A tool for finding error fractions of multiplier approximations available here.

To do: A tool for helping with a method I made up that lets you do mental arithmetic visually using a grid of factors. This method might be easier and faster, or it might just make things harder. Either way its pretty fun.

Query param example	Effect
simple	Remove the controls and coverage display
blind	Don't display the grid numbers
grid	Display the grid lines by default
axes=11,6,5	Provide a default to the axis lengths
hidden=2,2+-1,3	Display "???" over the numbers at the given co-ordinates. This is for practising using the system.