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.