Power To Weight Ratio Calculator

How It Works

Power to weight is simply power divided by mass. The calculator first converts your power figure to watts (1 hp = 745.6999 W, 1 PS = 735.49875 W, 1 kW = 1,000 W) and your weight to both pounds and kilograms (1 lb = 0.45359237 kg), then reports the ratio in the five ways people quote it: hp per lb, hp per kg, lb per hp, kW per tonne and W per kg. The last two are the same number, since a tonne is 1,000 kg. All conversion factors come from NIST Special Publication 811, Appendix B.9.

“hp” here means mechanical horsepower (550 ft·lbf/s), which is what US engine builders and sanctioning bodies use. PS is metric horsepower, about 1.4% smaller, and is what European engine sheets often quote. Pick the unit that matches the figure you have, and the tool converts the others for you.

Weight is where the karting number differs from the car number. Kart rulebooks set a minimum weight for kart plus driver in racing gear, so a class minimum is already the total. If you only know your kart’s weight alone, enter it and put your own weight in the driver field.

The reference table shows what published engine power and class minimums work out to. Only figures published by the engine manufacturer and the sanctioning body are used, so a Briggs 206, whose class weights are left to each sanctioning body, is not listed.

Worked Example

An IAME X30 is rated at 30 hp by IAME, and the SKUSA 2026 Rulebook sets the Pro X30 minimum at 360 lb with driver. 30 ÷ 360 = 0.0833 hp per lb, or 12.0 lb per hp. In metric, 30 hp × 745.6999 = 22,371 W and 360 lb × 0.45359237 = 163.3 kg, so 22,371 ÷ 163.3 = 137.0 W/kg, which is the same as 137.0 kW per tonne.

FAQ

Should I include the driver?

Yes if you want the number that decides lap time. The engine moves the kart and the driver together. Class minimums already include the driver, so enter those as the weight and leave the driver field at 0; otherwise add your weight in racing gear.

Why do a kart and a sports car land so close together?

The kart has far less power but also far less mass. An X30 at the SKUSA minimum comes out at 137 W/kg; a 2026 MX-5 Miata at Mazda’s 181 hp and 2,366 lb curb weight comes out at 126 W/kg, and that is before a driver climbs into the car.

Is hp the same as PS or bhp?

Not quite. 1 PS is 735.49875 W and 1 mechanical hp is 745.6999 W, so 100 PS is 98.6 hp. “bhp” usually means brake horsepower measured at the crank and is the same unit as hp; the tool treats hp and bhp as the same.

Why is my number lower than the table?

Engine ratings are peak figures on a fresh engine in the manufacturer’s test. Restricted junior classes, worn top ends, altitude and carburetion all take power away, and a heavy kart carries ballast only to reach the minimum, never below it. Use the table as a ceiling for the class, not a target.

Where do the reference weights come from?

Every row links to the rulebook or product page it was taken from: SKUSA 2026 Rulebook national class table for X30 and KA100, the Global Rotax MAX Challenge Sporting Regulation 2026 for Senior MAX, and Mazda USA for the MX-5. Class weights change year to year, so check your own series’ current rulebook.

Sources

• NIST SP 811, Appendix B.9 - horsepower (550 ft·lbf/s) = 745.6999 W, horsepower (metric) = 735.4988 W, pound = 0.45359237 kg
• IAME X30 Senior - max power 30 hp, 123.67 cc
• IAME Reedjet KA100 - max power 22 hp, 98.53 cc
• Rotax 125 Senior MAX Evo - 22 kW / 30 hp at 11,500 rpm
• SKUSA 2026 Rulebook - national class minimum weights: Pro X30 360 lb, KA100 Senior 355 lb, KA100 Master 390 lb (p. 16)
• Global Rotax MAX Challenge Sporting Regulation 2026 - 125 Senior MAX minimum weight 162 kg (p. 9)
• Mazda USA: 2026 MX-5 Miata - 181 hp, curb weight 2,366 lb (manual)