About Pebble Calculator

Pebble Calculator is a free library of calculators for the questions people actually have in the middle of a project: how much concrete to order, how much salt a pool needs, what size septic tank a house requires, how big an air conditioner should be.

Why "pebble"?

The word calculate comes from the Latin calculus, meaning a small pebble. Long before calculators, people counted and did arithmetic by moving small stones. We liked the idea of a tool that is just as simple and dependable.

What we promise

• The formula is always shown. You should be able to check any answer by hand.
• Numbers come from real references. Manufacturer data, building and health codes, government agencies and university extension services, all cited on the page.
• Every calculator is tested against worked examples before it is published. See how we test.
• No sign-ups, no paywalls. The calculators are free and work in US or metric units.

Get in touch

Found a mistake, or want a calculator we don't have yet? Contact us. Corrections are reviewed quickly and noted on the page.