Joules to Kilojoules Converter

How to convert joules to kilojoules?

Joules to Kilojoules Converter performs the SI prefix conversion every physicist, chemist, nutritionist, and engineer encounters daily. One kilojoule equals exactly 1000 joules by the 'kilo-' prefix definition. The joule is the SI unit of energy, equal to one newton-meter or one watt-second. Use this tool to read nutrition labels (a 400 kcal Snickers bar contains about 1673 kJ), to size mechanical brakes (kinetic energy of a 1500 kg car at 100 km/h is 579 kJ), to compute bond energies in chemistry (C-H bond is 414 kJ/mol), to measure laser pulse output (medical YAG laser shots deliver 50 to 1000 mJ), or to interpret electrical loads (a 60-watt bulb consumes 60 J/s, or 216 kJ over an hour). The conversion is exact, no rounding ever.

E_(kJ) = E_(J) ÷ 1000

Example

Convert 1000 J to kilojoules:

E_(kJ) = 1000J ÷ 1000 = 1kJ

Why does the European nutrition label show kilojoules instead of calories?

EU Regulation 1169/2011 requires both kJ and kcal on packaged food labels, with kJ listed first. The reason is that the SI energy unit is the joule, so kJ is the scientifically correct primary measure. Calories survive for cultural familiarity. The conversion: 1 kcal equals 4.184 kJ exactly (defined). A typical adult requires 8400 to 10500 kJ/day (2000 to 2500 kcal). A McDonald's Big Mac has about 2230 kJ (533 kcal). When converting US labels to EU format, multiply Calories by 4.184: a 100 kcal apple is 418.4 kJ. Note that 'Calorie' with a capital C in food contexts means kilocalorie, while 'calorie' (small c) usually means the 4.184 J unit used in chemistry.

What is the difference between a joule and a watt?

A joule is energy, a watt is power. One watt equals one joule per second. A 100-watt incandescent bulb uses 100 J of electrical energy every second, or 360 kJ per hour, or 8.64 MJ per day. Power is the rate of energy use. Conversely, energy equals power times time: a 1500 W microwave running 60 seconds delivers 90 kJ to your food. For battery sizing: a 5000 mAh phone battery at 3.7 V holds 18.5 watt-hours, equal to 66.6 kJ. For climbing energy: lifting a 70 kg person up a 3-meter staircase requires 70 x 9.81 x 3 = 2.06 kJ of mechanical work (more is required to account for body inefficiency, typically 4 to 5 times this).

How is joule related to other energy units like BTU, kWh, or electronvolt?

1 kWh equals exactly 3.6 MJ or 3600 kJ. 1 BTU equals approximately 1.05506 kJ. 1 electronvolt equals 1.602e-19 J or 1.602e-22 kJ. 1 foot-pound equals 1.356 J or 0.001356 kJ. 1 calorie (small c, thermochemical) equals exactly 4.184 J. For domestic electricity bills (kWh) to SI: a 30-kWh daily home use is 108 MJ. For HVAC sizing (BTU/hr to W): a 5000 BTU/hr air conditioner provides about 1465 W or 5275 kJ/hr cooling. Atomic physics uses eV almost exclusively because chemical bond energies (1 to 10 eV) and band gaps in semiconductors (silicon 1.12 eV) are awkward in joules.

Why is the kilojoule the unit of choice for chemical bond energies?

Bond dissociation energies typically fall in the 100 to 1000 kJ/mol range, which is convenient. A C-H single bond is 414 kJ/mol; a C=C double bond is 614 kJ/mol; a C-C triple bond (acetylene) is 839 kJ/mol. The conversion to per-bond energy: divide by Avogadro's number (6.022e23 /mol), so 414 kJ/mol equals 6.87e-22 kJ per bond, or about 4.29 eV per bond. Combustion enthalpies are also in kJ/mol: glucose oxidation yields about 2810 kJ/mol. The per-bond eV value is more useful in atomic physics, while the per-mole kJ is universal in introductory chemistry textbooks.

What is the kinetic energy of common moving objects in kJ?

Kinetic energy KE equals 0.5 times m times v squared, in joules when mass is in kg and velocity in m/s. A 70 kg cyclist at 5 m/s (18 km/h) has 875 J or 0.875 kJ of KE. A 1500 kg car at 100 km/h (27.78 m/s) has 579 kJ. A 70 kg pedestrian hit by that car receives the difference in KE as the impact: about 575 kJ. A 9-mm bullet (8 g) at 350 m/s has 490 J or 0.49 kJ. A baseball (145 g) at 90 mph (40.2 m/s) has 117 J. These numbers explain why a 100 km/h car crash is roughly 80 times more energetic than a 25 km/h fender-bender (the v^2 term), and why airbags and crumple zones need to absorb hundreds of kJ in 0.05 seconds.

How do nutrition kJ values match physical exercise energy expenditure?

Roughly speaking, 1 kg of body fat stores about 32000 kJ of metabolic energy (7700 kcal x 4.184). To lose 1 kg of fat you must burn that energy in excess of food intake. Running burns about 4 kJ per kg of body weight per km, so a 70 kg person running 10 km burns 2800 kJ (670 kcal), enough to offset one large fast-food meal. Cycling burns about 2 kJ/kg/km. Walking burns 1.5 kJ/kg/km. Resting metabolic rate is roughly 4 kJ/kg/hour. These are approximations; individual variation is large due to muscle mass, age, sex, and metabolism.

Popular joules to kilojoules conversion table