The heat loss calculator above adds up the steady-state heat leaving a building at your winter design condition. It takes each part of the envelope in turn — walls, ceiling, floor, windows and doors — multiplies area by U-value to get a conductance, sums those into a single UA figure, multiplies by the indoor-to-outdoor temperature difference, then adds the heat carried out by air leaking through the shell. The answer comes back in BTU per hour and in watts, with the fabric and infiltration halves shown separately so you can see which one is actually driving the number.
This is a preliminary sizing and teaching tool published by Arb Digital. It is not a stamped design, it is not a substitute for a full room-by-room load calculation, and no equipment should be selected from it. A licensed mechanical contractor or engineer performs the calculation that a heating system is actually sized on, and in most jurisdictions that calculation is required by code before a permit is issued. Read the number here as an order-of-magnitude check on someone else's work, or as a way to understand which part of a building is costing you the most.
What This Heat Loss Calculator Does
It computes a whole-building envelope heat loss using the conduction and infiltration terms only. Every element you enter contributes U × A to a total conductance, expressed in BTU per hour per degree Fahrenheit or in watts per kelvin. That conductance is the honest measure of how leaky the shell is, independent of the weather. Multiplying it by the design temperature difference converts it into a rate of heat flow at the coldest condition you have chosen to design for.
Two things it deliberately does not do. It does not publish a table of U-values, and it does not publish design temperatures. Both are location-specific and assembly-specific, and typing them from memory is exactly how estimating tools go wrong. The values pre-loaded in the fields are placeholders so the page shows a working example on load — replace every one of them with the rated figure for your actual assembly and your actual location. The window U-factor comes off the NFRC label. Opaque assembly U-values come from the ASHRAE Handbook of Fundamentals or from a manufacturer's tested assembly. Winter design dry-bulb temperatures come from the ASHRAE climatic design data set that ACCA Manual J is built on.
Where our thermal conductivity converter simply moves a k-value or an R-value between unit systems, this page uses those properties to produce a building-level result. If you want to work in the other direction and see how much insulation an area needs, the insulation calculator handles material quantity, and the wall area calculator gives you the net wall figure this page asks for.
How to Use It
- Choose your unit system first. The U-value fields change meaning between the two: imperial expects BTU/(h·ft²·°F), metric expects W/(m²·K). Mixing them silently produces an answer roughly 5.7 times wrong.
- Enter the two design temperatures. Indoor is the temperature you intend to hold. Outdoor is the winter design dry-bulb for your location, usually the temperature exceeded 99 or 99.6 percent of the time — not the coldest night on record.
- Enter each envelope element as net area plus U-value. Subtract window and door area from the gross wall area before entering it, so no square foot is counted twice.
- Set ceiling height and the air change rate. Height times floor area gives the volume the infiltration term uses. The air change rate should come from a blower-door test converted to natural conditions, not from a guess.
- Read the split, not just the total. The breakdown bars show which element dominates. That is usually more useful than the headline number.
The Formula / How It's Calculated
The conduction term is the classic steady-state form: Q = U × A × ΔT, summed over every element. U is the thermal transmittance of the assembly, the reciprocal of its total R-value including air films. A is the area of that assembly measured on the plane it occupies. ΔT is indoor design temperature minus outdoor design temperature. Adding the individual U×A products first gives the total envelope UA, and UA × ΔT is the whole fabric loss.
The infiltration term treats leaked air as a mass flow that must be heated from outdoor to indoor temperature. In imperial units the shortcut is Q = 1.08 × CFM × ΔT, where 1.08 bundles the density and specific heat of air at standard conditions with the minutes-to-hours conversion, and CFM is the building volume in cubic feet times the air change rate divided by 60. In SI the equivalent is Q = 0.33 × ACH × V × ΔT watts with volume in cubic metres. Both are approximations that assume dry air at ordinary density; at high altitude the constant drops because the air is thinner.
Worked example using the loaded defaults: walls 1,600 ft² at U 0.06 gives 96, ceiling 1,200 at 0.03 gives 36, floor 1,200 at 0.05 gives 60, windows 240 at 0.30 gives 72, doors 40 at 0.20 gives 8. The envelope UA is 272 BTU/h per °F. With a 65 °F design difference the fabric loss is 17,680 BTU/h. The volume is 1,200 × 9 = 10,800 ft³, so 0.35 air changes per hour is 63 CFM, and 1.08 × 63 × 65 is 4,423 BTU/h. Total design heat loss is 22,103 BTU/h, or about 6,477 watts.
Why Infiltration Is the Number Everyone Gets Wrong
In the example above, air leakage is only a fifth of the total. Change the air change rate from 0.35 to 1.0 and the infiltration loss triples to 12,636 BTU/h, pushing the total past 30,000 — a 37 percent increase from one field. That sensitivity is the whole reason blower-door testing exists. You cannot see air leakage, you cannot measure it with a tape, and the difference between a tight new build and a leaky older house is a factor of five or more.
The trap is that the number entered here is a natural air change rate, while a blower door reports air changes at 50 pascals of pressure difference. Those are not the same quantity and one cannot be substituted for the other. Converting between them requires a divisor that depends on climate, building height, shielding and exposure, and the correct method is set out in the relevant standard rather than being a single fixed number. If someone hands you an ACH50 figure and you type it straight into a heat loss calculation, the answer will be enormously too large.
There is a second subtlety. Below a certain leakage rate, code requires mechanical ventilation, and that deliberate ventilation air also has to be heated. A very tight house does not have a small ventilation load — it has a controlled one. Our air changes per hour calculator covers the ventilation rate side of that question in its own right.
Design Temperature Is a Choice With Consequences
Picking a colder outdoor design temperature makes every number on this page bigger, and it is tempting to add a margin "to be safe". Doing that is how heating systems end up two sizes too large. An oversized system short-cycles, which reduces efficiency, wears the equipment, and gives poor humidity control and uneven room temperatures. The design condition is deliberately not the worst case: it is a statistical value that is exceeded a small percentage of the hours in a year, and the system is allowed to run continuously for those few hours.
The indoor number matters too, and it is usually less dramatic than people expect. Going from 68 to 72 °F indoors with a 5 °F outdoor design raises ΔT from 63 to 67, about six percent more heat loss. Going from a 5 °F outdoor design to a −10 °F one raises it from 65 to 80, about 23 percent. The outdoor value is the lever. It should come from published climatic data for your specific location, which is why this tool asks for it rather than assuming a climate.
Reading the Element Breakdown
The bar chart under the results is the part worth acting on. In most older buildings the ranking is windows first per square foot, walls first in absolute terms because there is so much of them, and ceiling last because attic insulation is cheap and usually already there. In the default example, 240 ft² of window contributes 72 BTU/h/°F while 1,600 ft² of wall contributes 96 — the glass is less than a sixth of the area and more than two-fifths of the wall's contribution.
That ratio is the argument for looking at glazing before anything else on a retrofit, and it is also the argument for being sceptical of headline R-values. An assembly rated R-20 in the lab does not deliver R-20 once studs, plates and headers are counted, because the framing conducts heat around the insulation. The whole-assembly U-value that accounts for that is what belongs in this calculation. The Building America Solution Center, run by the US Department of Energy, documents assembly details and where those thermal bridges occur.
What This Calculation Leaves Out
Several real effects are absent, and each of them matters in a full load calculation. Ground-contact heat loss through slabs and basement walls does not follow the simple U×A form, because the soil is a large thermal mass with a temperature that varies with depth and season; slab-edge loss is normally handled as a per-linear-foot F-factor instead. Internal gains from people, lighting and appliances offset heating load and are subtracted in a proper Manual J calculation. Solar gain through south-facing glass does the same on a sunny winter day. Duct losses in unconditioned space can add a substantial fraction and are calculated separately.
Thermal mass is also ignored: this is a steady-state calculation, so it assumes the temperature difference has been held long enough for the flows to settle. That is a reasonable design assumption for a cold night, but it does not describe a recovery from setback. And the whole-building result here is a total, not a room-by-room distribution — you cannot size a radiator or a register from it, because that requires the load of each individual space. For a rough capacity cross-check on the cooling side, the AC BTU calculator approaches the problem from the opposite direction, and the furnace size calculator covers the heating equipment question.
Standards and Who Governs the Answer
The underlying physics is documented in the ASHRAE Handbook of Fundamentals, and the procedures that codes actually reference sit in ASHRAE standards and guidelines and in ACCA Manual J for residential load calculation. Manual J is an ANSI-approved standard and is the method most US jurisdictions require for residential equipment sizing. Commercial work is governed by the energy code adopted locally, typically derived from ASHRAE Standard 90.1 or the International Energy Conservation Code.
Local code governs, and amendments differ by state, province and country. The United Kingdom and Ireland work from different documents again, with U-value limits set in the national building regulations rather than by ASHRAE. Wherever you are, the authority having jurisdiction decides what calculation is acceptable, and a licensed installer or engineer signs off the result. This page is not that calculation.
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Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Entering R-value where U-value is asked for — they are reciprocals, so R-20 is U 0.05, and typing 20 into the U field overstates the loss by four hundred times.
- Using gross wall area with windows entered separately — the openings then appear twice and the wall loss is inflated.
- Typing an ACH50 blower-door result into the air change field — that is a pressurised-test figure, not a natural rate, and it can overstate infiltration by a factor of ten or more.
- Padding the outdoor design temperature — an added safety margin here produces an oversized system that short-cycles and controls comfort worse, not better.
- Treating the total as an equipment size — heating capacity selection follows a room-by-room load calculation, duct analysis and manufacturer performance data at the design condition.
Related Free Tools From Arb Digital
Pair this with the insulation calculator for material quantities, the thermal conductivity converter for moving R and U between unit systems, the wall area calculator for the net areas this page needs, the air changes per hour calculator for ventilation rates and the AC BTU calculator for the cooling side. The full free online tools hub lists every calculator we publish.
Frequently Asked Questions
No. Manual J is a room-by-room procedure that includes internal gains, solar gain, duct losses and ground-contact terms, and it is what codes require for residential equipment sizing. This page computes whole-building conduction plus infiltration only, as a preliminary estimate.
Window U-factors are printed on the NFRC label for the complete unit. Opaque assembly U-values come from the ASHRAE Handbook of Fundamentals or from a manufacturer's tested assembly. This page publishes no U-value table because the correct figure depends on the exact construction, including framing factor.
The winter design dry-bulb for your location, usually the 99 or 99.6 percent value from the ASHRAE climatic design data set that Manual J uses. It is not the record low, and adding a margin to it leads to oversized equipment.
One BTU per hour per square foot per degree Fahrenheit equals 5.678 watts per square metre per kelvin. Switch the units selector before entering values, because the U fields mean different things in each system.
Almost always because a blower-door ACH50 figure has been entered instead of a natural air change rate. Those are different quantities, and converting between them requires a climate and exposure dependent divisor set out in the applicable standard.
Not properly. Ground-contact heat loss does not follow a simple area times U-value form and is normally handled with a per-linear-foot slab-edge factor. Enter a floor only where it faces an unheated space such as a vented crawl space.
No. Equipment selection needs a room-by-room load, duct or emitter analysis and manufacturer capacity data at the design condition, and it must be done by a licensed installer. Treat the figure here as a sanity check only.
Because they are fixed by different things and improved by different work. Fabric loss responds to insulation and glazing; infiltration responds to air sealing. Seeing the split tells you which intervention is worth costing first.
This tool gives preliminary envelope estimates for education and planning only. It is not a structural or mechanical design, it is not stamped, and it does not replace a load calculation by a licensed engineer or mechanical contractor. Local code governs.
