Room Heating & Cooling Load Calculator
Enter room dimensions, insulation quality, window area, and climate zone to get the BTU/hr and kW required for heating and air conditioning. Nothing uploaded.
Room Settings
Load Results
Cooling load
Simplified estimate. Oversizing heaters or AC units by more than 25% causes short-cycling and humidity issues. Consult an HVAC professional for system selection.
Learn more: room heating and cooling load
How the heating load is built
The heating load is the heat lost through the walls, windows, ceiling and floor. Each loss is area times U-value times the design temperature difference for your climate zone, which runs from 15 K for tropical to 55 K for very cold. Walls use the U-value for your insulation level, from 2.5 W/m2K for poor down to 0.3 for excellent. Windows use a fixed 3.0 W/m2K, and the ceiling and floor count at 70% and 40% of the wall rate.
Take a 5 m by 4 m room with 2.5 m ceilings, good insulation (0.8 W/m2K), 3 m2 of window and a temperate climate (25 K). Walls lose 840 W, the window 225 W, the ceiling 280 W and the floor 160 W, so the heating load is 1,505 W, or 5,135 BTU/hr.
Windows weigh heavily
A window at 3.0 W/m2K loses heat at about 3.75 times the rate of a wall at 0.8. In the example the window is 7% of the wall and window area but 21% of the transmission loss before the ceiling and floor. Doubling it to 6 m2 raises the heating load from 1,505 W to 1,670 W. Energy Vanguard's guide to reading Manual J reports makes the same point, noting that entering too much window area is a way to inflate a load.
The cooling load is built differently
Cooling takes 80% of the transmission figure and adds solar gain of 150 W per square meter of window, 80 W per person and your equipment load. The example comes to 1,204 + 450 + 160 + 200 = 2,014 W, or 6,872 BTU/hr, which is 0.57 tons of cooling at 12,000 BTU/hr per ton.
The calculator is a room-level estimate. It has no infiltration or ventilation load, ignores window orientation and shading, and has no latent load. Energy Vanguard notes that Manual J reports show heating and cooling loads separately and divide cooling into sensible and latent parts. Use a full Manual J calculation before you buy equipment.
FAQ
Why does the ceiling get a 0.7 multiplier?
The calculator assumes the roof above adds some insulation benefit, so the ceiling loses 70% of what the same area of wall would at the same U-value and temperature difference. The floor uses 40%.
How is the BTU figure worked out?
Watts are multiplied by 3.412 to get BTU/hr, and cooling BTU/hr is divided by 12,000 to give tons of cooling.
Is the climate zone temperature difference an everyday value?
No. It stands for design conditions, the coldest expected day, so it sizes equipment for peak demand and does not estimate a typical day's load.