FPS to Joules: Formula, Table and Limit by BB Weight

A chronograph measures speed in FPS (feet per second), while fields and the law talk in Joules. Going from one to the other takes a single extra piece of information: the weight of the BB used during the measurement. Without it, an FPS figure says nothing about the real energy of the replica.

The FPS to Joules formula

A BB's kinetic energy is E = 0.5 × m × v², where m is the mass in kilograms and v the speed in metres per second. Since the chrono shows FPS, convert first: v (m/s) = FPS × 0.3048. For a 0.20 g BB (0.0002 kg) at 300 FPS, v = 91.44 m/s, so E = 0.5 × 0.0002 × 91.44² = 0.84 J.

Conversion table with a 0.20 g BB

• 280 FPS = 0.73 J
• 300 FPS = 0.84 J
• 328 FPS = 1.00 J
• 350 FPS = 1.14 J
• 360 FPS = 1.20 J
• 400 FPS = 1.49 J
• 402 FPS = 1.50 J
• 464 FPS = 2.00 J

Why BB weight changes everything

At equal energy, a heavier BB leaves slower. A replica set to 1 J therefore shows fewer FPS with a heavy BB than with a light one. Conversely, a replica that passes the chrono at 328 FPS with a 0.20 g BB goes over 1 J as soon as you test it with a heavier BB, if the energy stayed the same: that is the principle behind joule creep.

FPS equal to 1 Joule by BB weight

• 0.20 g BB: 328 FPS
• 0.25 g BB: 293 FPS
• 0.28 g BB: 277 FPS
• 0.30 g BB: 268 FPS
• 0.36 g BB: 245 FPS

Which limit to apply in practice

French law caps freely sold replicas at 2 Joules, but most fields set a lower limit and specify the weight of the control BB. The only reference that matters before a game is your field's: ask for it, then check your replica on the chrono with the same BB as the one used at the gate.

Enter your own reading and your BB weight to get the exact result. Use the FPS to Joules converter