About

Keisan Lab is a collection of free online calculators for everyday questions about money, dates and numbers: how much a loan really costs, how many days are left until a deadline, what a percentage change means. Every calculator shows the formula it uses and walks through worked examples, so you can see where the answer comes from.

How each page is checked

Every calculator and guide goes through the same steps before it is published:

1. Primary sources: formulas and rules are checked against government agencies, regulators and other authoritative references, which are listed at the bottom of each page.
2. Automated tests: each calculation has tests covering published examples, edge cases and invalid inputs.
3. Automated checks: broken links, missing information and exaggerated claims are flagged automatically.
4. Pre-publication review: a separate review step checks the accuracy of the math, the clarity of the explanation and the wording. Only pages that meet the standard are published.

If you find a mistake, please let us know and we will correct it promptly.

Your inputs stay in your browser

All calculations run in your browser. The numbers you enter are not sent to or stored on our servers. If you use "Copy link with these inputs", your inputs are included in the copied URL.

Who runs this site

Keisan Lab is run independently by an individual. Contact: contact form.