Window U-Value and Energy Savings Calculator

What U-value actually measures

U-value (also called U-factor) is how fast heat escapes through a window: British Thermal Units per hour, per square foot, per degree Fahrenheit of difference across the glass. Lower is better. It is the inverse of the R-value used for insulation, which is why the two numbers move in opposite directions and confuse people constantly. An R-3 wall and a U-0.33 window are describing the same performance.

The formula:

Heat loss (BTU/hr) = U-value × Window area × Temperature difference

That is it. The comparison this calculator runs is just that formula twice, once for what you have and once for what you are considering, with the gap between them being the saving.

Typical whole-window U-values

The frame matters as much as the glass, which surprises people. An aluminium frame conducts heat so well that it drags a decent glass package down:

• Single pane, aluminium frame: 1.20 to 1.30
• Single pane, wood or vinyl frame: 0.95 to 1.10
• Double pane, clear glass: 0.45 to 0.55
• Double pane, low-emissivity coating: 0.30 to 0.35
• Triple pane, low-emissivity: 0.15 to 0.25

The jump from single to double pane roughly halves the loss. The jump from double to triple saves about a third again, which is a much smaller gain for a much larger price, and is why triple glazing earns its keep in genuinely cold climates and rarely anywhere else.

Reading the result honestly

The annual figure assumes roughly 5,000 heating hours at the average temperature difference you entered. That is a reasonable approximation for a cold-climate heating season and generous for a mild one, so treat the yearly saving as an upper bound rather than a promise.

Two things the arithmetic cannot see. Air leakage around a bad frame often loses more heat than the glass itself, so a draughty old window performs worse than its U-value suggests and replacing it beats the prediction. And solar gain runs the other way: a south-facing window earns heat back on sunny winter days, which the Solar Heat Gain Coefficient describes and this calculation ignores entirely.

How this relates to heating degree days

Engineers usually size a heating season in HDD (Heating Degree Days) rather than in hours at an average difference. The two are the same idea in different clothing: your temperature difference multiplied by 5,000 hours, divided by 24, is the degree-day figure this page is effectively assuming. Enter 30°F and you are modelling about 6,250 degree days, which is colder than Boston at roughly 5,500 and milder than Minneapolis at 7,500. If that does not match your town, scale the answer by the ratio.

Which of the three window pages you want

They look similar and answer different questions, so it is worth being blunt about it. This page compares two window packages across a whole house for a whole season, which is the upgrade decision. The window heat loss calculator does one window at one moment in either unit system, which is what you want when reading an NFRC (National Fenestration Rating Council) sticker or checking a single draughty room. The window replacement payback calculator is the one that asks whether the job is worth the money, using real degree days and the install quote.