Pick a symbol, enter your stop loss in pips and your account size. The calculator returns the lot size that risks the percentage you choose.
Lot size is rounded down to 0.01 so the real risk never goes above what you chose. One pip is 0.0001 (0.01 for yen pairs; 0.10 for gold). Gold assumes a 100 oz contract. Check your broker's contract specification, because some brokers define pips differently.