R-Value to U-Value Calculator
Convert between R-value and U-value, and between the metric and imperial versions of each — with the heat flow through a square meter at a chosen temperature difference.
R-value is thermal resistance; U-value is its reciprocal, the heat flow per square meter per degree.
How the r-value to u-value calculator works
R-value is thermal resistance; U-value is its reciprocal, the heat flow per square meter per degree. Metric R is in m²·K/W and imperial R in ft²·°F·h/BTU, and one imperial unit is 0.1761 metric — an R-13 batt is R-2.3 in metric. Heat loss through a wall is U × area × temperature difference, which is what the last row shows per square meter.
Formula: U = 1 ÷ R; R(metric) = R(imperial) × 0.17611; heat flow = U × ΔT (W/m²)
Worked examples
| Inputs | U-value (W/m²·K) | Note |
|---|---|---|
| R-2.5 metric | 0.4 | U 0.4; R-14.2 imperial |
| R-13 imperial batt | 0.437 | R-2.29 metric, U 0.437 |
| A double-glazed window, U 2.8 | 2.8 | R-0.36 metric |
FAQFrequently asked questions
Which is better, high R or low U?
Both say the same thing: high R-value and low U-value mean less heat passes. Insulation is sold by R-value; windows and whole-wall performance are usually quoted as U-values.
Why do US and metric R-values differ so much?
Different units — an imperial R-value is 5.68 times the metric number for the same material. R-19 in a US catalogue is R-3.3 in Australia or Europe.
Do R-values add up?
Yes, in series: a wall’s total R is the sum of its layers (plus surface air films). U-values do not add; convert to R, add, and convert back.
What R-value do I need?
It depends on climate and building code: roughly R-2 to R-3 metric for walls and R-4 to R-7 for ceilings in temperate climates, more in cold ones. Local codes give the minimums.
Where these figures come from
- International Energy Agency — Energy Efficiency — appliance consumption and standby power context
- US Energy Information Administration — the federal energy statistics agency
Last checked: September 2026. Appliance power figures are typical ratings; check the label on your own equipment, which is authoritative for it.