MonsiTools

What it does

MonsiTools is a calculator builder. You type a formula in plain words, for example price - product cost - shipping, and each word becomes an input field. The answer updates as you type, and you can save the calculator and share it.

It also comes with ready-made calculators for ecommerce fees, margins, VAT, break-even, SaaS metrics, loans and payroll. Calculators can be chained into workflows, so one result feeds the next step: monthly revenue into runway into a rough valuation, or landed cost into price into profit per order.

How it's built

A Python static site generator builds every calculator page from a YAML file (formula, inputs, labels) plus written content.
Formulas are evaluated in the browser by a small parser, without eval.
A Cloudflare Worker serves the site, handles accounts and usage limits, and exposes a JSON API.
A remote MCP server at monsitools.com/mcp lets AI assistants search calculators, read their exact formulas and run them, so they return a deterministic number instead of doing the arithmetic themselves.