MT5 Lot Size Formula: How to Calculate Position Size in MetaTrader 5

If you trade MetaTrader 5 and you've ever asked "how big should this trade be?", you need a lot size formula you can trust. Not a vibe. Not "what feels right". A formula.

This post gives you the formula, walks through worked examples for FX, gold, and indices, and shows you where most retail traders break the math. By the end you'll know exactly how to size every trade — or how to let a tool do it for you in one click.

The MT5 lot size formula (in one line)

Lot size = (Account balance × Risk %) ÷ (Stop-loss in pips × Pip value per lot)

That's it. Three inputs, one division. Everything else is detail.

• Account balance — your real account equity, in your account currency
• Risk % — what percentage of your account you're willing to lose on this single trade (1-2% for most retail traders, never more than 3%)
• Stop-loss in pips — distance from entry to stop, in pips
• Pip value per lot — how much one pip moves your P&L per 1.00 standard lot

The trick is that pip value depends on the instrument and your account currency. EURUSD with a USD account is different from EURUSD with a GBP account. Gold is different from EURUSD. Indices are different again.

That's where retail traders get stuck. The formula is simple. Getting the right pip value is where mistakes happen.

Worked example #1: EURUSD on a £10,000 GBP account

You want to risk 1% per trade. Stop-loss is 25 pips. You're long EURUSD.

• Account balance: £10,000
• Risk %: 1%
• Stop-loss: 25 pips
• Pip value per 1.00 lot of EURUSD on a GBP account: ~£8 per pip (varies slightly with GBPUSD rate)

Lot size = (10,000 × 0.01) ÷ (25 × 8)
        = 100 ÷ 200
        = 0.50 lots

Trade 0.50 lots. If the stop hits, you lose 25 × 8 × 0.50 = £100, which is 1% of your account. Maths works.

Notice: the pip value of ~£8 already accounts for converting USD pip movement back to GBP. If you used a USD pip value (£/lot ≈ $10), you'd get a different lot size and risk too much.

Worked example #2: XAUUSD (gold) on the same account

Gold is the trade where most beginners blow up, because the pip math is unintuitive.

• Account balance: £10,000
• Risk %: 1%
• Stop-loss: 100 points (gold moves in points, not pips — 1 point = $0.01 of price movement)
• Pip value per 1.00 lot of XAUUSD on a GBP account: ~£0.80 per point (1 point on 100oz contract × GBP conversion)

Lot size = (10,000 × 0.01) ÷ (100 × 0.80)
        = 100 ÷ 80
        = 1.25 lots

Trade 1.25 lots of gold. Risk is £100, which is 1%.

Now imagine you used the EURUSD pip value of £8 by mistake:

Wrong lot size = 100 ÷ (100 × 8) = 0.125 lots

You'd trade 1/10th the position you should be trading. Not catastrophic, but you've under-sized — your win didn't pay you what it should have.

The opposite mistake — using gold's pip value to calculate an EURUSD trade — would mean trading 10x too big. That's how accounts go to zero in a week.

Worked example #3: NAS100 (Nasdaq) on the same account

Indices are confusing because they're often quoted in points and pip value depends on the broker's contract size.

• Account balance: £10,000
• Risk %: 1%
• Stop-loss: 50 points
• Pip value per 1.00 lot of NAS100 (1 point per contract on a typical CFD): ~£0.80 per point (depends on broker)

Lot size = (10,000 × 0.01) ÷ (50 × 0.80)
        = 100 ÷ 40
        = 2.5 lots

You'd trade 2.5 lots of NAS100. But — and this is critical — many brokers cap NAS100 lot sizes, or have minimum increments of 0.01 or 0.1. Always check your broker's contract spec.

The three places retail traders break the formula

The math itself is primary-school arithmetic. The mistakes are:

1. Using the wrong pip value

The biggest one. EURUSD pip value is not gold's pip value. Cross pairs (EURJPY, GBPNZD) have different pip values again. Account currency matters. Most online "pip calculators" round badly or default to USD-account assumptions.

Fix: Either learn the pip values for the 3-5 instruments you actually trade, or use a tool that pulls them live from MT5. Don't trust generic web calculators for anything cross-currency.

2. Using a fixed lot size every trade

This is the most common retail blunder. "I always trade 0.1 lots." Sounds disciplined. It isn't.

A 0.1-lot trade with a 20-pip stop on EURUSD risks roughly £16. The same 0.1-lot trade with a 200-pip stop on GBPJPY risks roughly £120. Same lot size. Roughly 7-8x the actual risk.

If you do this on a £10k account, your "I always risk 1%" rule is a lie. Some trades are 0.16% risk, others are 1.2%, and you have no idea which until afterward.

Fix: Risk a fixed percentage of account per trade, not a fixed lot size. Lot size is the output of the formula, not the input.

3. Calculating manually under pressure

Setup forms. You see the entry. You start dividing in your head. The price ticks. You miss the entry, or you punch in the wrong number, or you skip the math entirely and just "feel" the trade size.

This is how 4% risk happens on a "1% trade".

Fix: Pre-calculate before the session, or use an MT5 tool that does it live as you draw your stop. Don't do mental arithmetic when money's on the line.

How to do this properly inside MT5

Three options, in order of effort:

Option 1: Manual, with a spreadsheet

Build a spreadsheet that takes account balance, risk %, stop in pips, and pip value, and outputs lot size. Tab between MT5 and your spreadsheet on every trade.

Works. Slow. Error-prone in fast markets. Fine for swing traders.

Option 2: Free MT5 indicator/script

There are free position-sizing scripts on MQL5.com. Quality varies wildly. Read the reviews. Some only support FX. Some break on cross-pairs. Most don't update live as you drag the stop.

Works for some. Frustrating for others.

Option 3: A proper risk-management EA

A small Expert Advisor that sits on the chart and does the calculation in real time. You set risk % once. You drag your stop. The lot size updates live. Click BUY or SELL — it places the trade, sized correctly, with stop and take-profit pre-calculated.

This is what Rical does. £25 one-time, lifetime updates, works on any MT5 instrument with the right pip data.

I'm biased — I built it because I was tired of doing the math wrong under pressure. But the principle is what matters: automate the formula or run the formula manually every trade. The one thing you cannot do is skip it and hope.

Quick reference: pip values you should memorise

For a USD account, 1.00 standard lot, approximate pip values:

• EURUSD, GBPUSD, AUDUSD, NZDUSD: ~$10/pip
• USDJPY, EURJPY, GBPJPY: ~$10 per 100 yen (a "pip" in JPY pairs is 0.01, not 0.0001)
• XAUUSD (gold): ~$1 per cent of price ($100 per dollar of price move per 100oz lot)
• NAS100, US30, SPX500: ~$1 per point (broker-dependent)
• BTCUSD: $1 per dollar of price move per 1 BTC contract (broker-dependent)

For a GBP account, multiply USD pip values by GBPUSD ≈ 0.80 (i.e., a USD pip is worth ~£0.80).

These are approximate — always verify with your broker's contract spec or pull them live from MT5.

TL;DR

• Lot size = (Account × Risk %) ÷ (Stop × Pip value)
• Pip value depends on the instrument and your account currency. Get this wrong and the formula lies to you.
• Don't trade fixed lot sizes. Risk a fixed % of account per trade.
• Don't do mental math in fast markets. Pre-calculate or automate.

If you want the math automated inside MT5 with live updates as you drag the stop, Rical does exactly that for £25 one-time. If you'd rather build it yourself, the formula above is all you need — just be ruthless about getting the pip values right.

Either way, size every trade by your real account risk, every time. That's the difference between traders who survive and traders who don't.