U-factor ↔ R-value converter
Convert a window’s NFRC-rated U-factor to its R-value and back with the reciprocal identity R = 1 ÷ U. Enter either number, get the other.
A whole-window U-factor of 0.300 is R 3.33 (R = 1 ÷ U) — far below a wall’s cavity R-value, which is why the wall and the window are rated separately. Lower U (higher R) means less heat loss.
Calculator inputs
Windows are rated by U-factor — the rate of non-solar heat flow through the whole assembly (frame, spacer and glass), in BTU/hr·ft²·°F. Lower U is better. Walls and attics are rated by R-value (thermal resistance), and the two are simple reciprocals: R = 1 ÷ U and U = 1 ÷ R. That is why a good replacement window at U 0.30 works out to only about R 3.3 — a fraction of a well-insulated wall’s cavity R-value, which is exactly why the window and the wall are rated on different scales and specified separately.
Use this to compare a marketing R-value claim against the NFRC U-factor on the label, or to sanity-check a spec written in one convention against a code table written in the other.
Formula
R = 1 ÷ U and U = 1 ÷ R
The identity is exact for whole-window ratings; there is no unit conversion, only the reciprocal.
Worked example
A double-pane low-E vinyl window is rated U 0.30:
R = 1 ÷ 0.30 = 3.33
Going the other way, a claimed R 5.0 is:
U = 1 ÷ 5.0 = 0.20
So an “R-5 window” is a U-0.20 window — premium triple-pane territory. Most quality replacement windows land near U 0.25–0.30 (R 3.3–4.0).
What to check before you trust the number
Two traps worth avoiding. First, center-of-glass vs whole-window: glass-only U-factors look better than the NFRC whole-window number that includes the frame and spacer — always convert the whole-window rating, which is what code and ENERGY STAR reference. Second, a window’s R-value is not comparable to a wall’s: a window can never match cavity insulation, so a low U-factor is about cutting the biggest loss in the wall, not matching it.
Reference table
| Frame & glazing | Typical U-factor | R-value (1÷U) |
|---|---|---|
| Aluminum, single pane (no thermal break) | 1.20 | 0.83 |
| Wood/vinyl, single pane | 0.90 | 1.11 |
| Aluminum, double pane | 0.65 | 1.54 |
| Vinyl/wood, double pane (air) | 0.48 | 2.08 |
| Vinyl/wood, double pane low-E + argon | 0.30 | 3.33 |
| Fiberglass/composite, double low-E + argon | 0.28 | 3.57 |
| Vinyl/fiberglass, triple low-E + argon/krypton | 0.18 | 5.56 |
Labeled typical planning values — use the product’s NFRC-rated U-factor for the real number. See the U-factor & SHGC by frame & glazing table.
Frequently asked questions
What is a good U-factor for windows?
For most of the US, look for a whole-window U-factor of 0.30 or lower (about R 3.3 or higher). Cold Northern zones reward U 0.22–0.25 (triple-pane or premium double-pane low-E); hot Southern zones can meet code up to U 0.40 but usually prioritize a low SHGC instead. Confirm the target with the ENERGY STAR climate-zone checker.
How do I convert U-factor to R-value?
Divide one into 1: R = 1 ÷ U. A U-0.25 window is R 4.0; a U-0.30 window is R 3.33. It reverses the same way — U = 1 ÷ R.
Why is a window’s R-value so much lower than a wall’s?
Glass and frames conduct far more heat than insulated cavities. Even a triple-pane window (about R 5) is well below a code wall’s R 13–21, which is why windows are the weak point in the envelope and are rated on their own U-factor scale.
Is a lower or higher U-factor better?
Lower is better — U-factor measures heat loss, so a smaller number means the window insulates better (a higher R-value).