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Calcimator

Window U-Factor Calculator

Calculate overall window U-factor from frame and glass components, with R-value, annual heat loss, and energy cost impact.

Inputs

%
in

Results

Overall U-Factor

0.26

R-Value

3.85

Annual Heat Loss31,200 BTU/sq ft
Energy Cost/sq ft/yr$0.94
How to Use This Calculator
  1. Enter the Frame U-Factor, Glass (center-of-glass) U-Factor, and Frame Percentage of total window area.
  2. Select the Spacer Type: aluminum (thermally conductive) or warm-edge (less bridging).
  3. Enter the Air/Gas Gap Width in inches.
  4. Review the Overall U-Factor — a weighted average of frame and glass performance.
  5. Check R-Value, Annual Heat Loss per sq ft, and Energy Cost Impact per sq ft per year.

How the result changes with Glass U-Factor

Glass U-FactorOverall U-FactorR-Value
0.240.253.97
0.590.531.88
1.010.871.15
1.361.150.87

What each input means

Frame U-Factor
U-factor of the window frame material (vinyl ~0.3, aluminum ~0.6, wood ~0.35).
Glass U-Factor
Center-of-glass U-factor (double Low-E ~0.25, triple ~0.15).
Frame Percentage
Frame area as a percentage of total window area (typically 15-30%).
Spacer Type
Warm-edge spacers reduce thermal bridging at the glass-frame junction.
Air/Gas Gap Width
Width of the insulating air or gas gap between glass panes.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Frame U-Factor = 0.3, Glass U-Factor = 0.25, Frame Percentage = 20, Spacer Type = 2 = 5 input(s) provided
  2. Calculate Overall U-Factor
    Overall U-Factor
    0.26 = 0.26
  3. Calculate R-Value
    R-Value
    3.85 = 3.85
  4. Calculate Annual Heat Loss
    Annual Heat Loss
    31200 = 31200
  5. Calculate Energy Cost/sq ft/yr
    Energy Cost/sq ft/yr
    0.94 = $0.94

Engine last updated . Checked against 1 independently-derived test — how we verify calculators.

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