What is CastroTools
CastroTools is a hub of free online calculators, converters, and tools covering finance, health, math, and everyday utilities. The concept is simple: you arrive with a calculation to make, enter your values, and see the result instantly—no registration, no installation, and no hassle.
Why it exists
Most online calculators are slow, cluttered with ads covering the results, or require registration for something that should be instant. CastroTools was created as the exact opposite: fast, lightweight tools that respect the time of anyone who just wants to solve a calculation.
How it works
Every tool runs entirely in your browser. The values you type are calculated locally and are neither sent to nor stored on any server. You can read more details in our privacy policy.
The site is supported by ads, allowing us to keep all tools free and without any registration requirements.
What you will find here
- Finance: compound interest, simple interest, and other calculators for daily money decisions
- Health: BMI and other wellness calculators
- Math: rule of three and quick calculation tools
- Utilities: practical generators and converters
We are constantly adding new tools. If there is a calculator you would like to see here, or if you found an error in any calculation, let us know via the contact page.