BTU Calculator: Cooling and Heating by Room or Home Size

Adjust floor area and climate zone to explore an illustrative cooling-load split across walls, roof, windows and sun, infiltration, people and appliances, and ceiling height. The stacked bar and legend show how each share contributes to the total as you change the inputs. This example divides the area-based estimate into fixed shares and adds separate ceiling-height and extra-occupant adjustments, using sunny exposure, 9-ft ceilings, four occupants, and average insulation. Compare the categories to understand the sources of heat, then use the main calculator to explore how insulation, sun exposure, ceiling height, and occupancy affect your screening estimate.

The implemented formula is an area-rate screening model, not a sum of independently calculated wall, window and infiltration loads. It multiplies area by the selected climate rate, insulation and cooling sun factors, applies the ceiling multiplier, then adds extra-occupant cooling heat. The conduction equation beside it explains the physical calculation a more detailed method would use; this tool does not ask for surface U-values and areas.

These examples all use zone 4, average insulation and sun, 8-ft ceilings and two occupants. Room names describe the example area; they do not add cooking, appliance or bathroom loads. Larger equipment and whole-house systems require their own design check.

• Vaulted, cathedral, or 12+ ft ceilings — measure actual envelope surfaces and consider stratification; volume × ACH calculates airflow, not a complete thermal load
• 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
• High-performance assemblies — use measured or specified whole-assembly U-values rather than inferring load from a construction label
• Open floor plans or connected unconditioned spaces — establish the served boundary and account for heat and air transfer across it

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 has sensible and latent portions. Sensible heat changes air temperature; latent heat is associated with moisture that a cooling coil must remove. Outdoor-air leakage, ventilation and occupants can contribute to both. The proportions depend on climate, indoor humidity and the building, so a fixed latent percentage should not be treated as measured performance. This calculator produces one total screening estimate and does not separate sensible from latent load. For equipment selection, obtain both figures from a detailed load calculation, then check manufacturer capacity at the chosen airflow and entering-air conditions. Repeated short cycles can make moisture control harder even when the thermostat reaches setpoint.

The popular "20 BTU per square foot" shortcut is a rough screening assumption, while this tool uses 25 for its zone-4 baseline. 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 — compare the actual window U-values rather than assuming a fixed ratio between single glazing and low-E double panes, and a high solar heat gain coefficient (SHGC) on west glass admits late-afternoon sun when outdoor temperatures peak
• Sun exposure — orientation, shading and time of day affect solar gain; the tool uses 0.90 for shaded exposure and 1.20 for very sunny exposure; real solar gain needs glazing data
• Ceiling height — this model scales the area-based load above the 8-ft baseline; a 10-ft ceiling adds 25% before occupant gains
• 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 — activity changes sensible and latent occupant gains; this tool uses 600 BTU/h per person above two as a total screening allowance, and a kitchen oven or a rack of computers can swamp the envelope load entirely

Room use matters as much as room size. A kitchen can have cooking, refrigeration and exhaust makeup-air loads that are absent from a bedroom. A home office may have substantial electronics heat; a sunroom may be governed by glazing and solar exposure. This calculator has no appliance or ventilation inputs, so changing the room name cannot account for those loads. As an electrical heat-gain illustration, equipment releasing 500 W entirely into the room adds about 1,706 BTU/h, using 1 W = 3.412 BTU/h. Do not add a universal kitchen allowance: appliance use, exhaust capture and operating schedules determine what belongs in the room load.

Divide a BTU/h rate by 12,000 to express it in refrigeration tons: 30,000 BTU/h is 2.5 tons and 48,000 is 4 tons. The tonnage calculator compares nominal half-ton central-AC references, raising the nearest step if it would fall below 95% of load. Actual equipment lines may use different sizes or modulation ranges. The heat load calculator adds adjustable winter temperature and infiltration to an estimated envelope loss; it still does not itemize your wall and window assemblies. Use measured envelope data in a full Manual J when you need a design report.

Supply airflow must also match the selected equipment. At the common reference of 400 CFM/ton, a 3-ton system corresponds to 1,200 CFM, but the approved blower setting comes from the manufacturer rather than the ton label alone. Use the CFM calculator to compare tonnage-based airflow with sensible-load airflow. SEER2 and EER2 describe cooling efficiency, HSPF2 describes heat-pump heating efficiency, and AFUE describes fuel-fired heating efficiency. None is a substitute for capacity. A furnace's input fuel rating differs from its useful heating output, and a heat pump must be checked at winter design temperature.

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.

The area estimate multiplies floor area by a site climate-zone rate, insulation and sun factors, and max(1, ceiling height / 8). It adds 600 BTU/h per occupant above two. Zone 4 uses 25 BTU/h per sq ft for cooling and 35 for heating. These are screening assumptions, not rates prescribed by ACCA or ASHRAE.

At the default 500 sq ft in zone 4, average insulation and sun, 8-ft ceilings, and two occupants: cooling = 500 x 25 = 12,500 BTU/h (1.04 refrigeration tons). Heating = 500 x 35 = 17,500 BTU/h. Changing only the ceiling to 10 ft multiplies each base load by 10 / 8 = 1.25: 15,625 cooling and 21,875 heating BTU/h.

The heating tab is an area-rate estimate at the zone profile's 70 deg F indoor reference. It does not separately calculate infiltration. Refrigeration tons can express any heat-transfer rate, but a heating equivalent in tons is not a furnace input rating. For a temperature-sensitive heating estimate, use the heat-load calculator. Nominal central AC references use half-ton steps. The nearest step is raised if it falls below 95% of the estimate; values outside 1.5-5 tons are flagged. This is not equipment selection. Commonly cited ACCA Manual S guidance is about 95-115% of total cooling load, with heat-pump allowances up to about 125% in heating-dominated climates. Verify the applicable edition, equipment type, sensible and latent capacity, and manufacturer performance at design conditions.

Include winter temperature and infiltration.

ACCA Manual J is the residential load-calculation reference, Manual S covers equipment selection, and Manual D covers ducts. The area rates and adjustment percentages in this tool are site assumptions, not values supplied by those standards.

ACCA Manual S: residential equipment selection. For the load-calculation workflow, read how to calculate HVAC load.