About Opal Calculator

Opal Calculator is a collection of free online calculators for everyday questions: how much soil a garden bed needs, what a percentage change really is, what it costs to form an LLC, or how many calories are in your coffee order.

What we believe

A calculator should give you an answer fast, then show you how it got there. Every tool on this site comes with the formula, a worked example and plain-language notes about common mistakes, so you can check the result yourself instead of trusting a black box.

How we build and review

Formulas are written as small, tested functions. Where a tool relies on reference data, such as state filing fees or nutrition values, we record where the number came from and when it was last checked. You can read the details in our editorial policy.

Free to use

All calculators are free and need no signup. The site is supported by advertising, which never changes how a result is calculated.

Get in touch

Found a mistake, or want a calculator we don't have yet? Visit the contact page. We read every message.