You find a perfect setup. RSI oversold, price at support, volume confirming. You go in heavy — 20% of your account on a single trade.
It drops. Your stop hits. You just lost 20% of your capital on one trade.
Two more like that and you're done. Not because your strategy was bad, but because your sizing was.
This is the most common way trading accounts die. Not from bad strategies — from bad position sizing.
Why Position Sizing Matters More Than Win Rate
Here's a thought experiment.
Trader A: 70% win rate, risks 25% per trade
Trader B: 45% win rate, risks 1% per trade
Who survives?
Trader A hits three losers in a row (which happens to every 70% system eventually). They're down 75%. They need a 300% return just to break even. They're finished.
Trader B hits ten losers in a row (unlikely at 45%, but possible). They're down 10%. Uncomfortable, but completely recoverable.
The maths is brutal and asymmetric:
| Loss | Return Needed to Recover |
|---|---|
| 10% | 11% |
| 20% | 25% |
| 30% | 43% |
| 50% | 100% |
| 75% | 300% |
After a 50% drawdown, you need to double your remaining capital just to get back to where you started. Position sizing exists to make sure you never get there.
The 1-2% Rule
The most widely used position sizing approach: never risk more than 1-2% of your total account on a single trade.
This doesn't mean you invest 1-2% of your account. It means the amount you'd lose if your stop loss gets hit is 1-2% of your total capital.
Example:
- Account: £10,000
- Risk per trade: 1% = £100
- Stock price: £50
- Stop loss: £48 (£2 below entry)
- Position size: £100 ÷ £2 = 50 shares (£2,500 position)
You're investing £2,500 (25% of your account), but you're only risking £100 (1%). If the trade goes against you and hits your stop at £48, you lose £100 — exactly 1% of your account.
This is the crucial distinction. Position size depends on where your stop is, not on how much capital you have.
The Formula
Position sizing comes down to one formula:
Position Size = Risk Amount ÷ Risk Per Share
Where:
Risk Amount = Account Size × Risk Percentage
Risk Per Share = Entry Price - Stop Loss Price
Practical examples:
Stock Trade
- Account: £20,000
- Risk: 1.5% = £300
- Buy AAPL at £180, stop at £175
- Risk per share: £5
- Shares: £300 ÷ £5 = 60 shares (£10,800 position)
Crypto Trade
- Account: £5,000
- Risk: 2% = £100
- Buy BTC at £42,000, stop at £41,000
- Risk per unit: £1,000
- Size: £100 ÷ £1,000 = 0.1 BTC (£4,200 position)
Forex Trade
- Account: £10,000
- Risk: 1% = £100
- Buy EUR/USD at 1.0850, stop at 1.0800
- Risk: 50 pips
- £100 ÷ 50 pips = £2 per pip → approximately 0.2 standard lots
The pip calculator handles the forex maths, and the position size calculator works for all asset classes.
Why Not a Fixed Position Size?
Many beginners use a fixed approach: "I'll always buy £1,000 worth" or "I'll always buy 100 shares."
The problem: fixed sizing ignores the stop distance.
- Buy 100 shares with a £1 stop = £100 risk
- Buy 100 shares with a £10 stop = £1,000 risk
Same position size, 10x different risk. Your volatile trades would dwarf your conservative ones, making your results entirely dependent on whether the risky trades happen to work.
Risk-based sizing keeps every trade's potential loss equal in percentage terms. Win or lose, each trade has the same impact on your account. That's how you stay in the game long enough for your edge to play out.
Adjusting for Volatility
Not all markets move the same. BTC can swing 5% in an hour. A blue-chip stock might move 0.5% in a day.
Some traders use ATR (Average True Range) to set stops, which automatically adjusts position size for volatility:
- Stop = Entry - (2 × ATR)
- Higher ATR → wider stop → smaller position size
- Lower ATR → tighter stop → larger position size
This keeps your actual risk consistent even across wildly different instruments. A volatile crypto trade and a steady stock trade both risk the same 1% of your account.
Adjusting for Conviction
Some traders use a tiered system:
| Conviction | Risk |
|---|---|
| A+ setup (everything aligns) | 2% |
| Standard setup | 1% |
| Speculative / uncertain | 0.5% |
This makes intuitive sense — risk more when conditions are best, less when you're less sure. But be honest with yourself. If every trade is an "A+ setup," you're not being selective.
Your trading journal should track which conviction level each trade was. Review monthly to see if your A+ trades actually perform better than your standard ones. If they don't, drop the tiering and stick to a flat percentage.
The Kelly Criterion: For the Mathematically Inclined
The Kelly Criterion calculates the mathematically optimal bet size to maximise long-term growth:
Kelly % = W - (1-W)/R
Where:
W = Win rate (decimal)
R = Average win ÷ Average loss
Example: 55% win rate, average win £200, average loss £150
- Kelly = 0.55 - (0.45 / 1.33) = 0.55 - 0.338 = 21.2%
Full Kelly says risk 21.2% per trade. In practice, nobody should use full Kelly. The formula assumes perfectly known probabilities (you don't have those) and zero emotional impact (you're human).
Most practitioners use quarter Kelly (divide by 4) or half Kelly as a more conservative approach. Quarter Kelly on our example: 21.2% ÷ 4 = 5.3%, which is still aggressive for most retail traders.
The Kelly Criterion is useful as a ceiling — "I should definitely risk less than this amount" — rather than a target.
Position Sizing for Different Strategies
Day Trading
Day traders often use slightly higher risk (1.5-2%) because:
- Tighter stops (smaller time frame = less distance to stop)
- More trades per day = smaller positions needed per trade
- No overnight gap risk
But be careful with multiple concurrent positions. Three open trades at 2% each = 6% total risk. If the market drops and all three hit their stops, you're down 6% in one session.
Rule of thumb: Total open risk across all positions shouldn't exceed 5-6% at any time.
Swing Trading
Swing traders hold for days to weeks. Wider stops are needed to avoid getting shaken out by normal volatility, which means smaller position sizes.
1% risk is typical. The position might represent 10-15% of your account, but the risk (stop distance × shares) stays at 1%.
Crypto
Crypto is more volatile than stocks. Period.
- Consider using 0.5-1% risk instead of 1-2%
- Always account for slippage — stops in fast-moving crypto can fill well below your stop price
- Liquidation on leveraged positions is the ultimate position sizing failure — know your liquidation price before entering
Common Mistakes
1. Sizing Based on How Much You Want to Make
"If I buy 500 shares and it goes up £2, I'll make £1,000!"
You've calculated the upside and ignored the downside. What if it goes down £2? Can your account handle a £1,000 loss? Size based on what you can afford to lose, not what you hope to gain.
2. Doubling Down on Losers
"It dropped 10%, so I'll buy more to lower my average!"
This is anti-position-sizing. You're increasing your risk on a trade that's already going against you. If your thesis was wrong, adding to the position makes it worse. If your stop was at -5% and you average down at -10%, you never respected your original stop in the first place.
3. Going All-In on "Sure Things"
There are no sure things in trading. Every trade has a probability of loss. Size accordingly.
The graveyard of blown accounts is full of traders who were "absolutely certain" about a trade and sized accordingly. Being right 9 times and wrong once doesn't matter if the one time you're wrong you bet the farm.
4. Ignoring Correlation
Buying BTC, ETH, and SOL — each at 2% risk — isn't 2% risk. It's closer to 6% risk because crypto assets are highly correlated. When BTC drops, ETH and SOL usually follow.
If your positions are correlated, reduce the per-trade risk. 1% each on three correlated crypto trades keeps your total risk at roughly 3% — but if they all dump together, that's still your real exposure.
5. Not Adjusting After Losses
If your account was £10,000 and drops to £8,000, your 1% risk is now £80, not £100. Recalculate after every trade. This is called fixed fractional sizing — the position naturally gets smaller as your account shrinks, protecting you from ruin.
Conversely, as your account grows, your position size grows proportionally. This compounds your gains without taking on extra risk.
A Quick Position Sizing Checklist
Before every trade:
- What's my account balance right now? (Not what it was last week)
- What percentage am I risking? (1-2% for most strategies)
- Where's my stop loss? (Based on chart structure, not arbitrary percentage)
- What's the risk per share/unit? (Entry minus stop)
- How many shares/units? (Risk amount ÷ risk per share)
- What's my total open risk? (This trade + all other open positions)
If the answer to #6 is above 5-6%, either reduce this trade or close something else first.
The position size calculator does steps 2-5 automatically. Plug in your account size, risk percentage, entry, and stop — it gives you the exact position size.
The Uncomfortable Truth
Position sizing is boring. It doesn't feel like trading. Finding setups, reading charts, spotting patterns — that's the exciting part.
But here's what separates accounts that survive their first year from accounts that don't: the survivors respected position sizing. They had losing streaks (everyone does) but none of them were catastrophic.
You can have the best strategy in the world. If you size one trade badly enough, you can lose months of gains in an afternoon.
Keep it small. Keep it consistent. Let the edge compound.
Nydar's position size calculator computes exact trade sizes based on your account, risk tolerance, and stop placement. Pair it with paper trading to practise disciplined sizing before risking real capital. Track your sizing discipline in the trading journal — it's the metric that matters most.
Originally published at Nydar. Nydar is a free trading platform with AI-powered signals and analysis.
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