What is the formula for R-value to U-value?

U = 1 / R. If R is in m²·K/W (SI), U is in W/(m²·K). If R is in ft²·°F·h/BTU (Imperial), U is in BTU/(ft²·°F·h). The relationship is always a simple reciprocal.

What is a good U-value for a wall?

In most climate zones a U-value below 0.30 W/(m²·K) (R > 3.3 SI) is considered reasonable. High-performance walls target U ≤ 0.15 W/(m²·K) (R ≥ 6.7 SI). Local building codes set the minimum standard.

Are SI and Imperial R-values directly comparable?

No. 1 ft²·°F·h/BTU ≈ 0.1761 m²·K/W. A North American "R-20" batt is about 3.52 in SI. Always confirm which unit system a spec uses before comparing products.

Does U-value include surface resistances?

Whole-assembly U-values (used in energy codes) include internal and external surface film resistances. Material-only U-values do not. This tool converts the numeric value you enter regardless — make sure your input is consistent (assembly or material-only).

Why is a lower U-value better?

U-value measures heat loss per unit area per degree of temperature difference. A lower U-value means less heat escapes, so the building stays warmer in winter (or cooler in summer) with less energy.

R Value To U Value

Convert between R-value (thermal resistance) and U-value (thermal transmittance) instantly — U = 1/R.

Conversion Input

Convert direction

Unit system

R-value (m²·K/W)

Result

Enter a value and click Convert

Common reference values (SI)

Material	R (m²·K/W)	U (W/m²·K)
Single-pane glass	0.18	5.56
Double-pane window	0.35	2.86
Cavity brick wall	0.45	2.22
Insulated stud wall	3.52	0.28
Passive-house wall	7.00	0.14

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Summary

R-value measures how strongly a material resists heat flow — the higher the R-value, the better the insulation. U-value (thermal transmittance) is simply the inverse: U = 1/R. A wall with R-20 has a U-value of 0.05 W/(m²·K), meaning it transmits 0.05 watts per square meter for every degree of temperature difference. This tool converts between the two in both SI and Imperial units.

How it works

Select whether you want to convert R-value to U-value or U-value to R-value.
Choose your unit system: SI (m²·K/W) or Imperial (ft²·°F·h/BTU).
Enter the value in the input field.
The result is calculated instantly using U = 1/R (or R = 1/U).
Use the unit toggle to see the converted result in the other unit system as well.

Use cases

Check insulation specs when comparing building materials from different regions.
Convert architect drawings that mix SI and Imperial R-values.
Calculate U-value for building energy compliance (e.g. ASHRAE, Part L).
Verify window or wall assembly thermal performance for LEED certification.
Translate between R-value labels on North American insulation batts and European U-value specs.
Quick sanity-check during HVAC load calculations.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu