Window U-Value and Energy Savings Calculator
Compare single, double, and triple pane windows with different frame types.
Estimate heat loss and potential energy savings for your home.
How Window Sizing Works
Window sizing involves both the rough opening (the framed hole in the wall) and the actual window unit size. Manufacturers specify windows by their unit dimensions — the rough opening is always slightly larger to allow for shimming and leveling.
Rough opening formula:
Rough Opening Width = Window Unit Width + 2 inches Rough Opening Height = Window Unit Height + 2.5 inches
The extra space allows for a double top plate, sill plate, and shimming on all sides.
Window-to-floor-area ratio (natural light standard):
Building codes require windows to provide minimum natural light and ventilation:
Minimum window area = Room floor area × 8% (light) Minimum operable area = Room floor area × 4% (ventilation)
Worked example:
- Bedroom: 12 ft × 14 ft = 168 sq ft
- Minimum glass area: 168 × 0.08 = 13.4 sq ft
- A 3×5 ft window = 15 sq ft — passes code
Standard window sizes by room:
- Bedroom (egress required): minimum 5.7 sq ft net opening, 24 in height, 20 in width
- Kitchen: typically 30×48 inches above countertop
- Living room: commonly 36×48 to 48×60 inches
- Bathroom: obscure glass common, 24×36 inches typical
Energy performance ratings:
- U-factor (lower is better): measures heat loss; < 0.30 is excellent for cold climates
- SHGC (Solar Heat Gain Coefficient): higher values wanted in cold climates (passive solar heat), lower in hot climates to reduce cooling load
Always order windows to the unit dimension and frame the rough opening accordingly — never the reverse.