How to Convert Units in Physics Problems

Short answer: convert every given value to SI units — metres, kilograms, seconds, kelvin, pascals — before you put it into a formula. Most physics formulas only give a correct answer in SI units, and the most common lost marks come from skipping this step.

The conversions you will use most

Speed: km/h → m/s, divide by 3.6 (72 km/h = 20 m/s). mph → m/s, multiply by 0.447.
Mass: g → kg, divide by 1000.
Length: cm → m, divide by 100; mm → m, divide by 1000.
Time: min → s, multiply by 60; h → s, multiply by 3600.
Temperature: °C → K, add 273.15. °F → K, (°F − 32) × 5/9 + 273.15.
Pressure: 1 atm = 101 325 Pa; 1 kPa = 1000 Pa.
Energy: 1 kWh = 3.6 × 10⁶ J; 1 eV = 1.602 × 10⁻¹⁹ J.

A habit that prevents mistakes

Write each given value on its own line as "symbol = value with unit = value in SI". For example: v₀ = 36 km/h = 10 m/s. It takes a few seconds and makes every later step easier to check.

Squared and cubed units

Area and volume conversions use the square or cube of the factor: 1 cm² = 10⁻⁴ m², 1 L = 10⁻³ m³. Converting 500 cm² by dividing by 100 is a common mistake.

Doing it on your phone

In Newtora each value has a unit picker. Enter 36 km/h or 20 °C as given; the solution shows a "Convert to SI units" step listing every conversion, and you can choose the unit of the answer. The Tools tab also has a stand-alone unit converter.

Frequently asked questions

Why divide km/h by 3.6?

1 km/h is 1000 m per 3600 s, which is 1/3.6 m/s. So 72 km/h ÷ 3.6 = 20 m/s.

Do I always have to use kelvin?

For gas laws and anything with absolute temperature, yes. For a temperature change ΔT, a change of 1 °C equals a change of 1 K, so you can use either.

Should I convert the answer back?

Solve in SI units, then convert the final answer to the unit the question asks for.

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