U Value to R Value Calculator
Convert a U-value to an R-value, or an R-value to a U-value — they are reciprocals (R = 1 ÷ U).
Calculator
Free, runs entirely in your browser, and your numbers never leave your device. Results are estimates for planning only.
What this calculator does
U-value measures how readily heat passes through an assembly, while R-value measures resistance to that heat flow — they are reciprocals. This tool converts a U-value into the equivalent R-value so you can compare products and assemblies quoted either way.
How to use it
- Enter the U-value of the window, door, or assembly.
- Read the equivalent R-value, calculated as 1 ÷ U.
- Use it to compare against insulation and other components quoted in R-value.
The formula
R-value = 1 ÷ U-value. U-value and R-value are reciprocals of each other.
Example calculation
An assembly with a U-value of 0.05:
- R-value: 1 ÷ 0.05 = 20
Result: A U-value of 0.05 equals about R-20.
Buying and planning tips
- Lower U-value means better insulation; higher R-value means the same thing.
- Make sure both figures use the same unit system before comparing (US and metric R/U differ).
- Windows and doors are usually rated by U-value; insulation by R-value — this converts between them.
Common mistakes to avoid
- Comparing a US R-value with a metric (RSI) value without converting units.
- Forgetting that U and R are reciprocals, not a simple offset.
- Treating a window's whole-unit U-value as if it were just the glass.
Assumptions and limits
- R-value = 1 ÷ U-value, using the value you enter.
- US customary units are assumed; metric U-values map to RSI, a different scale.
- This is a planning estimate only, not engineering or building-code advice.
Frequently asked questions
How do I convert U-value to R-value?
Take the reciprocal: R-value = 1 ÷ U-value. So a U-value of 0.05 is R-20, and a U-value of 0.25 is R-4.
How do I convert R-value to U-value?
It is the same reciprocal in reverse: U-value = 1 ÷ R-value. An R-20 assembly has a U-value of 0.05, and R-4 is a U-value of 0.25.
Why are U-value and R-value reciprocals?
U-value measures heat flow through an assembly while R-value measures resistance to it, so one is simply one divided by the other.
Which is better, a high or low U-value?
A lower U-value is better — it means less heat passes through. That corresponds to a higher R-value.
Do US and metric values use the same numbers?
No. Metric thermal resistance (RSI) and U-values use different units than US R-values, so convert units before you compare figures from different regions.
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