InstaCalc is described as 'An online calculator that allows the free form mixing of numbers and words, and saves calculations automatically' and is a Calculator in the education & reference category. There are more than 10 alternatives to InstaCalc, not only websites but also apps for a variety of platforms, including Mac, Windows, Android and iPhone. The best alternative is Numi, which is free. Other great sites and apps similar to InstaCalc are OpalCalc, Soulver, NoteCalc and Caligator.
InstaCalc is mainly a Calculator but alternatives to it may also be Note-taking Tools. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of InstaCalc.
OpalCalc is a brand new type of calculator for the PC. As easy to use as Notepad or a word processor, OpalCalc allows natural language in the sum and multi-line support so you can go back and adjust your previous calculations on the fly.
Soulver is a smart notepad with a built-in calculator. It gives instant answers to any calculations it finds in your text. It's a better way to work stuff out than with a traditional calculator, and a more lightweight tool for quick calculations than a spreadsheet.