reckoner

A plain-text notepad where math answers itself. You type notes and numbers together; any line that parses as math gets a live answer on a dotted "answer rail" to the right, and anything else is just prose. It handles the arithmetic of actual daily life: dinner + 18%, 6 ft 2 in in cm, 9:30am + 3h 45m, $18 / 2.5 h, 62 mph * 3.5 h, sum/avg/total, etc.