About HandyWren

HandyWren is a free set of calculators for small business owners and marketers. 3 tools are live today: Margin Calculator, Markup Calculator and Profit Margin Calculator (P&L). More are planned, including ad math such as CPM, CPC and ROAS. You don't need an account, and there is nothing to install.

How the tools are built

• Each tool starts as a written spec with its formulas, the inputs it accepts and how it rounds.
• Each spec comes with worked examples whose answers are known in advance. Automated tests run the calculator on every example and fail if any answer is off by a cent.
• The math uses exact decimals instead of floating point, so you won't see answers like 0.30000000000000004. Each result is rounded once, to 2 decimals, half away from zero.
• The calculations run in your browser, and the numbers you type are not sent anywhere. The privacy policy has the details.

Every tool page shows its formulas and a worked example, so you can check a result by hand.

Who runs it

HandyWren is built and run by Thuong Le, a marketing technologist in Hanoi, Vietnam, who has worked in digital marketing since 2010.

Contact

If a result looks wrong or you want a tool that isn't here yet, email hello@handywren.com. The contact page says what to include.