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Watts to BTU/hr Calculator

Convert a wattage figure into BTU per hour — useful for heat-gain totals from lighting, equipment and occupants.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Heat equivalent

5118.21 BTU/hr

High confidence

Converted using an exact defined factor of 3.412141633128 BTU/hr per W. Useful for internal gains, where wattage drawn inside a room becomes heat the cooling system has to remove.

Conversion factor applied
3.41 BTU/hr per W

Cooling load work often starts with a list of what is plugged in: lighting circuits, servers, kitchen equipment, and the people in the room. Inside a conditioned space almost all of that electrical power ends up as heat, so the wattage total converts directly into a BTU per hour gain the plant has to remove. The exception worth remembering is any equipment that exhausts its heat outside the space — a vented dryer or an extract-hooded appliance moves much of its load out of the room, and counting it as an internal gain oversizes the system. Occupant load is not electrical at all and comes from published sensible and latent figures per person by activity, so add those separately rather than trying to express people in watts.

Add the equipment this sizes

This result is a specification — 5,118.212 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • 1 W = 3.412141633 BTU/h, from the definition 1 BTU/h = 0.29307107017222 W
  • Sensible heat gain from equipment: for cooling load purposes, electrical input inside the conditioned space is treated as heat gain

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Frequently asked questions

Is all the wattage in a room really heat?
For cooling load purposes, effectively yes for anything whose energy stays in the space — lighting, computers, televisions. Energy leaves as light or sound and is absorbed by surfaces almost immediately. The exception is equipment that vents outside, such as a ducted dryer or a hooded range, which removes much of its own load and should not be counted in full.
How do I include people in the heat gain?
Not through this calculator. Occupant gain is published as sensible and latent watts per person by activity level — roughly 70 W sensible for seated office work, considerably more for physical labour — and the latent portion is moisture, which affects dehumidification rather than temperature. Add those from a load table separately.
Why do lighting loads matter less than they used to?
Because LED retrofits cut lighting wattage dramatically, and with it the internal gain. A space converted from fluorescent or halogen may now need noticeably less cooling than its original design assumed, which is worth rechecking before replacing plant like for like.