Window Heat Loss Calculator
The heat that flows through a window or door — from its area, its U-value and the temperature difference across it — in watts and over a heating season, with the energy cost and what upgrading to a lower U-value would save.
Heat flow through a window is U-value times area times the temperature difference: a 2 m² single-glazed window at U 5.
How the window heat loss calculator works
Heat flow through a window is U-value times area times the temperature difference: a 2 m² single-glazed window at U 5.8 loses about 230 W on a day 20 °C colder outside than in. Over a heating season of degree-hours that is kilowatt-hours, and at your tariff a cost. Double glazing at U 2.8 halves it; a good low-emissivity unit at 1.4 cuts it by three quarters, which is the arithmetic behind the payback.
Formula: Q = U × A × ΔT; season kWh = U × A × degree-hours / 1000
Worked examples
| Inputs | Heat loss at the design difference (W) | Note |
|---|---|---|
| Four single-glazed windows | 776 | hundreds of watts |
| Already double glazed | 375 | half the loss |
| A colder climate | 1,293 | a faster payback |
FAQFrequently asked questions
What is a U-value?
The heat flow through a square meter of the element per degree of temperature difference, in W/m²K. Lower is better: single glazing about 5.8, standard double 2.8, low-e argon-filled double 1.4, triple under 1.
What are degree-hours?
The sum over the heating season of the temperature difference between inside and outside, hour by hour. Mild climates have 20,000 to 40,000 °C·h; cold ones 60,000 to 100,000. Degree-days times 24 gives the same figure.
What does the heat cost?
The price of delivered heat — electricity at its tariff divided by a heat pump's coefficient of performance, gas at its price divided by the boiler's efficiency. The default is a typical blended figure.
Is the payback realistic?
For single-to-double it is usually ten to twenty years on heat alone; comfort, noize and condensation are the reasons people actually do it. For double-to-triple it is longer still.
What about frames and doors?
A window's whole-unit U-value includes the frame, and doors are the same sum with their own U-value. Enter the unit figure the maker quotes, not the glass alone.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Gypsum Association — gypsum board sizes — 4 × 8 and 4 × 12 ft sheets
- ADA Standards for Accessible Design — ramps (§405) — maximum running slope 1:12
- International Code Council — the International Residential Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.