A lot is the unit used to measure trade size: in forex, a standard lot is 100,000 units of the base currency, a mini lot is 10,000 units and a micro lot is 1,000 units. To calculate your lot size, divide the amount you are willing to lose on the trade by the product of your stop distance in pips and the pip value of one lot. The definition is the easy part, and on its own it barely helps you, because the question you actually face is: how many lots should I trade right now?

This article answers that question with three steps you take before every trade.

What your lot size actually decides

Your lot size decides how much money each unit of price movement is worth to you. On a currency pair where the US dollar is the second (quote) currency, one standard lot makes each pip worth about $10. A mini lot makes it about $1, and a micro lot about 10 cents.

In other words, lot size is the conversion factor between "the market moved" and "my account changed." Your stop distance is different on every trade, so that factor has to change from trade to trade as well. That is the one and only reason a fixed size is a mistake.

Why a fixed lot size is a mistake

Say you always trade one mini lot. One trade has a 15-pip stop and the next has a 60-pip stop. If both fail, the first costs you about $15 and the second about $60: four times as much. Yet you took both with the same level of confidence. Without noticing, you are risking four times more on some trades than on others.

Adjusting your size is what keeps your risk constant: the wider the stop, the smaller the position.

Three steps to the right number

Step 1: Decide your risk amount

Decide the most you are willing to lose on this trade. A common rule of thumb is 1% to 2% of your total account. On a $5,000 account, 1% is $50.

The number has to be small enough that a normal losing streak cannot put your account out of action. A few losses in a row are an ordinary part of any sound method, not a sign that it is broken; win rate vs expectancy shows why.

Step 2: Take your stop distance from the market

Find the price that, if reached, proves your original idea wrong, and count the distance from your entry to that point in pips. This number comes from your analysis, not from your account balance. What a stop loss is and how wide it should be explains how to find it.

Step 3: Divide

In one line: lot size = risk amount ÷ (stop distance in pips × pip value per lot). With $50 of risk, a 25-pip stop, and a pair where one standard lot is worth $10 per pip, that is 50 ÷ (25 × 10), or 50 ÷ 250 = 0.2 lots.

Pip value differs from one instrument to the next, especially on gold and indices, and it also depends on your account currency. So rather than memorizing the formula, open the risk calculator every time: you give it the same three numbers and it gives you the size.

Three traps that break the math

  • Rounding up. If the math says 0.17 lots and you trade 0.2, you are taking about 18% more risk than you decided to. Always round down.
  • Forgetting costs. The spread and commission come out of that same risk amount. On tight stops their share can be large; the real cost of every trade shows it with numbers.
  • Raising your size after a loss. That is no longer a calculation, it is a reaction. We describe the pattern in revenge trading.
Size is the one part of a trade that is entirely in your hands. The market sets the price; you set the size.

Summary

A lot is the unit of trade size, but the work that matters is choosing the number: set your risk amount yourself, take your stop distance from the market, and let the division give you the size. Done in that order, your risk stays the same on every trade, even when your stops are very different.

To get started, read market basics, then log a few practice trades using these three steps in the trading journal. If there is a big gap between the risk you decided on and the risk you actually took, the problem is not your math, it is your execution.