OtterCalc

Soulver is fantastic: a notepad where you rough out a problem in plain words and the answers appear beside every line. OtterCalc is my take on that idea for the browser. It’s the back of an envelope, with the arithmetic handled for you.

You write line by line (flights = $1,240, total to EUR, 20% of total) and every result updates instantly as you edit. It handles variables, unit conversions, date and time math, live currency rates from the European Central Bank, and you can drop pie, bar, and line charts straight into your notes.

It’s 100% web based and there’s nothing to install. Drafts stay in your browser until you save, and saving a notebook gives you a link that reproduces the whole thing for anyone you send it to. No accounts, no signup.

Try it at ottercalc.com.