Free tools

Prop position size calculator

In prop, it’s less "what risk %" and more "how many lots inside my remaining drawdown headroom." Enter your remaining headroom and the loss per lot, and this returns the **safe lot size** that won’t touch the loss limit (with a buffer so you don’t use all of it). Where the general [position size calculator](/en/tools/position-size-calculator) sizes by risk %, this one is **capped by drawdown headroom** — the prop-specific angle.

Advanced (optional)

Result

Safe lots
8.00 lots
Loss if the stop hits
800.00 USD
Headroom used
80%

How to use it

  1. 1 Enter your remaining drawdown headroom — the "safe headroom" from the prop drawdown calculator (the smaller of daily and overall).
  2. 2 Enter the loss if 1.00 lot hits its stop — from the pip calculator or your platform.
  3. 3 Set the safety buffer % (default 80% — use only 80% of the headroom).
  4. 4 Read the safe lots, the loss if the stop hits at that size, and the % of headroom used.

How it is calculated

Safe lots = floor( (remaining headroom × buffer%) ÷ loss per lot , lot step ). It floors to the lot step (default 0.01) and returns 0 below the minimum lot. The loss at that size = safe lots × loss per lot. The buffer is a safety margin so you don’t use all the headroom (default 80%). These are reference figures; the real rules and basis vary by firm.

Frequently asked questions

How is this different from the position size calculator?

The general one sizes by "what % of the account to risk." This prop one caps lots by "remaining drawdown headroom" — because in prop a single trade must not exceed the daily or max loss limit. It’s the same logic as the cockpit pre-trade check in the live terminal.

Where do I get "remaining headroom"?

Use the "safe headroom" from the prop drawdown calculator (the smaller of daily and overall remaining). It changes intraday, so recompute it from your current equity.

How do I find "loss per lot"?

It’s what a 1.00 lot position loses if price reaches your stop. Get it from the pip calculator (stop distance × pip value per lot) or your platform’s order preview.

Why is the buffer 80%?

If you use 100% of the headroom, a small overshoot from spread, slippage or fees breaches the limit. 80% leaves a margin so you don’t sit right on the line (adjust if you want). It does not guarantee a pass.

Put the numbers to work

Practice these positions risk-free on the Merini demo — real-time charts and paper trading with virtual funds, in your browser, no install.

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