A scientific calculator that evaluates a whole expression at once instead of one keystroke at a time. Type directly into the box or tap the buttons: it understands +, −, × (*), ÷ (/), parentheses, decimals, percentages, the power operator ^, square roots and the constants π and e, plus the functions sin, cos, tan, log (base 10) and ln (natural log). Multiplication and division bind tighter than addition and subtraction, and the ^ operator binds tightest of all and is right-associative, so 2^3^2 is 512 and −3² follows standard math rules.
The engine is a hand-written tokenizer and recursive-descent parser — it never calls JavaScript's eval, so pasting an arbitrary string cannot run code; anything it does not recognise simply shows "Error". Trigonometric functions take radians. A percent sign divides the preceding value by 100, so 200×15% gives 30. The result updates live as you type and the whole thing runs offline in your browser.