Position Size Calculator: Lot Size From Risk Percent

What this calculator does

Position sizing is the single most important risk control a trader has. It answers one question before every trade: how many lots can I open so that if my stop-loss is hit, I lose only the amount I decided in advance? This tool takes your account balance, the percentage of that balance you're willing to risk, your stop-loss distance in pips, and the currency pair, then returns the precise lot size.

The output is deliberately mechanical. You choose the risk; the maths chooses the size. That removes the temptation to "size up" on a setup that feels strong — the most common reason accounts blow up.

The formula

The tool computes two steps:

riskAmount = balance × (risk% / 100)
lots       = riskAmount / (stopPips × pipValuePerLot)

The only moving part is pipValuePerLot, the cash value of a one-pip move on one standard lot. For a standard lot, contractSize = 100,000 and pipSize = 0.0001 (or 0.01 for JPY pairs):

• USD-quoted pairs (USD is the second currency, e.g. EUR/USD, GBP/USD): pipValuePerLot = contractSize × pipSize, which is a clean $10 per pip per standard lot.
• Other pairs (USD is the base or absent, e.g. USD/JPY, EUR/GBP): pipValuePerLot = (contractSize × pipSize) / price, because the pip is denominated in the quote currency and must be converted back to your account currency at the current rate.

Multiply stop distance by pip value and you get the loss per lot; divide your risk budget by that and you get lots.

Worked example

• Account balance: $5,000
• Risk per trade: 1%
• Stop-loss: 25 pips
• Pair: EUR/USD (USD-quoted)

Step 1 — risk budget: 5,000 × 1/100 = $50. Step 2 — pip value per lot: 100,000 × 0.0001 = $10. Step 3 — lots: 50 / (25 × 10) = 50 / 250 = 0.20 lots.

So you open 0.20 lots (20,000 units). Each pip is worth 0.20 × $10 = $2, and a 25-pip stop loses exactly 2 × 25 = $50 — your planned 1%. Widen the stop to 50 pips and the size halves to 0.10 lots, keeping the dollar risk identical. That inverse relationship between stop width and position size is the heart of disciplined sizing.

Derivation, in one line

You want lots × stopPips × pipValuePerLot = riskAmount. Solve for lots and you divide both sides by stopPips × pipValuePerLot — exactly the formula above. Everything else is just getting pipValuePerLot right for the pair.

Edge cases and pitfalls

• JPY pairs use a different pip size. A pip in USD/JPY is 0.01, not 0.0001. The tool handles this, but if you ever size by hand, using 0.0001 on a yen pair overstates your size by 100x.
• Non-USD-account currencies need a second conversion. If your account is in EUR but you trade USD/JPY, the pip value lands in the quote currency and must be converted to your account currency. Sizing in the wrong currency quietly skews real risk — confirm the tool's account-currency setting matches your broker statement.
• Stops and spread. Your effective stop is wider than the chart distance because you cross the spread on entry and exit. On a tight 8-pip scalp, a 1-pip spread is over 10% of your stop. Size on the real stop distance, not the ideal one.
• Rounding to broker minimums. Brokers trade in 0.01-lot steps (micro lots). If the formula returns 0.236 lots, round down to 0.23 to stay under budget, never up.

FXNX traders often pair this with raw spreads from 0.0 pips on NX Pro, so the spread component of a tight stop barely moves the position size.

Putting it to work

Lock a fixed risk percent — many professionals sit between 0.5% and 2% per trade — and let the calculator vary the lot size as your stop and balance change. Consistency in risk matters far more than consistency in lot size.