Convert between joules, calories, kcal, kWh, BTU, MJ, eV and 15+ energy units — instantly.
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This free energy converter lets you instantly switch between joules (J), kilojoules (kJ), megajoules (MJ), gigajoules (GJ), small calories (cal), kilocalories (kcal), watt-hours (Wh), kilowatt-hours (kWh), megawatt-hours (MWh), BTU, MMBtu, therms, foot-pounds, ergs, electron volts (eV), megaelectron volts (MeV), and tons of TNT — 17 units covering every field from nutrition science and home electricity bills to particle physics and explosive yield calculations.
Energy is the single most fundamental quantity in all of physics, yet it is expressed in a bewildering variety of units depending on the context. The same amount of energy that a physicist describes as 4.184 joules, a nutritionist calls 1 calorie, a power engineer calls 0.000001163 kWh, a heating contractor calls 0.003966 BTU, and an astrophysicist might express as 2.613 × 10¹⁹ electron volts. These are not competing definitions — they are the same physical quantity described in different units that evolved independently across different scientific and engineering traditions. This guide explains every unit, every conversion formula, and every real-world scenario where they matter.
All units below are expressed in joules (the SI base unit). To convert from any unit to joules, multiply by the "joules per unit" factor. To convert from joules to any unit, divide by that factor.
| Unit | Symbol | Joules per Unit | Primary Use |
|---|---|---|---|
| Joule | J | 1 | SI base unit — physics, engineering, all scientific work |
| Kilojoule | kJ | 1,000 | Food energy (Europe, Australia), chemistry, thermodynamics |
| Megajoule | MJ | 1,000,000 | Industrial energy, vehicle fuel content, gas meters |
| Gigajoule | GJ | 1,000,000,000 | Natural gas billing, national energy statistics |
| Small calorie | cal | 4.184 | Chemistry heat of reaction, small-scale thermodynamics |
| Kilocalorie (food Cal) | kcal | 4,184 | Nutrition, food labels, dietary energy |
| Watt-hour | Wh | 3,600 | Battery capacity, small appliances, EV range |
| Kilowatt-hour | kWh | 3,600,000 | Electricity bills, home energy use, solar production |
| Megawatt-hour | MWh | 3,600,000,000 | Grid-scale electricity, power plant output |
| BTU | BTU | 1,055.06 | HVAC, heating systems, US gas appliances |
| MMBtu | MMBtu | 1,055,060,000 | Natural gas pricing, industrial heating |
| Therm (US) | therm | 105,480,400 | US natural gas billing unit (≈ 100,000 BTU) |
| Foot-pound | ft·lb | 1.35582 | Torque, mechanical work in US engineering |
| Erg | erg | 0.0000001 (10⁻⁷) | CGS system — older physics literature |
| Electron volt | eV | 1.60218 × 10⁻¹⁹ | Atomic physics, particle physics, photon energy |
| Megaelectron volt | MeV | 1.60218 × 10⁻¹³ | Nuclear physics, particle masses, radiation |
| Ton of TNT | tTNT | 4,184,000,000 | Explosive yield, nuclear weapon comparison |
The joule is the SI (International System of Units) base unit of energy, named after English physicist James Prescott Joule. It is defined as the energy transferred when a force of one newton acts through a displacement of one meter — or equivalently, the energy dissipated as heat when one ampere of current flows through a one-ohm resistance for one second. The joule is the foundation from which all other energy units are derived or converted. Every physics equation, engineering calculation, and scientific paper that crosses international borders uses joules as the reference unit. One joule is a relatively small amount of energy in everyday terms — roughly the energy needed to lift a small apple one meter off a table.
The calorie is the source of the most widespread unit confusion in everyday life. There are two completely different definitions in common use, and they differ by a factor of 1,000. The small calorie (cal) — used in chemistry and physics — is the energy required to raise the temperature of 1 gram of water by 1°C at standard pressure. It equals exactly 4.184 joules. The kilocalorie (kcal) — used in nutrition — is 1,000 small calories, or 4,184 joules. The critical point: food labels in the United States list "Calories" (capital C) which actually means kilocalories. A chocolate bar listed as "250 Calories" contains 250 kcal = 250,000 small cal = 1,046,000 joules. In Europe and Australia, food energy is often listed directly in kilojoules (kJ), which avoids this ambiguity entirely: 1 kcal = 4.184 kJ.
The kilowatt-hour is the unit your electricity provider uses to charge you, and it represents the energy consumed by a 1,000-watt device running for exactly one hour. 1 kWh = 3,600,000 joules = 3.6 MJ. It is a compound unit combining power (kilowatts) with time (hours), which is why energy and power are so frequently confused. Your electricity bill shows kWh consumed, not kilowatts — kilowatts is how fast you use energy at any moment, kWh is the total energy used over time. A 100W light bulb running for 10 hours uses 1 kWh. The average US household uses approximately 877 kWh per month (about 29 kWh per day). At $0.13/kWh — close to the US average — that is roughly $114/month.
The British Thermal Unit is the amount of energy needed to raise 1 pound of water by 1 degree Fahrenheit. 1 BTU = 1,055.06 joules = 0.000293 kWh. Despite the British name, the BTU is used almost exclusively in the United States today for heating, cooling, and gas appliance ratings. Air conditioner capacity is rated in BTU/hour (a 12,000 BTU/h unit is a "one-ton" AC — the ton referring to the cooling power of melting one ton of ice per day). Furnaces are rated in BTU/h output. Hot water heaters, gas ranges, and pool heaters all carry BTU ratings. Natural gas in the US is sold in therms (100,000 BTU) or MMBtu (1 million BTU).
The electron volt is the energy gained by a single electron when it accelerates through a potential difference of 1 volt in a vacuum. 1 eV = 1.602176634 × 10⁻¹⁹ joules — an almost incomprehensibly tiny amount of energy by everyday standards, but the natural unit for processes at the atomic and subatomic scale. Visible light photons carry 1.8–3.1 eV. X-ray photons carry 100 eV to 100 keV. The rest mass energy of a proton is 938.3 MeV. The Large Hadron Collider at CERN accelerates protons to energies of up to 6.5 TeV (teraelectron volts) per beam. Electron volts are also used to describe the bandgap energy of semiconductors — silicon's bandgap of 1.1 eV determines what wavelengths of light it can absorb, directly controlling solar cell efficiency.
Megajoules and gigajoules appear in industrial, transportation, and national energy accounting. The energy content of vehicle fuels is typically expressed in MJ/liter: petrol (gasoline) contains approximately 34.2 MJ/liter; diesel contains about 38.6 MJ/liter; hydrogen (compressed at 700 bar) contains about 5.6 MJ/liter. Natural gas is often billed in gigajoules in Canada and Australia. The energy content of a typical residential natural gas bill in Canada might be 50–100 GJ per year for heating. For comparison, 1 GJ = 277.78 kWh = 947,817 BTU.
| From | To | Multiply By | Example |
|---|---|---|---|
| Joules (J) | Calories (cal) | × 0.239006 | 100 J = 23.9 cal |
| Calories (cal) | Joules (J) | × 4.184 | 100 cal = 418.4 J |
| Joules (J) | Kilocalories (kcal) | × 0.000239006 | 4,184 J = 1 kcal |
| Kilocalories (kcal) | Joules (J) | × 4,184 | 2,000 kcal = 8,368,000 J |
| Kilocalories (kcal) | Kilojoules (kJ) | × 4.184 | 100 kcal = 418.4 kJ |
| Kilojoules (kJ) | Kilocalories (kcal) | × 0.239006 | 100 kJ = 23.9 kcal |
| Joules (J) | kWh | ÷ 3,600,000 | 3,600,000 J = 1 kWh |
| kWh | Joules (J) | × 3,600,000 | 5 kWh = 18,000,000 J |
| kWh | BTU | × 3,412.14 | 1 kWh = 3,412.14 BTU |
| BTU | kWh | × 0.000293071 | 10,000 BTU = 2.93 kWh |
| BTU | Joules (J) | × 1,055.06 | 100 BTU = 105,506 J |
| Joules (J) | BTU | × 0.000947817 | 1,055.06 J = 1 BTU |
| BTU | Kilocalories (kcal) | × 0.251996 | 100 BTU = 25.2 kcal |
| MJ | kWh | × 0.277778 | 36 MJ = 10 kWh |
| kWh | MJ | × 3.6 | 10 kWh = 36 MJ |
| Therm | kWh | × 29.3071 | 1 therm = 29.3 kWh |
| GJ | kWh | × 277.778 | 1 GJ = 277.8 kWh |
| Ton of TNT | GJ | × 4.184 | 1 tTNT = 4.184 GJ |
The single most widespread misconception in everyday energy discussions is confusing energy with power. They are related but fundamentally different quantities, and mixing them up leads to meaningless comparisons and misinformed decisions.
Energy is the total amount of work done or heat transferred over time — how much fuel you burned, how much electricity you used, how much food you ate. It is measured in joules, kWh, calories, or BTU.
Power is the rate at which energy is produced or consumed — how fast you are burning it at this moment. It is measured in watts (W) or BTU/hour. The mathematical relationship is simple: Energy = Power × Time.
Practical examples of the difference: A 60W light bulb is a power rating. Running that bulb for 1 hour uses 60 Wh = 0.06 kWh of energy. A solar panel rated at 400W peak output produces 400W of power in full sun — but over 5 hours of good sunlight it produces 2,000 Wh = 2 kWh of energy. A country's power station with 1 gigawatt (GW) capacity produces 1 GW of power, and in one year (8,760 hours) at full output it would generate 8,760 GWh = 8.76 TWh of energy. Headlines saying "this plant generates X gigawatts per year" are always wrong — gigawatts is an instantaneous power rating, not an annual energy figure.
Understanding kilowatt-hours in terms of real appliances makes electricity bills tangible. The table below shows typical energy consumption for common household appliances, and the approximate cost at a typical US electricity rate of $0.13/kWh.
| Appliance | Power (watts) | 1 hour use (kWh) | Cost/hour | Annual use (kWh)* |
|---|---|---|---|---|
| LED light bulb | 9 W | 0.009 | $0.001 | 33 |
| Incandescent bulb | 60 W | 0.06 | $0.008 | 219 |
| Laptop computer | 45–65 W | 0.055 | $0.007 | 200 |
| Desktop PC + monitor | 150–300 W | 0.225 | $0.029 | 821 |
| Refrigerator (modern) | 100–200 W | 0.04 (avg) | $0.005 | 365 |
| Washing machine | 500–2,000 W | 0.5 | $0.065 | 182 |
| Electric dryer | 5,000 W | 5.0 | $0.65 | 769 |
| Dishwasher | 1,200–2,400 W | 1.0 | $0.13 | 365 |
| Microwave oven | 1,000–1,500 W | 1.25 | $0.163 | 131 |
| Electric oven | 2,000–5,000 W | 2.0 | $0.26 | 292 |
| Air conditioner (window, 1 ton) | 1,200 W | 1.2 | $0.156 | 876 |
| Central AC (3 ton) | 3,500 W | 3.5 | $0.455 | 2,555 |
| Electric vehicle (charging) | 7,200 W | 7.2 | $0.936 | 3,285 |
| Electric water heater | 4,000 W | 4.0 | $0.52 | 4,800 |
| Hair dryer | 1,875 W | 1.875 | $0.244 | 68 |
*Annual estimates based on typical US usage patterns. Actual consumption varies with usage frequency, settings, and model efficiency.
Nutritional energy is listed as Calories (kcal) on US food labels and as kilojoules (kJ) on European and Australian labels. Both measure exactly the same thing — the chemical energy released when your body metabolizes food. The conversion is always: 1 kcal = 4.184 kJ. Below is a reference of common foods expressed in all three main energy units.
| Food / Serving | kcal (food Calories) | Kilojoules (kJ) | Joules (J) | kWh equivalent |
|---|---|---|---|---|
| Apple (medium, 182g) | 95 kcal | 397 kJ | 397,480 J | 0.000110 kWh |
| Banana (medium, 118g) | 105 kcal | 439 kJ | 439,320 J | 0.000122 kWh |
| White bread (1 slice, 28g) | 75 kcal | 314 kJ | 313,800 J | 0.0000872 kWh |
| Boiled egg (large, 50g) | 78 kcal | 326 kJ | 326,352 J | 0.0000907 kWh |
| Chicken breast (100g, cooked) | 165 kcal | 690 kJ | 690,360 J | 0.000192 kWh |
| Brown rice (100g, cooked) | 112 kcal | 469 kJ | 468,608 J | 0.000130 kWh |
| Olive oil (1 tbsp, 14g) | 119 kcal | 498 kJ | 497,896 J | 0.000138 kWh |
| Cheddar cheese (30g) | 120 kcal | 502 kJ | 502,080 J | 0.000139 kWh |
| Cola drink (330ml can) | 139 kcal | 582 kJ | 581,576 J | 0.000162 kWh |
| Chocolate bar (45g) | 235 kcal | 983 kJ | 983,240 J | 0.000273 kWh |
| Big Mac burger | 550 kcal | 2,301 kJ | 2,301,200 J | 0.000639 kWh |
| Daily adult diet (avg) | 2,000 kcal | 8,368 kJ | 8,368,000 J | 2.324 kWh |
Note the last row: a 2,000 kcal daily diet is equivalent to just 2.324 kWh of electrical energy — less than what it costs to run a standard 100W light bulb for 24 hours (2.4 kWh). The human body is an extraordinarily efficient machine, extracting useful work from a remarkably small energy budget.
Comparing fuel types and understanding vehicle range requires knowing the energy content (also called calorific value or energy density) of each fuel. These are critical numbers for anyone evaluating fuel costs, EV vs combustion vehicle comparisons, or energy transition decisions.
| Fuel Type | Energy per liter (MJ/L) | Energy per liter (kWh/L) | Energy per kg (MJ/kg) | Notes |
|---|---|---|---|---|
| Petrol / Gasoline | 34.2 | 9.5 | 46.4 | Most common global transport fuel |
| Diesel | 38.6 | 10.7 | 45.6 | Higher energy density than petrol |
| LPG (propane) | 25.3 | 7.0 | 49.6 | Autogas, cooking, heating |
| CNG (compressed natural gas) | 9.0 | 2.5 | 53.6 | At 200 bar; lower density than liquid fuels |
| Ethanol (E100) | 21.4 | 5.9 | 29.7 | About 65% of petrol's energy content |
| Hydrogen (liquid) | 10.1 | 2.8 | 141.8 | Highest energy per kg; very low density |
| Biodiesel (B100) | 34.5 | 9.6 | 39.9 | Similar to petroleum diesel |
| Kerosene / Jet fuel | 35.1 | 9.75 | 46.2 | Aviation and heating use |
| Natural gas (pipeline) | 38.7 MJ/m³ | 10.75 kWh/m³ | 55.5 | Per cubic meter at standard conditions |
For anyone in the United States dealing with air conditioning, furnaces, water heaters, or gas appliances, BTU is the daily language of energy. Understanding BTU ratings helps you select correctly sized equipment, compare running costs, and calculate efficiency.
Air conditioners are rated in BTU/hour of cooling capacity. The old rule of thumb is 20 BTU/h per square foot of floor area — a 500 sq ft room needs about 10,000 BTU/h. The "ton" unit used for larger AC systems refers to 12,000 BTU/h (the cooling power of melting one ton of ice over 24 hours). A 3-ton central air system delivers 36,000 BTU/h. Converting to electricity: 12,000 BTU/h ÷ 3,412 = 3.52 kW, so a perfect-efficiency 1-ton AC would draw 3.52 kW — though real units consume less actual power than their cooling output because they move heat rather than generate it (SEER efficiency ratings reflect this).
Gas furnaces list their output in BTU/h — a typical home furnace might be rated 60,000–120,000 BTU/h. At an efficiency of 95% (AFUE rating), a 100,000 BTU/h furnace actually delivers 95,000 BTU/h of useful heat. Converting to kWh: 95,000 BTU ÷ 3,412 = 27.8 kWh of heat output per hour. At a natural gas cost of $1.10/therm and 1 therm = 29.3 kWh of electricity equivalent, gas heating is often significantly cheaper per unit of heat than electric resistance heating.
Natural gas billing in the US uses therms (approximately 100,000 BTU) or CCF (hundred cubic feet of gas, which contains about 103,700 BTU). Converting for budget planning: 1 therm = 100,000 BTU = 29.3 kWh = 105.5 MJ.
The electron volt (eV) and the ton of TNT represent opposite extremes of the energy scale — the subatomic and the catastrophic — both useful in their respective domains.
The ton of TNT equivalent (tTNT) is a unit used to express the yield of explosives and nuclear weapons. By definition, 1 ton of TNT releases 4.184 × 10⁹ joules = 4.184 GJ = 1,162 kWh. Hiroshima atomic bomb yield: approximately 15 kilotons of TNT (15,000 tTNT = 62.76 TJ). The Tsar Bomba (largest nuclear weapon ever detonated): approximately 50 megatons = 50,000,000 tTNT = 209.2 PJ. For comparison, the daily solar energy striking Earth's surface is approximately 1.74 × 10¹⁷ joules — equivalent to about 41.6 billion tons of TNT per second.
At the other extreme, the electron volt enables nuclear and particle physics to describe particle energies, masses (via E = mc²), and photon wavelengths on a coherent scale. The rest mass energy of an electron is 0.511 MeV. A uranium-235 fission event releases approximately 200 MeV = 3.2 × 10⁻¹¹ joules. That sounds tiny — but 1 gram of U-235 contains roughly 2.56 × 10²¹ atoms, so complete fission of 1 gram releases approximately 8.2 × 10¹⁰ joules = 22,800 kWh — equivalent to burning about 3 tonnes of coal.
1 kilocalorie (kcal) = 1,000 small calories (cal). The confusion exists because food labels in the US use "Calories" (capital C) to mean kilocalories — so a snack labeled "200 Calories" actually contains 200,000 small calories = 836,800 joules = 836.8 kilojoules. In physics and chemistry, "calorie" always means the small calorie (4.184 J). European food labels use kilojoules (kJ) directly, which avoids the confusion entirely. The conversion is always: 1 kcal = 4.184 kJ = 4,184 J.
1 kWh = exactly 3,600,000 joules (3.6 megajoules). This follows directly from the definitions: 1 kW = 1,000 watts = 1,000 J/s, and 1 hour = 3,600 seconds. So 1 kWh = 1,000 J/s × 3,600 s = 3,600,000 J. For other multiples: 1 Wh = 3,600 J; 1 MWh = 3,600,000,000 J = 3.6 GJ. This is the most commonly needed conversion for anyone working with electricity bills, battery capacity, or renewable energy systems.
BTU (British Thermal Unit) is used primarily in the United States for heating, ventilation, and air conditioning (HVAC). It measures the energy required to raise 1 pound of water by 1°F. 1 BTU = 1,055.06 joules = 0.000293 kWh. Air conditioner capacities, furnace output ratings, water heater recovery rates, and gas appliance outputs are all expressed in BTU or BTU/hour. Natural gas is sold in therms (100,000 BTU) and priced by the therm or CCF. The UK and most of the world have transitioned to metric units for heating, but BTU remains dominant in American HVAC and energy markets.
1 kWh = 3,412.14 BTU. This is the key conversion for comparing electric vs gas heating costs. To convert: kWh × 3,412.14 = BTU; BTU × 0.000293071 = kWh. Practical example: a 10,000 BTU/h window air conditioner running for 8 hours uses 80,000 BTU = 80,000 ÷ 3,412.14 = 23.4 kWh. At $0.13/kWh that costs $3.05 to run for 8 hours. For comparing gas vs electric: if gas costs $1.10/therm (1 therm = 29.3 kWh of heat), and electricity costs $0.13/kWh, gas heat costs 1.10 ÷ 29.3 = $0.038 per kWh of heat vs $0.13/kWh for electric resistance — gas is about 3.4× cheaper per unit of heat delivered, ignoring efficiency differences.
A typical adult requires 2,000–2,500 kcal per day = 8,368,000–10,460,000 joules (8.4–10.5 MJ) = 2.32–2.91 kWh. This is less energy than running an average hair dryer for 2 hours. The human body converts this chemical energy into heat, mechanical work (movement), electrical signals (nerve impulses), and chemical synthesis (building proteins, DNA, etc.) with remarkable efficiency. Athletes and manual laborers may require 3,000–5,000 kcal/day = 12.6–20.9 MJ. A sedentary older adult may need only 1,600–1,800 kcal = 6.7–7.5 MJ.
Energy is the total amount of work done or heat transferred — measured in joules, kWh, or BTU. Power is the rate at which energy flows — measured in watts or BTU/hour. The relationship: Energy = Power × Time. A 2,000W electric kettle is a power rating. Running it for 3 minutes (0.05 hours) uses 2,000 × 0.05 = 100 Wh = 0.1 kWh of energy. A solar farm rated at 500 MW of power capacity generates 500 × 8,760 = 4,380,000 MWh = 4,380 GWh of energy in a full year at 100% capacity factor. The confusion between watts and watt-hours is one of the most common errors in energy journalism and policy debates.
A therm (US) = exactly 100,000 BTU = 105,480,400 joules ≈ 29.307 kWh. It is the unit used by US natural gas utilities for billing. Your gas bill shows therms consumed per month. The energy content of natural gas varies slightly by composition, but one therm always means exactly 100,000 BTU regardless. For budgeting: if your gas utility charges $1.20/therm, you are paying 1.20 ÷ 29.307 = $0.041 per kWh of energy — roughly one-third the cost of electricity in most US markets, which is why gas is typically preferred for space heating and water heating despite electricity's efficiency advantages in heat pump systems.