25Sport is a free library of performance calculators built specifically for endurance and amateur athletes — runners, cyclists, swimmers, and triathletes. Every tool on this site is grounded in established sports science: Coggan training zones, the Riegel race-time formula, Daniels running economy, Foster's Session-RPE model, and more.
The project started from a simple frustration: most sports calculators online are either buried inside apps that require a subscription, filled with ads that break the experience, or built on formulas that aren't clearly explained. We wanted something different — fast, clean, and honest about the math behind the result.
We focus on calculators that give athletes a concrete, actionable number rather than a generic recommendation. That means:
25Sport is built and maintained by Javier Revuelta, a software engineer and amateur triathlete based in Spain. The “25” is a nod to the 25-metre pool lane where the idea first took shape during an early morning swim session.
This is a one-person project, built in spare hours between training blocks. If you find a bug, have an idea for a new calculator, or just want to say hi, the contact page is always open.
Questions or suggestions? Get in touch.