Heat Loss Calculator — Walls, Windows & Air Leakage
Estimate design heat loss through walls, windows, ceiling, floor and air leakage, with an editable BTU/h and kW breakdown.
Required heat at 20°F outdoors
2.84 kW including a 10% planning reserve. Edit every envelope and leakage assumption below.
1. Building geometry and design temperature
Use exterior dimensions for one heated rectangular zone. For complex homes, calculate each zone and add the loads.
2. Whole-assembly U-factors
Use whole-assembly values, including framing and thermal bridges—not insulation-batt labels alone. U = 1 ÷ R in imperial units.
Where the heat goes
Geometry and calculation
| Net opaque wall | 720 ft² |
| Windows + doors | 80 ft² |
| Ceiling / floor | 600 ft² each |
| Heated volume | 4,800 ft³ |
| Air leakage flow | 40 CFM |
| Temperature difference | 50°F |
Infiltration: 1.08 × 40 CFM × 50°F = 2,160 BTU/h
Design: (6,662 + 2,160) × 1.10 = 9,704 BTU/h
Heat loss formula
Each surface uses Q = U × A × ΔT. U-factor describes heat flow through the complete assembly, area is the exposed surface, and ΔT is the indoor–outdoor design temperature difference. Air leakage is calculated separately from volume and ACH.
U-factor versus R-value
U and R are reciprocal only when they use the same unit system: U = 1/R. A whole wall usually performs worse than cavity insulation because studs, plates, fasteners and junctions bridge the insulation.
How to use the result
Compare the required output—not merely the nominal equipment label—at your winter design temperature. A blower-door-derived natural ACH, verified assembly U-factors and room-by-room geometry provide a stronger estimate.
What this does not calculate
This is a steady-state winter transmission and infiltration estimate. It does not calculate cooling/latent loads, solar gain, duct loss, slab edge geometry, ventilation heat recovery, intermittency or equipment cycling.
Frequently asked questions
Is this a Manual J calculator?
No. It exposes useful heat-loss physics but does not implement the full ACCA procedure or replace a local code calculation.
Can I enter ACH50 from a blower-door test?
No. ACH50 is measured under an artificial 50-pascal pressure. Convert it to an approved natural infiltration method before entering ACH here.
Should the floor always be included?
Only include heat flow across the thermal boundary. For a conditioned basement or slab, use the appropriate exposed assembly and a local ground-loss method rather than blindly using the default floor value.
Why not size a heat pump from square feet?
Two equal-size buildings can have very different windows, insulation, leakage and design temperatures. Capacity at cold outdoor conditions matters more than a generic BTU-per-square-foot rule.
