U-Value to R-Value Calculator

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Market

Imperial · sales tax

U-value (BTU/hr·ft²·°F)

The assembly's thermal transmittance, as specified.

Building regulations and window specifications are usually written as maximum U-values. Metric U-values (W/m²K) are a different unit and roughly 5.68 times the imperial figure — check which you have.

Equivalent R-value

20 hr·ft²·°F/BTU

High confidence

R = 1 ÷ U, the exact inverse relationship. The result is the whole assembly's resistance — subtract the other layers before choosing an insulation thickness.

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

R = 1 / U, the reciprocal of thermal transmittance
Imperial U (BTU/hr-ft2-F) and metric U (W/m2K) differ by a factor of about 5.678

Inputs used

U-value (BTU/hr·ft²·°F): 0.05

Final result20 hr·ft²·°F/BTU

Confidence note: R = 1 ÷ U, the exact inverse relationship. The result is the whole assembly's resistance — subtract the other layers before choosing an insulation thickness.

What this calculation does not cover

The conversion is arithmetic and takes the U-value at face value. It does not check that you have the right figure for the element: a window's whole-unit U-factor, its centre-of-glass value and its frame value are three different numbers, and only the whole-unit one describes what the opening actually loses.
The field is imperial only, and the site's metric/imperial toggle does not convert it. A W/m2K figure typed in unchanged returns the metric resistance in m2K/W, which is about a fifth of the imperial R the result is labelled as. Divide a metric U-value by 5.678 before entering it.
Thermal bridging is not in the number. A U-value quoted for a clear-wall build-up excludes studs, joists, rim boards, lintels and junctions, so the R returned describes the insulated bay rather than the whole element.
This is not a code compliance check. It does not test the figure against any energy code's prescriptive U-factor or R-value tables, and it does not perform the area-weighted or trade-off calculation a submission needs.
Steady-state conduction only: no air leakage, no thermal mass, no moisture. Insulation that is compressed, wind-washed or damp delivers less than its rated R, and a reciprocal cannot show that.

Add the equipment this sizes

This result is a specification — 20 hr·ft²·°F/BTU — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

QtyItemPrice each ($)

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations2

R = 1 / U, the reciprocal of thermal transmittance
Imperial U (BTU/hr-ft2-F) and metric U (W/m2K) differ by a factor of about 5.678

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Building regulations and window schedules are written as maximum U-values, while insulation on a merchant's shelf is labelled in R, so specifying to a regulation means crossing between them. The arithmetic is a reciprocal and takes a second. The two traps are unit systems and scope. A metric U-value in watts per square metre kelvin is roughly 5.68 times its imperial counterpart, and mixing them produces an assembly that appears five times better insulated than it is. And the converted R covers the entire build-up, not the insulation alone: sheathing, cladding, internal linings and surface films all contribute, so the product you buy only needs to close the remaining gap.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

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The void under a period floor is a drying machine. Insulate between the joists without switching it off, and the floor gets warm instead of rotten.

U-value (BTU/hr·ft²·°F)	Equivalent R-value (hr·ft²·°F/BTU)
0.005	200
0.01	100
0.02	50
0.05	20
0.1	10
0.2	5
0.5	2

U-value (BTU/hr·ft²·°F)

Equivalent R-value (hr·ft²·°F/BTU)

0.005

200

0.01

100

0.02

0.05

0.1

0.2

0.5

Frequently asked questions

My specification is in W/m²K. Can I use it here?

Not directly — that is the metric U-value and is about 5.678 times the imperial figure, so divide by 5.678 before entering it. Enter it raw and the R-value comes out roughly five times too LOW, not too high: a 0.18 W/m²K wall reads 5.6 instead of its true imperial R-31.5, making a well-insulated assembly look barely insulated at all. The error is pessimistic, which is why it tends to be noticed — but it is still wrong by a factor of 5.678.

Does the resulting R-value match the insulation I need to buy?

Not on its own. The converted figure is the whole assembly's resistance, and the insulation only has to make up the difference between that and what the other layers already provide. Subtract the existing layers before specifying a product.

Why do regulations specify U rather than R?

Because a maximum U-value sets a performance ceiling on the finished assembly regardless of how it is built up, which is what a regulator wants. R-value labelling describes a single product, which is what a manufacturer wants.