Quick answer
Power-to-weight = power ÷ mass. A 300 horsepower (mechanical) engine in a 3,200 lb vehicle gives 154.12 W/kg, or 6.49 kg per kW, or 10.67 lb per hp.
Enter the power and weight in whatever units you have. The calculator converts to watts and kilograms and reports the ratio three ways.
Related tools: Transmission Gear Ratio Calculator: Vehicle speed from engine rpm, gear ratio, final drive and tire size. Gear Ratio Calculator: Gear ratio, output speed and torque multiplication. Dimensional Weight Calculator: Dimensional (volumetric) weight using a carrier divisor you enter.
How to use the Power-to-Weight Ratio Calculator
- Enter power (above 0).
- Choose power unit: Horsepower (mechanical), Kilowatts and Watts.
- Enter weight (above 0).
- Choose weight unit: Pounds and Kilograms.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Power | A number | > 0 |
| Power unit | Choose one: Horsepower (mechanical); Kilowatts; Watts | Starts at Horsepower (mechanical) |
| Weight | A number | > 0 |
| Weight unit | Choose one: Pounds; Kilograms | Starts at Pounds |
Formula and method
Formula
ratio = power ÷ mass
IEEE-754 double precision; shown to the decimals indicated.
Mass in kilograms; 1 lb = 0.45359237 kg and 1 hp = 745.69987 W.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Power = 300; Power unit = Horsepower (mechanical); Weight = 3200; Weight unit = Pounds.
| Result | Value |
|---|---|
| Power-to-weight | 154.1238 W/kg |
| kg per kW | 6.4883 |
| lb per hp | 10.6667 |
Example 2
Inputs: Power = 150; Power unit = Kilowatts; Weight = 1400; Weight unit = Kilograms.
| Result | Value |
|---|---|
| Power-to-weight | 107.1429 W/kg |
| kg per kW | 9.3333 |
| lb per hp | 15.3439 |
Example 3
Inputs: Power = 200; Power unit = Watts; Weight = 1.5; Weight unit = Kilograms.
| Result | Value |
|---|---|
| Power-to-weight | 133.3333 W/kg |
| kg per kW | 7.5 |
| lb per hp | 12.3299 |
The first line is the headline result (power-to-weight). Below it the power-to-Weight Ratio Calculator lists kg per kW and lb per hp, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Three ways to express the same ratio
Watts per kilogram suits cyclists and engineers, kilograms per kilowatt is common in European car specifications, and pounds per horsepower is the US habit. The example shows all three from one input: 154.12 W/kg, 6.4883 kg/kW and 10.6667 lb/hp.
Which weight to use
Curb weight, kerb weight, and weight with driver all change the ratio. Compare vehicles using the same definition. A bigger number in W/kg is better; a bigger number in kg/kW or lb/hp is worse.
How the result changes with power
Holding every other input at the first worked example, this table changes only power. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Power | Power-to-weight | kg per kW | lb per hp |
|---|---|---|---|
| 150 | 77.0619 W/kg | 12.9766 | 21.3333 |
| 300 | 154.1238 W/kg | 6.4883 | 10.6667 |
| 600 | 308.2475 W/kg | 3.2441 | 5.3333 |
| 1200 | 616.4951 W/kg | 1.6221 | 2.6667 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Power: entering "0" shows "Power must be greater than 0." beside that field and no result.
- Weight: entering "0" shows "Weight must be greater than 0." beside that field and no result.
Choosing between the Power-to-Weight Ratio Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Transmission Gear Ratio Calculator](/engineering/transmission-gear-ratio-calculator/) | transmission gear ratio, final drive ratio and tire diameter | Vehicle speed |
| [Gear Ratio Calculator](/engineering/gear-ratio-calculator/) | driving gear teeth and driven gear teeth | Gear ratio |
| [Dimensional Weight Calculator](/engineering/dimensional-weight/) | length, width and height | Dimensional weight |
| [Weight Converter](/converters/weight-converter/) | value, from and to | Pound (lb) → Gram (g) |
| [Power Converter](/converters/power-converter/) | value, from and to | Horsepower (mechanical) → Watt (W) |
Assumptions and limitations
- Use curb or test weight consistently
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for power and weight to confirm this.
Frequently asked questions
What is a good power-to-weight ratio for a car?
Typical family cars are below about 100 W/kg, while sports cars exceed 200.
Is horsepower mechanical or metric?
This page offers mechanical horsepower (745.7 W); metric horsepower is about 735.5 W.
Does it consider drag or gearing?
No. It is a simple ratio.
What does the Power-to-Weight Ratio Calculator calculate?
Power per unit weight of a vehicle. It returns Power-to-weight, kg per kW and lb per hp.
What formula does the Power-to-Weight Ratio Calculator use?
ratio = power ÷ mass. Mass in kilograms; 1 lb = 0.45359237 kg and 1 hp = 745.69987 W.
What do I need to enter in the power-to-Weight Ratio Calculator?
You enter Power, Power unit, Weight and Weight unit.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Use curb or test weight consistently.
What does the “Power unit” option change?
It sets how the power-to-Weight Ratio is worked out. The choices are Horsepower (mechanical), Kilowatts and Watts.
What is an example of a power-to-Weight Ratio result?
With power 300, power unit Horsepower (mechanical), weight 3200 and weight unit Pounds, the power-to-weight is 154.1238 W/kg.
Where does the method come from?
The method follows: NIST SP 811 conversions.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Use curb or test weight consistently.
Related calculators
- Transmission Gear Ratio Calculator — Vehicle speed from engine rpm, gear ratio, final drive and tire size.
- Gear Ratio Calculator — Gear ratio, output speed and torque multiplication.
- Dimensional Weight Calculator — Dimensional (volumetric) weight using a carrier divisor you enter.
- Weight Converter — Convert kilograms, pounds, ounces, stone and more.
- Power Converter — Convert watts, kilowatts, horsepower and BTU per hour.
- Three-Phase Power Calculator — Real, apparent and reactive power of a balanced three-phase load.
Sources
- NIST SP 811 conversions
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.