Sumfern - Showcase Project
Listed October 20, 2026
About Sumfern
Free calculators that show the formula and a worked example you can check
A calculator that doesn't show its formula asks you to trust it. Sumfern doesn't ask. Every page shows the formula, a worked example with round numbers you can verify by hand, and the publication behind any official rate: IRS and Department of Labor for US pay and tax rules, the Canada Revenue Agency for GST and HST, and Ontario's Employment Standards guides for Ontario pay rules. Where a rate changes every year, the page names the figure and the year. It runs in your browser, so the numbers you type stay on your device. There is no account. If we get one wrong, the fix goes on a public corrections page. Sumfern is independent: not a bank, lender, payroll company or tax adviser, and it takes no commission for sending you anywhere. Feedback I would like: which calculator should I build next?
Project details
Website: https://sumfern.com
Project metrics
Average Monthly Visitors: Pre-launch