A Clock (Modular) Arithmetic Calculator

Here is a calculator for doing arithmetic (adding, subtracting, and multiplying) whole numbers on a clock with a given number of hours. If this doesn't make any sense to you, here's an explanation of what clock arithmetic is, with an interactive clock that shows the different names for a number on a clock in another way. Follow this link for another calculator for renaming numbers on a clock.

If you don't have a java-enabled browser, you won't be able to see this applet.


  1. Choose the operation you want (+, -, or x) by holding down the button with the pull-down menu, and releasing on the one you want.
  2. Type the first number you want to calculate with in the first box.
  3. Type the second number you want to calculate with in the second box.
  4. Type the number of hours on the clock you want to use in the third box (after "mod").
  5. Click the "=" button to get the smallest nonnegative "name" for the result of the operation on the clock.

Problems? Got an "E"?

Technical Comments

Original Web Site by Susan Addington