Every figure on this site came out of one of these four sums. They are here in the same form I use them: type your numbers, read the answer, check the arithmetic below it against a real test result. Nothing you type leaves the page.
Answers in lots, in money, and in the fraction of the account at risk — and says so when the answer is smaller than the smallest trade your broker will accept, which on a small account is the usual answer.
Win rate, average win and average loss in, one number out. On this site that number was negative for 149 of 190 EAs, and 145 of those 149 lost money.
The gain needed to recover, and how many months of your current return that is. The curve is not symmetric, which is why a 50% loss needs a 100% gain.
Exact probability of a run of 5 and of 10 inside N trades, plus the longest run you should expect. Computed, not simulated.
Four tools, four different problems. Picking the wrong one is how people end up sizing a position with the question “is this EA any good” still open.
Why a calculator and not another article. Everything else on this site is a judgement, and judgements can be wrong. These four are identity: given the inputs, the output is not a matter of opinion. If you find one that disagrees with your broker’s own calculator, I want to know — write to me and I will fix it or say why it is right.
Fixed fractional risk: each trade risks the same percentage of current equity, not of the deposit. That is the only sizing rule that survives a real losing run, and it is the rule every number on this site is quoted against. Fees, commission, swap and slippage are not in the arithmetic — they are in the notes on each page, because on a small account the spread is often the largest single term.
One more thing worth saying here rather than on every page: a calculator tells you the size that fits your risk. It cannot tell you whether the trade is worth taking. That is the rest of this site.