Physics Unit Converter — Convert SI and Imperial Physics Units

Physics Quantities and Their Units

How to Use the Physics Unit Converter

1. Open the physics unit converter.
2. Select the physical quantity you are converting (force, energy, pressure, etc.).
3. Enter the numerical value and select the from unit.
4. Select the to unit. The converted value appears instantly.
5. For temperature conversions, note that these are not simple multiplicative factors — the converter handles the offset correctly (°C to K, °C to °F).

Physical Intuition for Units

The SI System

The International System of Units (SI) is the modern form of the metric system, agreed internationally in 1960. It is built on seven base units:

• Metre (m) — length
• Kilogram (kg) — mass
• Second (s) — time
• Ampere (A) — electric current
• Kelvin (K) — thermodynamic temperature
• Mole (mol) — amount of substance
• Candela (cd) — luminous intensity

All other SI units are derived from these seven. A newton (force) is kg·m·s⁻², a joule (energy) is kg·m²·s⁻², a pascal (pressure) is kg·m⁻¹·s⁻². Understanding these derivations helps you verify dimensional consistency in calculations.

Force Units in Depth

Newton (N)

The newton is the SI unit of force, defined as the force that accelerates a 1 kg mass by 1 m/s². From Newton's second law: F = ma, so 1 N = 1 kg × 1 m/s². The weight of a 1 kg mass on Earth is approximately 9.81 N (g ≈ 9.81 m/s²).

Pound-force (lbf)

The pound-force is the weight of a 1-pound mass under standard gravity. 1 lbf = 4.448 N. Note the distinction between pound-mass (lbm, a unit of mass) and pound-force (lbf, a unit of force) — this distinction is frequently confused in engineering contexts.

Kilogram-force (kgf)

The kilogram-force is the weight of a 1 kg mass under standard gravity. 1 kgf = 9.80665 N. Despite being deprecated in SI, kgf is still used in many engineering contexts, particularly in older standards and in countries that transitioned late to metric.

Energy Units in Depth

Joule (J)

The joule is the SI unit of energy, equivalent to the work done by a force of 1 N over a displacement of 1 m. All forms of energy — kinetic, potential, heat, electrical, chemical — can be expressed in joules.

Calorie (cal) and kilocalorie (kcal)

1 calorie = 4.184 J (the energy needed to raise 1 g of water by 1°C). The food calorie (Cal, or kcal) = 1,000 calories = 4,184 J. Food packaging labelling uses kcal even when it says "Calories." A typical adult diet of 2,000 kcal/day = 8.37 MJ/day.

Electron-volt (eV)

1 eV = 1.602 × 10⁻¹⁹ J — the energy gained by an electron moving through 1 volt of electric potential. Used in atomic physics, particle physics, and semiconductor engineering where energies are tiny fractions of a joule.

BTU (British Thermal Unit)

1 BTU = 1,055 J. Used in the US for heating/cooling systems (HVAC), boiler capacity, and air conditioning ratings. A 10,000 BTU air conditioner = 2,930 W cooling capacity.

Pressure Units in Depth

Why so many pressure units?

Pressure measurement units proliferated because different industries developed their own standards: medical (mmHg for blood pressure), meteorological (millibar = hPa), tyres (psi in the US, bar in Europe), and industrial/scientific (Pa, kPa, MPa, atm). Converting between them is a constant practical need.

Gauge pressure vs. absolute pressure

Absolute pressure is measured from zero (perfect vacuum). Gauge pressure is measured from atmospheric pressure as the reference point. Tyre pressures are given as gauge pressure — a tyre at "35 psi" has 35 psi above atmospheric, or about 49.7 psi absolute. When using pressure in physics equations, always use absolute pressure. Remember to add atmospheric pressure (14.696 psi or 101,325 Pa) when converting from gauge readings.

Temperature Conversion

Temperature conversions involve a zero-point shift, not just scaling:

• Celsius to Kelvin: K = °C + 273.15
• Celsius to Fahrenheit: °F = (°C × 9/5) + 32
• Fahrenheit to Celsius: °C = (°F − 32) × 5/9
• Kelvin to Celsius: °C = K − 273.15

Absolute zero (0 K = −273.15°C) is the theoretical temperature at which all thermal motion ceases. All thermodynamic calculations (Carnot efficiency, gas laws) must use Kelvin, not Celsius.

Power Units

Watt (W)

1 watt = 1 joule per second. Power is the rate of energy conversion. Electrical devices are rated in watts; larger systems use kilowatts (kW) and megawatts (MW). Power stations are rated in gigawatts (GW).

Horsepower (hp)

1 mechanical horsepower = 745.7 W. Defined by James Watt as the power a horse could sustain over a full working day, used to market steam engines by comparison to the horse-power they replaced. Still used for vehicle engine ratings (particularly in the US) and for motor ratings.

Dimensional Analysis

Dimensional analysis tracks the dimensions (mass, length, time, etc.) of each quantity to check the correctness of equations and unit conversions. If the dimensions on both sides of an equation do not match, the equation is wrong. For example:

Checking E = mc²: [Energy] = [Mass] × [Speed]² = kg × (m/s)² = kg·m²·s⁻² = J ✓

Dimensional analysis is also used to derive conversion factors. If 1 m = 3.281 ft, then 1 m/s = 3.281 ft/s by multiplying both sides by 1/s. More complex conversions follow the same pattern of multiplying by appropriate unit fractions.

Common Mistakes in Unit Conversion

Confusing mass and weight

Mass (kg) is the amount of matter; weight (N) is the gravitational force on that mass. On the Moon (g ≈ 1.63 m/s²), a 70 kg person still has a mass of 70 kg but weighs only 70 × 1.63 = 114 N (vs. 686 N on Earth). In everyday language, "weight" often means mass — but in physics, they are distinct.

Using Celsius in thermodynamic equations

The ideal gas law (PV = nRT) and efficiency calculations (η = 1 − T_cold/T_hot) require absolute temperature in Kelvin. Substituting Celsius values gives wrong answers. Always convert to K before applying thermodynamic formulas.

Forgetting prefix multipliers

SI prefixes change the magnitude by powers of 10. A millijoule is 10⁻³ J, a kilopascal is 10³ Pa, a megawatt is 10⁶ W. In calculations, convert to base SI units first, compute the result, then convert back to appropriate prefix units for reporting.

Common Questions

Why does physics use so many different pressure units?

Different industries standardised around different reference points before SI was established. Blood pressure is measured in mmHg because mercury manometers were the historical standard in medicine. Tyre pressure in psi reflects US industry conventions. Weather pressure in hPa reflects meteorological standards. The converter handles all common variants.

What is the difference between kWh and kW?

kW (kilowatt) is a unit of power — the rate of energy use at any given moment. kWh (kilowatt-hour) is a unit of energy — the total energy consumed over time. 1 kWh = 1 kW × 1 hour = 3,600,000 J = 3.6 MJ. Electricity bills charge for kWh (total energy) not kW (instantaneous power).

Is electron-volt a unit of energy or voltage?

Electron-volt is a unit of energy, despite the name. 1 eV is the energy acquired by one elementary charge moving through a potential difference of 1 volt: 1 eV = 1.602 × 10⁻¹⁹ J. The confusion arises because voltage (measured in volts) and energy per unit charge (also volts) are related — but eV is specifically an energy unit.

Which unit converter should I use for everyday length and weight?

For length, weight, and volume conversions, the general unit converter covers metres to feet, kilograms to pounds, litres to gallons, and other common everyday conversions. The physics converter focuses on scientific units like newtons, joules, and pascals.