U-Value ↔ R-Value Converter
Flip between R-value and U-value. They are reciprocals: U = 1 ÷ R and R = 1 ÷ U.
Calculator
R-20 is a U-value of 0.0500; a U-value of 0.0770 is R-12.99. U-value is conductance — the exact electrical-thermal analogy of 1 ÷ resistance.
Insulation is sold in R-values; windows, doors and whole-assembly energy specs are often rated in U-values. They are the same physics inverted, so you never have to guess: U = 1 ÷ R. This converter runs both directions at once so you can put a window’s U-factor and a wall’s R-value on the same scale.
Think of it as Ohm’s law for heat: R is resistance, U is conductance, heat flow is current. Lower U (or higher R) means less heat leaking through.
Formula
U = 1 ÷ R and R = 1 ÷ U
For a whole assembly, invert the total R (all layers summed): U_total = 1 ÷ ΣR.
Worked example
Both directions. A wall at R-20 has a U-value of 1 ÷ 20 = 0.050. A window rated U-0.077 is equivalent to R = 1 ÷ 0.077 = R-13.0. Now the two are comparable — that R-20 wall resists heat about 1.5× better than the U-0.077 glass.
Background & practice
Parallel paths do not invert layer-by-layer. You can only take 1 ÷ R for a single series stack. A wall with studs beside insulation is two paths in parallel; average their U-values by area first, then convert — do not average the R-values.
Watch the units. US R-values (ft²·°F·h/BTU) and metric RSI (m²·K/W) are not the same number: RSI ≈ US-R ÷ 5.68. A "R-value" quoted far below what you expect is often a metric figure. This tool works in US units throughout.
Where it matters: matching a window U-factor to a wall assembly, reading a manufacturer’s U on a spec sheet, or checking a code table that lists maximum U instead of minimum R.
Reference table
| R-value | U-value (1 ÷ R) |
|---|---|
| R-11 | 0.0909 |
| R-13 | 0.0769 |
| R-19 | 0.0526 |
| R-21 | 0.0476 |
| R-30 | 0.0333 |
| R-38 | 0.0263 |
| R-49 | 0.0204 |
| R-60 | 0.0167 |
U is conductance — the exact electrical analogy of 1 ÷ resistance. Lower U = less heat flow. Whole assemblies with parallel paths (studs vs cavity) need an area-weighted average, not this single-layer swap.