Two loads, one system
An HVAC system does two jobs, and each has its own BTU load. Cooling capacity is what the AC or heat pump removes in summer; heating capacity is what the furnace or heat pump adds in winter. They are rarely equal - most homes need more heating BTU than cooling BTU, because the winter gap between indoor and outdoor temperature is usually larger than the summer gap, even in hot-summer regions like the Sacramento foothills.
BTU per square foot by climate
These are the planning factors this calculator uses. Insulation, windows, and ceiling height shift them, which is what a full Manual J captures.
| Climate | Cooling BTU/sq ft | Heating BTU/sq ft |
|---|---|---|
| Mild / coastal | 18 | 30 |
| Moderate | 22 | 40 |
| Hot inland (Sacramento) | 25 | 35 |
| Cold winters | 20 | 50 |
Planning factors only. A Manual J load calculation is the accurate way to size both loads.
BTU, tons, and furnace output
Cooling is often expressed in tons: 12,000 BTU per hour equals one ton, so 36,000 BTU is a 3-ton AC. Heating stays in BTU. Furnaces list a BTU input and an AFUE efficiency, and the usable heat is input times AFUE - a 90,000 BTU furnace at 95 percent AFUE delivers about 85,500 BTU of output. Size each to its load; an oversized furnace or AC short-cycles and heats or cools unevenly.
Turn BTU into equipment size. Convert the cooling number to tons with the AC Tonnage Calculator, or start whole-home with the HVAC Size Calculator. We confirm both loads with a Manual J before quoting equipment.
HVAC BTU Questions, Answered
How many BTU do I need to heat and cool my house?
As a planning estimate, cooling runs about 18 to 25 BTU per square foot depending on climate, and heating about 30 to 50 BTU per square foot. So a 1,800 sq ft home in a hot inland climate like Sacramento needs roughly 45,000 BTU of cooling (about 3.5 tons) and around 63,000 BTU of heating. A Manual J load calculation confirms both figures for your specific home.
What is a BTU in heating and cooling?
A BTU, or British Thermal Unit, is the amount of heat needed to raise one pound of water by one degree Fahrenheit. In HVAC it measures capacity per hour: cooling BTU is the heat an AC removes, and heating BTU is the heat a furnace or heat pump adds. 12,000 BTU per hour of cooling equals one ton.
How many BTU per square foot for heating versus cooling?
Cooling is roughly 18 BTU per sq ft in a mild climate up to about 25 in a hot inland one. Heating is higher, about 30 BTU per sq ft in mild areas and 40 to 50 in colder ones, because the winter indoor-to-outdoor temperature gap is usually larger. That is why most homes need more heating BTU than cooling BTU.
Is heating BTU the same as cooling BTU?
No. They are separate loads and rarely equal. A home usually needs more heating BTU than cooling BTU, so a furnace is sized to the heating load while the AC or heat pump is sized to the cooling load. On a heat pump, one unit must satisfy both, which is why proper sizing matters even more.
How do BTU and tons relate?
For cooling, 12,000 BTU per hour equals one ton. So 24,000 BTU is 2 tons, 36,000 is 3 tons, and 48,000 is 4 tons. Furnaces and heating are quoted in BTU rather than tons, which is why a heating figure looks larger even when it is the same size class of equipment.
How do I size a furnace by BTU?
Match the furnace's BTU output to your home's heating load. Furnaces list a BTU input and an AFUE efficiency; output equals input times AFUE. A 90,000 BTU input furnace at 95 percent AFUE delivers about 85,500 BTU of usable heat. Size to the load rather than oversizing, since an oversized furnace short-cycles and heats unevenly.
Get Both Loads Sized Right.
We run a Manual J for heating and cooling, then match the furnace and AC - or a single heat pump - to your home, with a flat-rate written quote.