A kilocalorie is 1,000 calories.
Kilocalories to Attojoules Converter — kcal to aJ
Convert Kilocalorie (kcal) to Attojoule (aJ) using the exact conversion factor (1 kilocalorie = 4.184e+21 attojoule). See the formula, worked examples, and conversion table.
Kilocalories to Attojoules converter
Converter
Engineering Converter
Convert common stress, force, torque, power, and energy units used in engineering.
Context values
Some unit families need one extra value for a physically accurate result.
Attojoule is 1e-18 joule units.
Result = input x 4184 / 1e-18.
Energy units are compatible.
Reserved above the conversion cards and below the converter.
About Converting Kilocalories to Attojoules
Kilocalorie and Attojoule both measure a quantity used in structural or mechanical engineering — commonly stress, pressure, force, torque, power, or energy — where converting between the unit conventions used in different regions, industries, or equipment specifications is a routine part of design work. Specifically: A kilocalorie is 1,000 calories, and attojoule is 1e-18 joule units.
Formula
attojoules = kilocalories × 4.184e+21
This factor comes from each unit's defined relationship to the category's base unit: 1 kilocalorie equals 4184 base units, and 1 attojoule equals 1e-18 base units, so dividing one by the other gives the direct kilocalorie-to-attojoule factor of 4.184e+21.
Simple example
1 kcal × 4.184e+21 = 4.184e+21 aJ
1 kilocalorie = 4.184e+21 attojoules.
Real-world example
100 kcal × 4.184e+21 = 4.184e+23 aJ
100 kilocalories = 4.184e+23 attojoules.
Conversion table
| Kilocalorie (kcal) | Attojoule (aJ) |
|---|---|
| 1 kcal | 4.184e+21 aJ |
| 10 kcal | 4.184e+22 aJ |
| 50 kcal | 2.092e+23 aJ |
| 100 kcal | 4.184e+23 aJ |
| 500 kcal | 2.092e+24 aJ |
| 1,000 kcal | 4.184e+24 aJ |
Reverse conversion: Attojoule to Kilocalorie
4.184e+21 aJ × 2.390057e-22 = 1 kcal
4.184e+21 attojoules = 1 kilocalorie.
kilocalories = attojoules × 2.390057e-22
For a page dedicated to this direction, see Attojoule to Kilocalorie.
Understanding the Kilocalorie (kcal)
A kilocalorie is 1,000 calories.
Understanding the Attojoule (aJ)
Attojoule is 1e-18 joule units.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many attojoules are in 1 kilocalorie?
1 kilocalorie equals 4.184e+21 attojoules, using the exact defined conversion factor rather than an estimate.
How do I convert kilocalorie to attojoule?
Multiply the kilocalorie value by 4.184e+21. The converter above does this instantly to whatever precision you set.
How do I convert attojoule back to kilocalorie?
Use the reverse factor: 1 attojoule equals 2.390057e-22 kilocalories. You can also use the swap control in the converter above to flip the direction instantly.
What is a kilocalorie?
Kilocalorie (kcal) is a unit of engineering.
What is a attojoule?
Attojoule (aJ) is a unit of engineering.
Is the kilocalorie to attojoule conversion exact?
Yes. Both kilocalorie and attojoule are defined by fixed standards rather than physical artifacts, so the factor of 4.184e+21 used above is exact to as many digits as you choose to display.
Why do engineering unit conventions vary so much by region and industry?
Pressure, force, and stress can be expressed in metric (pascal, newton), US customary (psi, pound-force), or older and industry-specific units (atmosphere, torr, kip) depending on the country, industry, and even the equipment manufacturer, which is why engineers routinely need to convert between them when working across specs from different sources.