BTU Per Square Foot To Watts Per Square Metre Converter
Convert BTU/h per square foot to W/m2 for heat flux, HVAC load density and building-services notes, with reverse conversion and total area power included.
Enter Heat Load Density
Converted Heat Flux
10 BTU/h per sq ft equals 31.55 W/m2.
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Quick Conversion Answer
For the common HVAC meaning, 1 BTU/h per square foot equals 3.15459 W/m2. Therefore 10 BTU/h per sq ft is 31.55 W/m2, 20 BTU/h per sq ft is 63.09 W/m2, and 30 BTU/h per sq ft is 94.64 W/m2. The reverse factor is 1 W/m2 equals about 0.316998 BTU/h per sq ft.
The time part is important. A BTU is energy, while a watt is power. To convert to watts, the BTU value must be tied to a time period. In building-services shorthand, “BTU per square foot” often means “BTU per hour per square foot”. If a document says BTU per day per square foot, choose the per-day basis so the page calculates an average W/m2 over 24 hours.
BTU/h/sq ft To W/m2 Formula
The default factor combines the International Table BTU, the hour, and the exact square-foot-to-square-metre conversion.
1 BTU = 1055.05585262 J
1 hour = 3600 seconds
1 sq ft = 0.09290304 m2
1 BTU/h/sq ft = 1055.05585262 / 3600 / 0.09290304 = 3.15459075 W/m2
For daily energy density, divide the hourly factor by 24 because the same BTU per square foot is spread over a full day.
When To Use This Converter
Use the converter when an imperial heat-load density needs to be entered in SI units, such as HVAC concept notes, radiant heating output, heat gain comparisons, older US design notes or product literature. It is also useful when comparing a W/m2 heating allowance with a BTU/h per sq ft rule of thumb.
Do not use it as a full heat-loss calculation. A real design may need room volume, fabric U-values, air change rate, outside design temperature, solar gain, occupancy, internal gains, control strategy, safety factors and professional judgement. The converter only changes units and totals the area if you enter one.
BTU/h Per Square Foot To W/m2 Table
| BTU/h per sq ft | W/m2 | kW Over 100 m2 | Use Note |
|---|---|---|---|
| 1 | 3.15 W/m2 | 0.32 kW | Factor check. |
| 5 | 15.77 W/m2 | 1.58 kW | Low heat density comparison. |
| 10 | 31.55 W/m2 | 3.15 kW | Common quick conversion example. |
| 15 | 47.32 W/m2 | 4.73 kW | Moderate load density. |
| 20 | 63.09 W/m2 | 6.31 kW | Higher heating or cooling allowance. |
| 30 | 94.64 W/m2 | 9.46 kW | Check against equipment and comfort limits. |
Worked Examples
Multiply 10 by 3.15459075. The result is 31.55 W/m2. Over 100 m2, that is 3.16 kW.
Multiply 50 by 0.316998. The result is 15.85 BTU/h per sq ft. This is useful when comparing SI room-load notes with US literature.
Choose per-day basis. The hourly average is 240 x 3.15459075 / 24 = 31.55 W/m2.
Unit Boundaries To Watch
- BTU/h per sq ft is power density, comparable with W/m2.
- BTU per sq ft without a time period is energy per area, not power density.
- BTU/h per ft is not the same as BTU/h per sq ft.
- W/m2 is not the same as W/mK, which is used for thermal conductivity.
- W/m2K includes a temperature difference and is used for U-values, not simple heat-load density.
- BTU/h, W and kW are total power units; divide or multiply by area only when area is part of the problem.
Reverse W/m2 To BTU/h Per Square Foot Table
| W/m2 | BTU/h per sq ft | kW Over 50 m2 | Comment |
|---|---|---|---|
| 10 W/m2 | 3.17 | 0.50 kW | Low output density. |
| 25 W/m2 | 7.92 | 1.25 kW | Small room comparison. |
| 50 W/m2 | 15.85 | 2.50 kW | Common heating-output check. |
| 75 W/m2 | 23.77 | 3.75 kW | Higher output density. |
| 100 W/m2 | 31.70 | 5.00 kW | Check comfort, surface and equipment limits. |
FAQ
How many W/m2 is 1 BTU per square foot?
If the phrase means 1 BTU per hour per square foot, the answer is 3.15459 W/m2.
Why do you add per hour?
A watt is one joule per second. BTU is energy, so a time period is needed before it can be converted to power.
Is BTU/h/sq ft the same as BTU/ft2?
No. BTU/h/sq ft is power density. BTU/ft2 without time is energy per area.
How do I get total kW from W/m2?
Multiply W/m2 by the area in m2, then divide by 1000. The calculator does this when you enter area.
Can this size a radiator or heat pump?
No. It converts units. Equipment sizing needs heat loss, ventilation, design temperatures, controls and professional checks.
Which BTU definition is used?
The page uses the International Table BTU value of 1055.05585262 J, which is the common engineering conversion factor.
Sources
- National Institute of Standards and Technology. (2014). NIST Guide To The SI, Appendix B.9: Factors For Units Listed By Kind Of Quantity Or Field Of Science. NIST. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9
- National Institute of Standards and Technology. (2024). British Thermal Unit (Btu). NIST. https://www.nist.gov/glossary-term/19511
- Bureau International des Poids et Mesures. (2019). The International System Of Units (SI), 9th Edition. BIPM. https://www.bipm.org/en/publications/si-brochure
- National Institute of Standards and Technology. (2008). Guide For The Use Of The International System Of Units (SI), NIST Special Publication 811. NIST. https://doi.org/10.6028/NIST.SP.811e2008