This heat density converter changes thermal energy per unit volume between joules per cubic meter, kilojoules and megajoules per cubic meter, BTU per cubic foot, calories per cubic centimeter and therms per cubic foot. Formula: 1 langley = 41840 J/m²; 1 cal(th)/cm² = 41840 J/m². The SI unit is J/m³. 1 BTU/ft³ is about 37,258.9 J/m³ (0.0372589 MJ/m³), and 1 cal/cm³ is exactly 4,184,000 J/m³.
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How to use the heat density converter
- Enter the value, for example 1,036 from a natural gas heat content figure in BTU/ft³.
- In the heat density converter, choose its unit and the target unit, such as BTU/ft³ to MJ/m³.
- Read the heat density converter result: 1,036 BTU/ft³ ≈ 38.6 MJ/m³.
For MJ/m³ to BTU/ft³, multiply by 26.8392. Gas heating values depend on the reference temperature and pressure of the cubic meter or foot, so compare like with like.
What is heat density?
Heat density is thermal energy per unit volume, in joules per cubic meter in SI. It is used for fuel energy content by volume and thermal storage, and in older references as BTU per cubic foot.
Conversion formula
The heat density converter stores every unit as a multiple of the base unit, the joule per cubic meter (J/m³). To convert:
value in target unit = value × (factor of source unit ÷ factor of target unit)
where each factor is the size of the unit in joules per cubic meter.
Common heat density conversions
| From | To | 1 unit equals |
|---|---|---|
| BTU per cubic foot (BTU/ft³) | joule per cubic meter (J/m³) | 37,258.95 J/m³ |
| megajoule per cubic meter (MJ/m³) | BTU per cubic foot (BTU/ft³) | 26.83919 BTU/ft³ |
Heat density converter worked example
Convert 25 megajoules per cubic meter to BTUs per cubic foot:
- Factor of MJ/m³ = 1,000,000 J/m³
- Factor of BTU/ft³ = 37,258.946 J/m³
- 25 × 1,000,000 ÷ 37,258.946 = 670.9798 BTU/ft³
Heat density unit reference
| Unit | Symbol | Group | Equals (J/m³) |
|---|---|---|---|
| Joule per Cubic Meter | J/m³ | SI | 1 |
| Kilojoule per Cubic Meter | kJ/m³ | SI | 1,000 |
| Megajoule per Cubic Meter | MJ/m³ | SI | 1,000,000 |
| BTU per Cubic Foot | BTU/ft³ | US | 37,258.94581 |
| Calorie per Cubic Centimeter | cal/cm³ | CGS | 4,184,000 |
| Therm per Cubic Foot | thm/ft³ | US | 3,725,894,581 |
Reverse factors for each heat density unit
The heat density converter reference table gives each unit in joule per cubic meters. To go the other way, divide by that factor; the list below does the division for you.
- Kilojoule per Cubic Meter (kJ/m³), SI: 1 kJ/m³ = 1,000 J/m³, so 1 J/m³ = 0.001 kJ/m³.
- Megajoule per Cubic Meter (MJ/m³), SI: 1 MJ/m³ = 1,000,000 J/m³, so 1 J/m³ = 1 × 10−6 MJ/m³.
- BTU per Cubic Foot (BTU/ft³), US: 1 BTU/ft³ = 37,258.95 J/m³, so 1 J/m³ = 2.68392 × 10−5 BTU/ft³.
- Calorie per Cubic Centimeter (cal/cm³), CGS: 1 cal/cm³ = 4,184,000 J/m³, so 1 J/m³ = 2.39006 × 10−7 cal/cm³.
- Therm per Cubic Foot (thm/ft³), US: 1 thm/ft³ = 3.72589 × 109 J/m³, so 1 J/m³ = 2.68392 × 10−10 thm/ft³.
Natural gas and heat storage examples
Volumetric energy content is how gaseous fuels and thermal stores are compared. The natural gas figure is the US EIA estimate of 1,036 BTU per cubic foot for 2026; the storage row assumes water at about 4.18 MJ per m³ per kelvin:
| Fuel or store (approximate) | BTU/ft³ | MJ/m³ | kWh/m³ |
|---|---|---|---|
| Natural gas (EIA 2026 US average delivered) | 1,036 | 38.6 | 10.72 |
| Water heated by 50 K (thermal storage) | 5,609 | 209 | 58.1 |
A cubic meter of natural gas holds about 10.7 kWh, while a cubic meter of water heated by 50 K stores about 58 kWh.
Choosing a unit
Values are quoted in MJ/m³ in SI work, BTU/ft³ in US gas billing and engineering, and kWh/m³ for heat storage. For MJ/m³ to kWh/m³, multiply by 0.277778. For energy alone, use the energy unit converter.
Thermal storage examples
Heat density is also the right measure for storage. A water tank that swings 40 K between charged and discharged holds about 4.19 MJ/(m³·K) × 40 K ≈ 167 MJ/m³. In the heat density converter that is about 4,490 BTU/ft³, or 46.5 kWh per cubic meter.
Ice storage does better because of latent heat. Freezing water releases 334 kJ/kg, and ice has a density of about 917 kg/m³, so the latent part alone is roughly 306 MJ/m³, or about 8,200 BTU/ft³. That is why ice tanks are compact compared with chilled-water tanks.
For fuels, compare these with natural gas at about 38 MJ/m³ and heating oil near 38,000 MJ/m³. Liquid fuels are around a thousand times denser in energy than gas at room pressure.
Related converters and calculators
Related heat and energy tools:
- Energy unit converter: J, kWh, BTU
- Specific heat capacity converter: J/(kg·K)
- Heat flux converter: W/m²
- Volume converter: m³ and ft³
- BTU calculator: room heating and cooling
Sources
NIST Special Publication 811, Appendix B; US EIA: Energy conversion calculators.
Questions about this calculator
What is heat density?
Thermal energy per unit volume, measured in J/m³ in SI.
How do I convert MJ/m³ to BTU/ft³?
Multiply by 26.8392. For example, 38 MJ/m³ = 1,019.9 BTU/ft³.
How do I convert BTU/ft³ to MJ/m³?
Multiply by 0.0372589. For example, 1,036 BTU/ft³ = 38.6 MJ/m³.
How much energy is in a cubic foot of natural gas?
The US EIA estimates about 1,036 BTU per cubic foot for natural gas delivered to consumers in 2026.
How do I convert MJ/m³ to kWh/m³?
Divide by 3.6. For example, 36 MJ/m³ = 10 kWh/m³.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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