Two traders take the same setup. Same entry, same stop, same strategy, same win rate over the next twenty trades. One trader is up for the month. The other has blown through a third of their account.

The difference was never the setup. It was how many contracts, lots, or shares each of them bought — a decision most traders make by feel, in a few seconds, right before clicking buy. Position sizing is the one variable in a trade that a trader controls completely, before the market has any say in the outcome, and it is also the variable most traders never actually calculate.

This article covers the position sizing formula that works across any market, worked examples in futures, forex, stocks and crypto, and the specific mistakes that quietly turn a correctly-sized plan into an oversized one.

What Is Position Sizing?

Position sizing is the process of deciding how many units of an instrument — contracts, lots, shares, or coins — to buy or sell on a given trade, based on how much money you are willing to lose if your stop-loss is hit.

It is not the same question as "how much can I afford to buy." A trader with $50,000 can afford to buy far more than one contract of most futures products. Position sizing asks a narrower, more useful question: given my stop distance and my risk tolerance for this specific trade, what size keeps my loss at exactly the amount I decided on in advance, no more and no less?

Get that number right and a losing streak costs exactly what you planned for. Get it wrong — even while using the same entries, same stops, and same strategy — and the same losing streak can end the account.

The Position Sizing Formula

The formula is the same regardless of what you trade. Only one input changes between markets: the dollar value of one unit of price movement.

Stop Distance = |Entry Price − Stop-Loss Price|

Risk Per Unit = Stop Distance × Dollar Value Per Unit

Position Size = Risk Per Trade ÷ Risk Per Unit

"Risk per trade" is a number you decide before the trade exists — typically a fixed percentage of account balance, such as 1%. "Risk per unit" is what one contract, lot, or share actually costs you if the stop is hit. Divide the first by the second and the result is the largest position size your risk tolerance allows for that specific stop distance.

The formula always rounds down. A calculation of 5.7 contracts means 5 contracts — the fractional part isn't a position you can take, and rounding up defeats the entire purpose of the calculation.

Position Sizing Examples by Market

The formula doesn't change. What "dollar value per unit" means does, and getting that part wrong is where most sizing mistakes actually happen.

Futures

Every futures contract has a fixed dollar value per point, set by the exchange. Micro E-mini S&P 500 (MES) is worth $5 per point; Micro E-mini Nasdaq-100 (MNQ) is worth $2 per point. These are not approximations — they're exact, published contract specifications, which is what makes futures sizing the cleanest example to work through.

Risk per trade: $250. Instrument: MES. Entry: 6500. Stop: 6490.

Swap MES for MNQ with a 50-point stop instead:

Same $250 risk budget, same trader, two completely different position sizes — because the two instruments have different dollar values per point and the trades used different stop distances. Treating "5 contracts" as a number that transfers from MES to MNQ, or from a 10-point stop to a 50-point stop, is a sizing error that has nothing to do with market analysis.

Forex

Forex sizing runs on lot size instead of contract multipliers. A standard lot is 100,000 units of the base currency; a mini lot is 10,000; a micro lot is 1,000.

For a pair quoted directly against the dollar (EUR/USD, GBP/USD), the math is straightforward: one standard lot moves roughly $10 per pip (0.0001). Risk $500 with a 50-pip stop:

For a pair where the dollar is the base currency instead of the quote currency (USD/JPY, USD/CHF), the dollar value per pip depends on the current exchange rate rather than being a fixed number — which is exactly why a correct calculator asks for the entry price on these pairs specifically: it's needed to convert the pip value into dollars correctly, not just for reference. Skipping that conversion and assuming every pair behaves like EUR/USD is one of the more common — and more expensive — forex sizing mistakes.

Stocks

Stocks are the simplest case: one share is one unit, so the dollar value per unit is always 1.

Risk per trade: $250. Entry: $200. Stop: $195.

Crypto

Crypto follows the identical stocks logic — one coin (or fraction of one) is one unit.

Risk per trade: $250. Entry: $100,000. Stop: $99,000.

The stop distance in dollar terms looks large next to a $250 risk budget, and that's the point of running the calculation rather than guessing — a wide stop on a high-priced asset shrinks the position size sharply, exactly as it should.

Why Position Sizing Matters More Than Your Win Rate

A strategy's win rate gets checked constantly. Position sizing rarely does, which is backwards, because win rate alone can't tell you whether a strategy is profitable — but incorrect position sizing can turn a genuinely profitable strategy into a blown account regardless of what the win rate says.

Here's why: win rate and reward-to-risk describe the shape of a strategy's edge over many trades. Position sizing determines whether the account survives long enough for that edge to actually show up. A strategy with a real, positive edge and consistent 1% risk per trade can survive an eight-loss streak and barely notice. The identical strategy, sized inconsistently — 1% on some trades, 4% on others because a setup "felt stronger" — can be functionally ended by the same eight-loss streak, even though nothing about the edge itself changed.

Win rate answers "is this strategy good." Position sizing answers "will I still have an account by the time that answer becomes statistically clear." Both questions matter. Only one of them gets checked by most traders before every single trade.

Common Position Sizing Mistakes

Sizing by a fixed dollar amount instead of a fixed stop-adjusted amount. Buying "10 contracts" or "$5,000 worth" regardless of stop distance means a tight-stop trade risks far less than intended and a wide-stop trade risks far more — the position size stopped reflecting the actual trade.

Increasing size after a loss to "get it back faster." This is a sizing decision driven by the last trade's result instead of the current trade's risk, and it's one of the more direct ways a trader ends up violating their own risk rules under pressure without necessarily framing it to themselves as rule-breaking — it just feels like conviction.

Confusing similar-looking instruments. MES and ES look almost identical on a chart and are worth $5 and $50 per point respectively — a ten-times difference. Sizing an MES trade using ES's multiplier, or the reverse, isn't a rare typo; it's an easy mix-up with a real dollar cost the moment it happens.

Rounding up instead of down. A calculation that comes out to 3.4 contracts means 3 contracts. Rounding up to 4 "because it's close enough" quietly increases risk per trade above what was actually decided on — a small-looking decision that compounds across every trade sized that way.

Not recalculating for every trade. Stop distance changes trade to trade even within the same strategy and instrument. A position size that was correct for a 10-point stop is wrong for a 25-point stop on the next setup — reusing yesterday's number is reusing yesterday's stop distance, whether or not that's what actually happened.

Position Sizing and Account-Level Risk Limits

Risk per trade and account-level risk are two different constraints, and conflating them is a specific mistake worth calling out on its own. Risk per trade asks "how much am I willing to lose on this one trade." A prop firm's drawdown limit, or a trader's own maximum daily loss, asks a completely separate question: "how much room does the account have left before a rule is breached, or before I've decided to stop for the day."

A trade can be sized correctly against its own risk budget and still be too large for the account's remaining room — five contracts might be the right size for a $250 risk-per-trade budget, while the account only has $100 of drawdown room left before hitting a firm's limit. In that case, the smaller of the two numbers is the one that should govern the trade, not the risk-per-trade calculation in isolation. A prop firm trading plan that only accounts for per-trade risk and never checks it against the account's actual remaining drawdown room is missing exactly this layer.

Calculating Position Size Without Doing the Math by Hand

The formula above is simple, but doing it correctly, every time, for every stop distance and every instrument, is where most traders quietly stop bothering — which is usually the moment sizing reverts to a guess.

This is the specific problem TradeProof AI's Prop Rules position size calculator is built to remove. Enter a risk-per-trade amount, pick the market (futures, forex, stocks, or crypto) and instrument, and enter the entry and stop-loss prices — it calculates the stop distance, the risk per contract, lot, share, or coin, and the maximum position size automatically, using the same formula worked through above. It also checks that number against the account's remaining drawdown room, so the position size respects both constraints from the section above — the trade's own risk budget and what the account can actually still absorb — without needing to run two separate calculations by hand before every entry.

None of that replaces having a real edge. What it removes is the specific, avoidable failure mode where a good strategy gets undermined by an arithmetic mistake made in the ten seconds before clicking buy.

Final Thoughts

Position sizing is the one part of a trade that is fully decided before the market has any input, which makes it the easiest place to be precise and one of the most common places traders instead guess. The formula is short — risk per trade divided by risk per unit — but every input has to be right: the correct stop distance, the correct dollar value for the specific instrument, and the correct account-level constraint checked alongside it.

Run the calculation before every trade, not just the ones that feel like they matter. The trades that don't feel important are exactly the ones where sizing quietly drifts, and a strategy with a genuine edge only gets to prove it over enough trades to survive the drawdown along the way.