Window U-Value to R-Value Converter
Convert between U-value and R-value for windows, doors, and insulated glass units.
Includes both US and metric (SI) systems.
Type in any field — the others update instantly. Supports both US and metric systems.
Understanding U-Value and R-Value for Windows
U-value and R-value both measure how well a material insulates, but they work in opposite directions. R-value measures thermal resistance (how well it blocks heat flow). U-value (also called U-factor) measures thermal transmittance (how easily heat passes through). A higher R-value means better insulation. A lower U-value means better insulation.
The Conversion (US / Imperial):
R-value = 1 / U-value
U-value = 1 / R-value
For example, a window with a U-value of 0.30 has an R-value of 1/0.30 = 3.33.
US vs Metric (SI) Systems:
The US and metric systems use different base units, so a U-value in one system is not the same number in the other:
- US U-value is in BTU/(hr·ft2·F)
- Metric U-value is in W/(m2·K)
US U-value × 5.678 = Metric U-value
Metric U-value × 0.1761 = US U-value
Similarly for R-values:
US R-value × 0.1761 = Metric R-value
Metric R-value × 5.678 = US R-value
Window Performance Ratings:
| Window Type | US U-Value | Metric U-Value | US R-Value | Rating |
|---|---|---|---|---|
| Single pane, clear | 1.00-1.10 | 5.7-6.2 | 0.9-1.0 | Poor |
| Single pane + storm window | 0.50-0.60 | 2.8-3.4 | 1.7-2.0 | Below average |
| Double pane, clear, air fill | 0.45-0.55 | 2.6-3.1 | 1.8-2.2 | Average |
| Double pane, Low-E, air fill | 0.30-0.40 | 1.7-2.3 | 2.5-3.3 | Good |
| Double pane, Low-E, argon fill | 0.25-0.32 | 1.4-1.8 | 3.1-4.0 | Very good |
| Triple pane, Low-E, argon fill | 0.15-0.22 | 0.85-1.25 | 4.5-6.7 | Excellent |
| Triple pane, Low-E, krypton fill | 0.10-0.15 | 0.57-0.85 | 6.7-10.0 | Superior |
What the Numbers Mean Practically:
For comparison, a standard insulated wall (2x4 with fiberglass batts) has an R-value around 13-15. Even the best windows are R-6 to R-10 — windows are always the weakest thermal point in a building envelope. This is why window placement, size, and quality have such a large impact on heating and cooling costs.
ENERGY STAR Requirements (US, as of 2023):
| Climate Zone | Maximum U-Value | Description |
|---|---|---|
| Northern | 0.25 | Cold climates (heating dominated) |
| North-Central | 0.28 | Mixed-cold climates |
| South-Central | 0.30 | Mixed-warm climates |
| Southern | 0.40 | Hot climates (cooling dominated) |
Factors That Affect Window U-Value:
- Number of panes: Each additional pane adds an insulating air (or gas) gap
- Low-E coating: Microscopically thin metallic coating that reflects infrared heat. Reduces heat transfer by 30-50%.
- Gas fill: Argon and krypton are denser than air and transfer less heat through convection. Argon is cost-effective. Krypton is more effective but significantly more expensive.
- Frame material: Vinyl and fiberglass frames insulate better than aluminum. Wood is in between.
- Spacer bars: Warm-edge spacers reduce heat conduction at the edge of the glass where the panes meet the frame
Practical Note:
When comparing windows, always compare whole-window U-values (which include the frame and edge effects), not just center-of-glass values. Center-of-glass numbers look better but do not reflect real-world performance.
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This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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