator uses the starting balance.
- When the day resets. A daily limit resets at a set time, often in the firm’s own time zone, and not necessarily at midnight where you are.
- Pause or fail. On some evaluations hitting the daily limit fails the account; on others it only locks trading until the next day.
- Trailing locks. Whether a trailing drawdown stops moving at the starting balance, at a profit milestone, or never.
- Consistency rules. Some firms cap how much of your total profit can come from a single day, which limits how large your best trades can be.
Sizing for the whole evaluation, not one trade
The limits tell you the most you can risk. What you should risk depends on the profit target and the time allowed. Risking the maximum the rules allow makes the target reachable in a few trades, but it makes failing reachable in a few trades as well.
A useful check: divide the profit target by your risk per trade. If you need, say, 8% to pass and risk 1% a trade, you need roughly eight winning trades more than losers. Our risk-reward calculator shows the win rate each ratio needs, and the risk of ruin guide covers why smaller risk per trade survives streaks that larger risk does not.
For contract values, see futures tick values and contract sizing, and for forex, forex lot sizes. For a personal account without prop limits, the standard position size calculator is simpler.