BTU Calculator

The cooling load isn't one number — it's a sum of envelope conduction (walls, roof, windows), solar gain, infiltration, and internal gains from people and equipment. The chart below shows the typical share of each source, and how it shifts with home size and climate zone.

BTU/hr is the sum of conduction through the envelope, infiltration losses, solar gain, and internal gains. The simplified rule-of-thumb form is:

• Vaulted, cathedral, or 12+ ft ceilings — use volume × ACH instead of area × factor
• Walls of glass facing west — solar gain dominates; size on glazing schedule, not floor area
• Server rooms or kitchens with ovens — internal heat gain can exceed envelope load
• Insulated metal/SIP walls — much lower load than the rule of thumb suggests
• Open floor plans connected to unconditioned space — ignore the connecting room and use volume

A British Thermal Unit is a fixed quantity of energy: the heat required to raise one pound of water by one degree Fahrenheit. HVAC equipment is rated in BTU per hour — a rate of energy transfer — which is why a window air conditioner is labeled 5,000 or 8,000 and a central system is labeled in tons, where one ton equals 12,000 BTU/hr. The number this tool produces is an estimate of that rate at design conditions, not the energy your system will consume over a season. Two homes of identical floor area can need very different BTU/hr because the load is driven by how fast heat crosses the building envelope, not by square footage alone.

Every cooling load splits into two parts. Sensible heat is the temperature load — conduction through walls and the roof, solar gain through glass, and warm air leaking in. Latent heat is the moisture load — humidity entering through infiltration and exhaled by occupants that the coil must condense out. A simple square-foot rule lumps these together, but in a humid Gulf Coast climate the latent share can be a quarter of the total, which is one reason an oversized unit that short-cycles often leaves a house cold and clammy. The ASHRAE Handbook of Fundamentals and ACCA Manual J both separate these loads explicitly; this calculator approximates them through its occupant and climate factors.

The popular "20 BTU per square foot" shortcut is only a starting point for an average envelope in a temperate IECC climate zone. The variables below explain why your real load can land far above or below that figure, and why a thorough estimate weights them rather than treating floor area as the whole story.

• Insulation and U-value — walls and ceilings are rated by R-value (resistance) or its inverse U-value; doubling attic R-value can cut the roof conduction term substantially in hot zones
• Windows and orientation — single glazing has roughly five times the heat flow of a low-E double pane, and a high solar heat gain coefficient (SHGC) on west glass admits late-afternoon sun when outdoor temperatures peak
• Sun exposure — south- and west-facing rooms collect the most solar gain; the same room on a shaded north wall may need 15–20% less cooling
• Ceiling height — load scales with air volume, so a 10-foot ceiling adds about a quarter over the 8-foot baseline
• Infiltration — gaps around rims, ducts, and penetrations let outdoor air in; a leaky house in a cold zone can lose more heat to infiltration than through its insulated walls
• Occupancy and equipment — each person adds roughly 230 BTU/hr sensible plus latent load, and a kitchen oven or a rack of computers can swamp the envelope load entirely

Room use matters as much as room size. A bedroom is occupied while people sleep and generates little internal heat, so it sizes close to the envelope load. A kitchen runs ovens and a refrigerator and often carries west-facing glass, so it can need 4,000 BTU/hr more than a bedroom of the same dimensions. Home offices full of monitors, sunrooms wrapped in glazing, and media rooms with heat-shedding electronics all break the floor-area rule for the same reason — internal and solar gains rise faster than the walls can shed them.

Once you have a BTU/hr figure, converting to equipment language is arithmetic: divide by 12,000 to get tons. A 30,000 BTU/hr cooling load is 2.5 tons; a 48,000 BTU/hr load is 4 tons. Manufacturers build in half-ton steps, so you round to the nearest available size rather than the nearest convenient one. Our tonnage calculator does that conversion directly, and the heat load calculator works the conduction side in more detail when you know your wall and window assemblies.

Matching airflow to capacity is the other half of the job. A well-designed system delivers about 400 CFM of air per ton, so a 3-ton unit moves roughly 1,200 CFM through ductwork that ACCA Manual D is meant to size. Equipment efficiency ratings — SEER2 and EER2 for cooling, HSPF2 for heat pumps, and AFUE for furnaces — describe how much energy a unit uses to deliver that capacity, not the capacity itself, so a higher SEER2 does not change the BTU/hr you need to meet. DOE minimums and the EPA ENERGY STAR program set the thresholds those ratings are measured against, while AHRI publishes the certified performance data manufacturers cite.

Treat this estimate as a sanity check, not a permit-ready design. A rule-of-thumb number tells you whether a quote is in the right ballpark and helps you avoid the oversizing that causes short cycling and poor humidity control. For the actual installed equipment, a contractor should run an ACCA Manual J load calculation on your specific envelope, then Manual S to select the matched unit. If you are starting from scratch and want a plain-language walkthrough of the sizing decision, our guide on what size AC you need covers the same trade-offs without the worksheet.