How to Calculate Lot Size on a Prop Firm Challenge (With Examples)
The lot size formula, pip value for USD-quoted, USD-base and cross pairs, and worked examples on a $100K simulated account.
OctoFunded Trading DeskUpdated ยท 8 min read
To calculate lot size on a prop firm challenge, start from the dollar amount you accept to lose, not from a lot size you are used to. Divide that amount by the stop distance in pips times the pip value of one lot. The answer is the position size that loses exactly your planned risk if the stop is hit.
This guide gives the formula, the pip value for each type of pair and worked examples on a $100K simulated account.
Key takeaways
- Start from dollars: decide the risk per trade first, then derive lots from it.
- Stop distance comes before size: place the stop where the trade idea is wrong, then size to it.
- Pip value is not always $10: it depends on the quote currency and the current rate.
- Round down: a slightly smaller position keeps the loss inside your plan.
- Count all open trades: their combined risk must fit inside today's remaining loss room.
Why does lot size matter more on a prop firm challenge?
Lot size matters more on a prop firm challenge because the loss limits are fixed and a breach ends the account. On a personal account an oversized trade hurts; on a challenge it can use a full day's limit in one move.
At OctoFunded, the daily loss limit is 5% on the 2-Step, 4% on the 1-Step and 3% on Instant Funded. On $100,000 that is $5,000, $4,000 and $3,000. A trade sized by habit instead of by math can take a large part of that in one stop.
Position size is the link between your plan and those limits. For the bigger picture, see prop firm rules explained.
What is the lot size formula?
The lot size formula turns a dollar risk and a stop distance into a position size.
Formula: lots = dollar risk / (stop distance in pips x pip value per standard lot)
Three inputs feed it:
- Dollar risk. Account size times your risk percentage. 0.5% of $100,000 is $500. To choose the percentage, read how much to risk per trade.
- Stop distance. The pips between your entry and your stop loss. Set it from the chart, where the trade idea is invalidated, not from the size you want.
- Pip value per lot. What one pip is worth on one standard lot, in your account currency.
A quick reminder on units. A standard lot is 100,000 units of the base currency, a mini lot 0.10 and a micro lot 0.01. A pip is 0.0001 on most pairs and 0.01 on pairs quoted in Japanese yen.
How do you find the pip value?
Pip value depends on which currency the pair is quoted in. The examples below use round prices for illustration only; use live rates when you trade.
| Pair type | Example pair | Pip value formula per standard lot | Illustrative rate | Pip value per lot ($) |
|---|---|---|---|---|
| USD is the quote currency | EURUSD | 100,000 x 0.0001 = 10 USD | Not needed | $10.00 |
| USD is the base currency | USDJPY | 100,000 x 0.01 / USDJPY price | 150.00 | $6.67 |
| USD is the base currency | USDCAD | 100,000 x 0.0001 / USDCAD price | 1.3600 | $7.35 |
| Cross pair | EURGBP | 10 GBP x GBPUSD price | 1.2700 | $12.70 |
USD as quote currency. On EURUSD, GBPUSD, AUDUSD and similar pairs, one pip on a standard lot is $10, whatever the price.
USD as base currency. On USDJPY, the pip is worth 1,000 yen per lot. Divide by the price to get dollars: 1,000 / 150.00 = $6.67. On USDCAD, the pip is 10 Canadian dollars, so 10 / 1.3600 = $7.35.
Cross pairs. On EURGBP, the pip is worth 10 pounds. Multiply by GBPUSD to convert: 10 x 1.2700 = $12.70.
Because rates move, pip values on non-USD-quoted pairs move too. A calculator or your platform's symbol details do this conversion live.
How do you calculate lot size on a $100K simulated account?
You calculate it by applying the formula to each trade with that pair's own pip value. Here are four worked examples, all with the same $500 risk (0.5% of $100,000). Prices, stops and contract sizes are examples, not trade ideas.
| Example | Stop distance | Pip or point value per lot ($) | Loss per lot at the stop ($) | Lot size (rounded down) |
|---|---|---|---|---|
| EURUSD | 25 pips | $10.00 | $250 | 2.00 |
| USDJPY at 150.00 | 30 pips | $6.67 | $200 | 2.50 |
| EURGBP at GBPUSD 1.2700 | 20 pips | $12.70 | $254 | 1.96 |
| Gold, 100 oz per lot | $4.00 move | $100 per $1 | $400 | 1.25 |
Step by step:
- EURUSD: 25 x $10 = $250 per lot. $500 / $250 = 2.00 lots.
- USDJPY: the exact pip value is 1,000 / 150.00 = $6.667, so 30 pips cost $200 per lot. $500 / $200 = 2.50 lots. If you round the pip value to $6.67 first, you get 2.499: round down to 2.49.
- EURGBP: 20 x $12.70 = $254 per lot. $500 / $254 = 1.968, rounded down to 1.96 lots.
- Gold: with a contract of 100 ounces, a $1 move is worth $100 per lot. A $4.00 stop costs $400 per lot. $500 / $400 = 1.25 lots.
Gold, indices and crypto have their own contract sizes, which differ between providers. Always check the contract size and tick value of the exact symbol before sizing, rather than assuming forex conventions.
How do the loss limits cap your size?
Your loss limits cap the total risk you can have open, not only the risk of each trade. Every open trade counts at its stop, together with any loss already taken today.
Take an illustrative 1-Step day on $100,000, with a 4% daily limit of $4,000:
| Moment | Closed loss today ($) | Open risk at stops ($) | Total if all stops hit ($) | Room left to the daily limit ($) |
|---|---|---|---|---|
| Start of day | $0 | $0 | $0 | $4,000 |
| Two trades open at $500 each | $0 | $1,000 | $1,000 | $3,000 |
| One stop hit, one trade still open | $500 | $500 | $1,000 | $3,000 |
| Three more trades at $500 each | $500 | $2,000 | $2,500 | $1,500 |
Firms measure the daily limit in different ways, and floating losses usually count, so read how your daily loss limit is calculated before you rely on these numbers. The rule of thumb still holds: add every open stop to today's loss before you size the next trade.
The max drawdown works the same way over the life of the account. Under a trailing floor, the room to the floor can shrink even after a profitable run, so size from the current distance to the floor, not from the starting balance.
What mistakes break the lot size math?
Most sizing errors come from skipping a step, not from the formula itself.
- Sizing from habit. "I always trade 2 lots" means a different dollar risk on every pair and every stop.
- Using $10 per pip everywhere. On EURGBP at the example rate, assuming $10 instead of $12.70 gives 2.50 lots instead of 1.96, a position about 27% too large.
- Moving the stop after sizing. A wider stop at the same size raises the loss. Re-run the formula.
- Forgetting costs. Spread, commission and slippage come on top. Take a little off the risk before sizing.
- Ignoring correlated trades. Long EURUSD and long GBPUSD often move together. Treat them as one bigger position.
- Rounding up. 1.968 lots becomes 1.97 or 2.00 and the loss at the stop is larger than planned.
- Not checking the symbol. Gold, indices and crypto have their own contract size. Assuming forex conventions can be off by a large factor.
If you trade with an EA, put the same formula in the code: read the pip value from the symbol, compute lots from the stop and round down. OctoFunded allows EAs; see trading with an EA for the rest of the setup.
FAQ
What is the formula for lot size?
Lot size equals your dollar risk divided by the stop distance in pips times the pip value of one lot. Round the result down to the smallest lot step your platform allows.
How much is one pip worth on a standard lot?
On pairs quoted in US dollars, such as EURUSD, one pip on a standard lot of 100,000 units is worth about $10. On other pairs the value depends on the exchange rate, so convert it to your account currency.
Does account size change the lot size formula?
No. Account size only sets the dollar risk, for example 0.5% of $100,000 is $500. The formula from dollar risk to lots is the same on any account.
Should I round lot size up or down?
Round down. Rounding up makes the loss at your stop larger than planned, and on a funded account that extra amount comes out of a fixed loss limit.
Does the daily loss limit affect lot size?
Yes. The combined loss of all open trades at their stops, plus any loss already taken today, must stay inside your remaining daily room. Size each new trade from what is left.
Size every trade from the rules
Lot size is where your plan meets the loss limits. OctoFunded runs the 2-Step, 1-Step and Instant Funded on simulated capital, with daily limits of 5%, 4% and 3%, a one-time fee and no time limit. Check the OctoFunded program rules, then size your first trade with the formula above.
