How to use it
- Type the Engine power and choose its unit in Power unit: kW, PS or hp, as the car's papers give it.
- Enter the Mass of the car in kilograms, with a driver and a full tank.
- Set the Target speed in km/h, for example 100.
- Read Time from a standstill and the table Time to each speed, then read the note on what the model leaves out.
How the time is estimated
With the power held constant, the work done grows in a straight line with time. The work equals the change in kinetic energy, one half of the mass times the speed squared, so power × time = ½ × mass × speed².
Solving for the time gives the estimate. The speed must be in metres per second: 100 km/h is 100 ÷ 3.6 = 27.78 m/s, and the power must be in watts. Time grows with the square of the speed, so 130 km/h takes about 1.69 times as long as 100 km/h.
Time (s) = mass (kg) × speed (m/s)² ÷ (2 × power (W))
Speed (m/s) = speed (km/h) ÷ 3.6Units of power
- One PS (metric horsepower) is 735.4988 W, and one hp (mechanical horsepower) is 745.6999 W (NIST SP 811, Appendix B.8).
- So 100 PS is about 98.6 hp, and 100 PS is about 73.5 kW.
- Use the figure from the car's papers or the maker, and check which unit it is given in.
What this calculator does not include
- Drag and rolling resistance, which the model ignores.
- Gearing, the grip of the tyres at the start and the losses in the drivetrain. Each one can only add time, so the figure is a best case.
- The difference between engine power and power at the wheels, and the driver's reaction. The result is a model, not a test figure.
Worked example: 100 kW, 1,500 kg, 100 km/h
The speed is 100 ÷ 3.6 = 27.78 m/s. The time is 1,500 × 27.78² ÷ (2 × 100,000) = 5.79 s.
The same car reaches 50 km/h in 1.45 s and 130 km/h in 9.78 s, because the time grows with the square of the speed.
| Speed (km/h) | Time (s) |
|---|---|
| 50 | 1.45 |
| 100 | 5.79 |
| 130 | 9.78 |
Frequently asked questions
How long does it take a car to reach 100 km/h?
It depends on the power, the mass and the gearing. This model gives 5.79 s for 100 kW and 1,500 kg; a real car takes longer, because the model leaves out the things that add time.
Why is this estimate shorter than a real test result?
The model assumes that all the power goes into speed from the first moment. Drag, rolling resistance, drivetrain losses, gearing and the grip of the tyres at the start all add time, and the model leaves them out.
What is the difference between PS and hp?
Both are horsepower units. One PS is 735.4988 W and one hp is 745.6999 W (NIST SP 811), so 100 PS is about 98.6 hp.
Can I use this for a speed far above 100 km/h?
The result is least reliable at high speed, because the model leaves out drag, which grows quickly with speed. Use it as a rough guide for the time to 100 km/h and the speeds near it.
Does the calculator keep the numbers I type?
The page works in your browser and sends nothing to a server. The numbers stay in the page address, so a copied link shows the same figures.
Sources
- NIST SP 811, Appendix B.8: conversion factors (horsepower, kilometre per hour, metre per second)
- OpenStax, College Physics 2e, section 7.2: kinetic energy and the work-energy theorem
- OpenStax, Physics, section 9.1: work, power and the work-energy theorem
Figures and text last checked on Thursday, November 12, 2026.