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Heating & Cooling Load Screening Calculator

Estimate whole-house heating and cooling loads from envelope areas and U-values — a screening check against a contractor's figure.

Computed in your browser — nothing you enter is uploaded. Figures are presented in United States units and terminology, and each formula’s basis is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Design heat loss

5,710 W

Check your inputs

Steady-state whole-house loss. Sound as a screening figure and as a check on a quote; not a substitute for a room-by-room calculation, which is what sizes emitters.

Fabric loss
4321.11 W
Ventilation loss
1386.05 W
Kilowatts
5.71 kW
BTU/h
19473.64 BTU/h
Ventilation as a share of the total
24.29 %

Running these inputs gives 5707 W as the design heat loss. Fabric loss carries the most weight in this calculation, at 4321 W. Expect some drift against the real job; the calculation is solid but conditions on site are not. Currently reading for United States — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 5,710 W — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Regulatory standards & verification citations

  • Steady-state fabric heat loss: Q = Σ(U × A × ΔT), the basis of every load method including ACCA Manual J and CIBSE Guide A
  • Ventilation heat loss: Q = 0.33 × n × V × ΔT (W), CIBSE Guide A — 0.33 Wh/m³K is the volumetric heat capacity of air
  • ACCA Manual J (8th ed.) for the full room-by-room procedure this approximates

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why is this not called a Manual J calculator?
Because it is not one, and naming it that would be a misrepresentation. Manual J is a defined room-by-room procedure with specific tables for glazing, orientation, shading, infiltration and duct losses. This is the steady-state equation underneath it applied to the whole house. It will tell you whether a quoted boiler is roughly right; it will not tell you what size radiator the back bedroom needs.
Why does ventilation matter so much?
Because heating outdoor air to indoor temperature costs energy, and a draughty house does it continuously. The breakdown shows ventilation as a share of the total — in an old house it is routinely 40-50%, which is why airtightness work often beats adding insulation for the same money.
Should I size the boiler to this figure?
Close to it, plus an allowance for domestic hot water and for warm-up after setback. What you should not do is add a large arbitrary margin: an oversized boiler cycles, runs below its condensing temperature, and delivers worse efficiency than the label promises. Modern condensing plant performs best when it is running near its calculated load, not far below it.
Where do I get a U-value?
From the assembly. A single-glazed window is around 5.0 W/m²K, double glazing 1.2-2.8, modern low-E around 1.0. Uninsulated solid brick is roughly 2.0, a filled cavity 0.5-0.6, modern insulated wall 0.2-0.3. Area-weight them across your envelope — glazing pulls the average up sharply even at modest area.