How the Forex Lot Size Calculator works
Professional traders size every position so that a stop-loss costs only a small, fixed part of the account — usually 0.5–2%. Lots = (balance × risk %) ÷ (stop-loss in pips × pip value per lot).
Round down to the lot step your broker allows (0.01 = micro lot). A smaller lot with a sensible stop is always safer than a big lot with a tight stop.