Physics Unit Converter
Convert between physics units across five categories: energy (Joules, kWh, eV), force (Newtons, lbf), power (Watts, horsepower), pressure (Pascal, bar, PSI), and velocity (m/s, km/h, mph). No signup, runs entirely in your browser.
⏱ 6 min read · Complete guide below
How to Use the Physics Unit Converter
- 1Pick a category: energy, force, power, pressure, or velocity.
- 2Select the from and to units — e.g. kilowatt-hours to Joules, or PSI to bar.
- 3Enter the value; the conversion updates instantly. E-notation is fine for very large or small quantities.
Worked Example: Making Sense of an EV Spec Sheet
An electric car brochure lists a 60 kWh battery, a 150 kW motor, and recommends 36 psi tire pressure — three different unit systems on one page. Converting the battery: 60 kWh × 3.6 MJ/kWh = 2.16 × 10⁸ J of stored energy. The motor: 150 kW ÷ 0.7457 ≈ 201 hp, which is the number a petrol-car buyer can compare against. The tires: 36 psi × 6.895 ≈ 248 kPa, or about 2.5 bar — the unit European pressure gauges display.
A useful sanity check ties energy and power together: at highway cruise the car might draw 20 kW, so the 60 kWh pack lasts 60 / 20 = 3 hours. Units are the grammar of physics — most calculation errors are unit errors, and converting everything to SI base units (Joules, Watts, Pascals, m/s) before working a problem is the single most effective way to avoid them.
Energy vs Power — the Most Confused Pair
The single most common mix-up in physics units is treating energy and power as the same thing. They are not. Energy (measured in joules) is a quantity — the total amount of work done or stored. Power (measured in watts) is a rate — how fast energy is delivered or used. A watt is defined as one joule per second, so the two are linked by time: energy = power × time. This is exactly why your electricity bill is in kilowatt-hours (a unit of energy) rather than kilowatts: a 2 kW heater running for 3 hours consumes 6 kWh of energy. Keeping the distinction straight — a quantity versus a rate — resolves a huge share of everyday confusion around batteries, engines, and appliances.
Why SI Base Units Are Worth the Effort
Physics has many units for each quantity — energy alone can be written in joules, calories, kilowatt-hours, electronvolts, or BTU — because different fields grew up using whatever was convenient. That convenience comes at a cost: mixing units mid-calculation is the leading cause of errors, and famously caused the loss of a Mars spacecraft when two teams used metric and imperial figures. The reliable habit is to convert every quantity to its SI base unit — joules, newtons, watts, pascals, metres per second — before you calculate, do the arithmetic, and only convert back to a friendly unit at the very end. This converter exists to make that first and last step painless.
Metric, Imperial, and Everyday Units
Much of the reason this converter is useful in daily life is the uneasy coexistence of measurement systems. Tyre pressure is quoted in psi in the US and bar or kPa in Europe; engine power appears as horsepower on a petrol car and kilowatts on an electric one; speeds switch between mph and km/h at borders. None of these are “wrong,” they are just different conventions for the same physical quantity. Rather than memorising dozens of conversion factors, you can pick the category, choose the two units, and read the answer — whether you are decoding a spec sheet, solving a physics problem, or comparing figures across regions.
Physics Unit Tips
Energy in everyday life
A typical home uses ~900 kWh/month. One kWh = 3,600,000 J. A cup of food contains ~200 kcal = ~837,000 J of chemical energy — but our bodies convert it at ~20–25% efficiency.
Pressure at altitude
Atmospheric pressure is 1 atm = 101,325 Pa at sea level. It drops ~1.2% per 100 m gain. At cruising altitude (10,000 m), cabin pressure is ~75 kPa or about 0.75 atm.
Force in engineering
Structural engineers often work in kN or MN. A typical car weighs ~15 kN (about 1500 kg × 9.81). Bridge loads are calculated in MN and kN/m distributed loads.
Speed of sound in m/s
343 m/s = 1235 km/h = 767 mph. Supersonic aircraft exceed Mach 1 (343 m/s). The Concorde cruised at Mach 2 ≈ 686 m/s ≈ 2470 km/h.
Frequently Asked Questions
What units of energy does this converter support?
Joules (J), kilojoules (kJ), calories (cal), kilocalories (kcal), watt-hours (Wh), kilowatt-hours (kWh), electronvolts (eV), BTU, and ergs. The SI base unit is the Joule.
What is the difference between a calorie and a kilocalorie?
One kilocalorie (kcal) = 1000 calories (cal). Food calories are actually kilocalories — a "200-calorie" snack contains 200 kcal or 200,000 cal. One kcal = 4184 J.
How many Watts is one horsepower?
One mechanical horsepower = 745.7 Watts. So a 100 hp engine produces about 74.57 kW. Electric motors are often rated in kW while combustion engines use hp — this converter bridges the two.
What pressure units are supported?
Pascals (Pa), kilopascals (kPa), megapascals (MPa), bar, atmospheres (atm), PSI (pounds per square inch), and millimeters of mercury (mmHg/Torr). 1 atm = 101,325 Pa = 14.696 PSI.
How do I convert km/h to m/s?
Divide by 3.6: 100 km/h = 100/3.6 ≈ 27.78 m/s. The converter does this automatically. 1 m/s = 3.6 km/h.
Is my data stored?
No. All calculations run locally in your browser. No data is sent to any server.
What is the difference between energy and power?
Energy is a total amount of work (measured in joules), while power is the rate at which energy is used or delivered (measured in watts, where one watt equals one joule per second). They are related by time: energy = power × time. This is why electricity is billed in kilowatt-hours, a unit of energy — a 2 kW device running for 3 hours uses 6 kWh. Confusing the two is one of the most common unit mistakes.
Why are there so many different units for the same quantity?
Different fields and regions adopted whatever was convenient at the time. Energy, for example, is measured in joules by physicists, calories by nutritionists, kilowatt-hours by utilities, and electronvolts by particle physicists — all describing the same physical quantity. This converter lets you move between them so figures from different sources can be compared directly, without memorising the conversion factors.
How do I avoid unit errors in a calculation?
The most reliable method is to convert every quantity to its SI base unit — joules, newtons, watts, pascals, metres per second — before doing any arithmetic, carry out the calculation, then convert the final answer back to whatever unit you need. Mixing units mid-calculation is the single biggest source of errors in physics and engineering, and doing all the maths in consistent SI units eliminates it.
Does the converter handle both metric and imperial units?
Yes. Within each category it includes common metric and imperial (and other everyday) units — for example psi alongside bar and pascals for pressure, horsepower alongside kilowatts for power, and mph alongside km/h and m/s for velocity. That makes it well suited to decoding spec sheets, comparing figures across regions, or checking homework that mixes systems.