Lot size calculator for prop firm challenges in MT5

In a challenge the lot size decides how many losing trades you can take before a rule ends the account. Work it out from money at risk, not from a habit.

The formula

Lot size = money you are willing to lose ÷ (loss per lot at your stop + commission per lot).

• Money at risk: a percentage of the balance, or a fixed amount.
• Loss per lot at the stop: stop distance × value of one price step per lot. MT5 knows this value for every symbol (tick value and tick size).
• Commission: the round-turn commission per lot belongs in the risk, because it is paid even when the stop is hit.

Round the result down to the broker's lot step.

A worked example

Account 100,000 USD, risk 0.5 % = 500 USD. EURUSD, stop 20 pips, about 10 USD per pip and lot, commission 7 USD per lot round turn.

• Loss per lot at the stop: 20 × 10 = 200 USD, plus 7 USD commission = 207 USD.
• Lot size: 500 ÷ 207 = 2.415, rounded down to 2.41 lots.
• Without the commission you would trade 2.50 lots and risk about 517 USD instead of 500.

The numbers are an illustration; your broker's values decide.

Size from the room you have left today

Prop firms limit the loss per day. A fixed 1 % per trade ignores how much of that limit you have already used. Sizing from the room left keeps one stop from breaking the rule:

• Daily limit 5 % of 100,000 = 5,000 USD. Today you are down 3,200 USD (closed and open trades), so 1,800 USD is left.
• Risking a quarter of the room on the next trade means 450 USD, and the room shrinks with every loss.
• Check how your firm measures the day: balance or equity at the reset, and at which time the reset happens. See equity vs balance drawdown.

Gold, indices and crypto

A "pip" means different things on XAUUSD, US100 or BTCUSD, and contract sizes differ between brokers. A calculator that uses the symbol's own tick value works on every symbol; a fixed "10 USD per pip" table does not.

Common mistakes

• Rounding the lot up.
• Leaving out commission, spread and slippage at the stop.
• Sizing each trade alone while several positions are open: if all stops are hit at once, the losses add up.
• Copying the lot from a 100k account to a 10k account without scaling it.

Free tool for MT5

The free PropBlaster Position Sizer does this on the chart: drag the stop loss line and it shows the lot from your risk with commission and expected slippage included, plus hard caps for lot size and risk; the risk mode "% Day room" sizes the trade from the daily loss room left today. The PropBlaster Trade Panel adds up to three take-profit levels, break-even, trailing and partial closes. To see the daily room and max. loss in one panel, use the free PropBlaster Drawdown Meter, the PropBlaster Guard or the PropBlaster Cockpit.

Questions

Should I round the lot size up or down?

Always down to the broker's lot step. Rounding up puts more than your planned risk on the trade.

Why is my lot size on gold so different from EURUSD?

Gold and indices have other contract sizes and tick values. Let MT5 use the symbol's own tick value instead of a fixed pip value.

How much of the daily loss room should one trade use?

That is your decision. Many traders keep a single trade well below the room left today, so that one stop does not end the day.

Related: Daily drawdown rules explained · Equity vs balance drawdown · Copy trading rules · Trading journal for prop firms · Daily loss limit EA · Lot size for prop firms