FitAge – functional age from 8 physical tests (open source)

FitAge is an open-source calculator that estimates your "functional age"—how well your body functions relative to a typical healthy person of your sex—based on results from 8 simple physical tests (grip strength, sit-to-stand, walking speed, etc.). It was created by Dr. Peter Attia, a prominent longevity-focused physician and author of *Outlive*, whose work has popularized the idea that chronological age is a poor proxy for health, and that measuring and improving functional capacity is key to extending "healthspan" (years of healthy life). The project is part of The Human Runtime (thehumanruntime.com), Attia's broader platform. The calculator is open source, meaning anyone can inspect, modify, or run the code themselves. This matters because mainstream medicine rarely offers such composite, actionable physical-age estimates, and Attia's audience (tech-minded, health-optimizing, often affluent) treats this as a practical tool rather than a clinical diagnostic.