Percentage Lab is a free percentage calculator site organized into multiple categories. Core tools cover increases, decreases, changes, differences, percent error, and conversions between percentages, fractions, decimals, and ratios. Finance tools cover VAT, discounts, margins, markups, and salary changes. Education tools help with grades, marks, and attendance. Every calculator includes the formula, a clear explanation, worked examples, and FAQs. Free, mobile friendly, no login needed.