Heat Loss Calculator

A heat loss calculator estimates how much heat a room loses — in BTU per hour and watts — from its size, the temperature difference you want to hold, and how well it is insulated. That number is what a heater or heating system has to replace to keep the room warm.

Your measurements

Result

It is a quick sizing guide for a space heater, radiator or mini-split: bigger rooms, colder outside, and poorer insulation all push the figure up. For buying actual HVAC equipment, treat it as a starting estimate and get a full load calculation.

How it is calculated

Simplified estimate: heat loss ≈ floor area × temperature difference × an insulation coefficient (higher for poorer insulation), adjusted for ceiling height. It approximates the steady-state output needed to hold the indoor temperature. A full Manual J adds window/wall U-values, infiltration, orientation and internal gains — use one for equipment selection.

Steps

Enter the room floor area and ceiling height.
Enter the indoor-minus-outdoor temperature difference for your design day.
Pick the insulation level.
Read the heat loss in BTU/h (and watts).
Size a heater at or a little above this; verify with a full load calc for whole-home systems.

Worked example

A 200 sq ft, 8 ft-ceiling room at a 40°F difference with average insulation loses roughly 3,200 BTU/h (about 940 watts).

Common questions

How do I calculate heat loss?

Roughly, floor area × the indoor-outdoor temperature difference × an insulation factor. The calculator does it and converts to watts.

How many BTUs to heat a room?

It depends on size, climate and insulation. A 200 sq ft room often needs a few thousand BTU/h — this estimates yours; round up a little.

Is this a Manual J calculation?

No — it is a simplified estimate. A Manual J load calc accounts for each window, wall and air leak and is what you should size whole-home HVAC from.

How do I reduce heat loss?

Air-seal and add insulation first — they shift the result more than a bigger heater, and lower running costs.