Heat Loss Calculator

Heat Loss Calculator works out how fast your building loses heat on a design winter day — the parent calculation behind every boiler, furnace, heat pump and insulation decision. Enter each part of the envelope with its own area and R-value, add your air leakage rate, and read the total in BTU/h and kW along with a ranked breakdown of exactly where the heat is going. It works completely offline and nothing you enter ever leaves your phone.

HOW IT WORKS
The app follows the standard steady-state heat-loss chain:

Design temperature difference. ΔT is your indoor set point minus the 99 percent winter design outdoor temperature for your location — the value exceeded 99 percent of the hours in a year. That number is published by ASHRAE and adopted by local building and energy codes. Because every other term is multiplied by it, getting it wrong scales the whole answer.
Fabric loss, element by element. For each part of the envelope, Q = U x A x ΔT, where U is 1 divided by the whole-assembly R-value. Walls, roof or ceiling, floor, windows and doors are entered separately because their U-values differ by an order of magnitude — a window at R-3 leaks more than eight times as fast per square foot as an R-25 wall.
Infiltration loss. Every cubic foot of cold air that leaks in has to be heated. Standard air takes 0.018 BTU to warm one cubic foot by one degree, so Q = 0.018 x air changes per hour x heated volume x ΔT. The metric form is 0.33 x ACH x volume in cubic metres x ΔT, in watts. This is the same constant behind the trade shortcut Q = 1.08 x CFM x ΔT.
Totals and a sanity check. The app adds the fabric and infiltration UA values, multiplies by ΔT, and reports the design heat loss in BTU/h, the same figure in kW, and the loss per square foot of floor so you can compare it against typical values.

A 1,600 square foot house with R-13 walls, an R-38 ceiling, an R-19 floor, 200 square feet of R-3 glazing and 0.5 air changes per hour, held at 70°F against a 5°F design day, comes out at 25,800 BTU/h — 7.55 kW, or 16.1 BTU/h per square foot. Air leakage alone is 7,488 BTU/h of that, 29 percent, and the single biggest line on the list.

WHAT YOU CAN DO

Enter walls, roof, floor, windows and doors separately, each with its own area and assembly R-value
Set your indoor set point and your published 99 percent winter design outdoor temperature
Adjust air changes per hour on a slider and watch the infiltration line move
Read the design heat loss in BTU/h and kW, the total UA, and the loss per square foot
See the ranked breakdown of where the heat goes, biggest first — the view that shows which upgrade actually pays
Open Details for the full step-by-step chain and a reference table of assembly R and U values
Learn the method, the 0.018 constant and the design temperature in plain language

BUILT FOR CLARITY
A warm plum design puts the big number first, with the loss ranking right beside it. Light and dark themes, a tiny download, no clutter and no account required.

AN ESTIMATE, NOT A MANUAL J
This is a simplified steady-state whole-house estimate. It has no internal or solar gains, no thermal mass, no room-by-room distribution, no duct losses and no moisture. It stops at the heat loss and does not select equipment. The correct basis for sizing a boiler, furnace or heat pump is an ACCA Manual J room-by-room load calculation, or an equivalent national method, by a qualified contractor — with Manual S for the equipment and Manual D for the ducts. Use this app to understand where the heat goes, plan a retrofit and sanity-check a proposal. For general information and education only; not professional engineering advice.

PRIVACY
The app collects no personal data and works fully offline. Ads are served by Google AdMob, which may use an advertising identifier as described in Google's policies.

Download Heat Loss Calculator and find out where your building's heat is really going.