BTU Calculator · Practical guide
Heating BTU per Square Foot by Climate
Plan on 45 BTU per square foot in a mixed climate, 60 in cold regions, and 30 in hot ones.
At a glance
The Rule of Thumb
Heating load is driven by how fast your home loses heat through its shell, so it scales with floor area rather than air volume. The standard factors this calculator uses are 45 BTU per square foot in a mixed climate, 60 in a cold climate, and 30 in a hot climate, assuming average insulation. A 1,500-square-foot home in a mixed climate therefore needs about 67,500 BTU/hr of heating capacity.
Interactive tool
BTU Calculator
Enter your own values, then compare the result with the examples below.
BTU Estimate
Results are estimates. Consult an HVAC professional for precise sizing.
Sizing Table by Climate
| Floor Area | Hot (30/sq ft) | Mixed (45/sq ft) | Cold (60/sq ft) |
|---|---|---|---|
| 500 sq ft | 15,000 | 22,500 | 30,000 |
| 1,000 sq ft | 30,000 | 45,000 | 60,000 |
| 1,500 sq ft | 45,000 | 67,500 | 90,000 |
| 2,000 sq ft | 60,000 | 90,000 | 120,000 |
How Insulation Shifts the Factors
Insulation quality adjusts the base factor by a fifth in either direction. Poor insulation — older windows, uninsulated walls, drafty attics — multiplies the factor by 1.20, turning 45 into 54 per square foot. Good insulation — modern windows, sealed envelope, adequate attic depth — multiplies it by 0.90, bringing 45 down to 40.5. On a 2,000-square-foot home in a cold climate, that gap is the difference between a 108,000 BTU/hr system (good insulation) and a 144,000 BTU/hr one (poor insulation), which is why weatherization often pays for itself before a furnace upgrade.
Why Heating and Cooling Use Different Math
Cooling must condition the full volume of air plus internal heat gains, so this calculator uses 25 BTU per cubic foot for cooling. Heating is dominated by envelope losses through walls, windows, and roof, which track floor area — hence the per-square-foot factors. Do not be surprised when the two estimates for the same room differ by a wide margin; they are answering different physical questions. For whole-home equipment sizing, a Manual J calculation refines both numbers.
Frequently Asked Questions
Which climate factor should I use?
Use hot (30/sq ft) for the Gulf Coast and Southwest desert, mixed (45/sq ft) for most of the mid-latitude US, and cold (60/sq ft) for the northern tier and mountain states. When in doubt, size to the colder factor for heating.
Does ceiling height affect heating BTU?
This calculator's heating estimate uses floor area only, since heat loss is dominated by the building envelope. In practice, very high ceilings add some load through stratification and extra wall area.
Is the furnace size the same as the BTU load?
Not exactly. Furnaces are rated by output capacity, and efficiency matters: a 90,000 BTU/hr load calls for roughly a 100,000 BTU/hr input furnace at 90% AFUE efficiency.
Do heat pumps follow the same sizing?
Heat pumps must cover both the heating and cooling load, and their capacity drops in cold weather. In cold climates, contractors often size to the heating load with supplemental heat as backup.