Heat Loss Calculator

How the heat loss calculation works

Heat leaves a heated space two ways: through the surfaces (conduction) and with the air that leaks out and is replaced by cold air (infiltration). For each surface, the loss is Q = U × A × ΔT: the U-factor, the area in square feet and the difference between the indoor and outdoor design temperatures. For the air, it is 0.018 × volume × air changes per hour × ΔT. Add them all for the design heat loss in BTU per hour: the heat the system has to replace on a design-cold day.

Worked example: a 1,500 sq ft single-story house with 8 ft ceilings, 70 °F indoors and 5 °F outdoors, so ΔT = 65 °F. Walls 1,200 sq ft at R-13: 1,200 ÷ 13 × 65 = 6,000 BTU/h. Windows 200 sq ft at U 0.50: 200 × 0.50 × 65 = 6,500. Doors 40 sq ft at U 0.17: 442. Ceiling 1,500 sq ft at R-30: 3,250. Floor 1,500 sq ft at R-19 over a vented crawl space: 5,132. Air: 12,000 cubic feet × 0.5 × 0.018 × 65 = 7,020. Total: about 28,344 BTU per hour.

Enter wall area net of the windows and doors, which have their own lines. For one room, enter only the surfaces that face outdoors or an unheated space, and the room's own floor area and ceiling height for the air.

R-value and U-factor

Insulation is rated by R-value, its resistance to heat flow; the higher, the better. Windows and doors are rated by U-factor, the heat that gets through per square foot, per hour, per degree; the lower, the better. They are inverses: U = 1 ÷ R. Penn State's window label guide puts it this way: an R-value of 4.0 is the same as a U-factor of 0.25.

The R-value on an insulation bag is for the insulation, not the whole wall: studs, plates and headers carry heat around it, so a framed wall's whole-assembly R is lower than the batt's. If you know the assembly value, use it; the calculator takes what you type.

U-factors and R-values to start from

Starting points for the inputs, from the sources below. Use the label, the spec sheet or what you find on site whenever you can.

Air changes per hour

Air changes per hour (ACH) is how many times the air in the heated volume is replaced by outside air in an hour under normal conditions. The Los Alamos solar heating handbook puts the usual range at one-half to one air change per hour. Build It Solar's calculator suggests 0.33 for a very tight house, 0.5 for new, careful construction and 1.0 for typical existing construction, with a question mark on the last.

A blower door test measures leakage at 50 pascals (ACH50), which is not the same number: it is many times the natural rate. Do not type an ACH50 figure here. If you only have ACH50, ask the energy auditor for the natural air change rate.

Design temperatures

The result is only as good as ΔT. The indoor design temperature is the one the system has to hold; the outdoor design temperature is a cold value for the location that is exceeded only a small share of winter hours, taken from ACCA Manual J or the ASHRAE climate data, not the coldest day on record. The NGMA heat loss standard notes that the 97.5% winter dry bulb from the ASHRAE data is common practice.

Floors over a crawl space or basement lose heat to that space, not to the outdoors. If you know its winter temperature, type it under "Temperature under the floor"; left empty, the calculator uses the outdoor design temperature, which overstates the floor loss for most basements. Slab-on-grade floors lose heat mainly at the edges and are not modelled here.

Before you size equipment

This is the heat-loss half of a load calculation, done with your numbers. ANSI/ACCA Manual J is the standard method for residential loads: it works room by room with tables for each assembly, counts duct losses and sets the design conditions, and Manual S then selects the equipment. This page is not a substitute for that calculation. Use it to understand where the heat goes, to check a quote, and to see what an upgrade such as new windows or attic insulation would change.

For HVAC and home-services reps

The table under the result is a sales conversation: it shows the homeowner which part of the house loses the most heat. Change one line, say the windows from U 1.0 to U 0.30 or the attic from R-19 to R-49, and the drop in BTU per hour shows what the upgrade is worth before anyone talks price. Measure, note what you see in each room, and run the full load calculation before you quote equipment.

Sources, each read on 2026-10-28. All results are estimates:

• National Greenhouse Manufacturers Association: Heat Loss standard (Q = UA(Ti − To), Q = 0.018NV(Ti − To), 97.5% winter dry bulb)
• Los Alamos LA-6242-MS: Pacific Regional Solar Heating Handbook, building thermal load (0.018 BTU/°F ft³, one-half to one air change)
• Penn State EGEE 102: Understanding Window Performance Labels (U = 1/R, window U-factors)
• ENERGY STAR Residential Windows, Doors and Skylights Version 7.0 specification (U-factor limits)
• ENERGY STAR: Recommended home insulation R-values (attic, floor, basement wall by zone)
• Build It Solar: Home Heat Loss Calculator (air change guidance, 18,000 cu ft at 0.5 ACH = UA 162)

Picking the furnace? The furnace size calculator turns a load into an input rating at your AFUE. For cooling, the BTU and AC size calculator gives the rule-of-thumb size, and the insulation calculator works out the bags or batts for a new R-value.

Related tools

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• Insulation CalculatorWork out how much insulation an attic, wall or floor needs from its area and target R-value, in batts, rolls or bags, with cost.Open the tool
• CFM and Duct Size CalculatorWork out the CFM a room needs from air changes per hour, tons of cooling, bathroom size or whole-house ventilation, and size a round or rectangular duct at a chosen velocity.Open the tool