Thermal Conductivity Converter — W/(m·K), BTU and R-value

The thermal conductivity converter above moves a k-value between the SI unit, the calorie-based units still common in older thermal literature, and the BTU-based units used throughout North American building and process work. It also does the step most converters skip: it turns conductivity into thermal resistance for a thickness you specify, in both the metric RSI form and the US R-value form.

Arb Digital publishes this converter as part of a free technical reference library. The BTU and calorie factors are computed from their exact defined values, the International Table BTU of 1055.05585262 joules and the thermochemical calorie of 4.184 joules, rather than copied from a rounded table.

What This Thermal Conductivity Converter Does

Thermal conductivity, symbol k or λ, measures how readily a material conducts heat: watts of heat flow per metre of thickness per kelvin of temperature difference. The converter supports the SI unit and its multiples, the centimetre form, the calorie units in both thermochemical and International Table flavours, the kilocalorie per hour metre degree Celsius still used in European heating documentation, and the two BTU forms that dominate North American practice.

Alongside the headline conversion, the four supporting panels show the SI value, the BTU inch form used on insulation labels, and the resulting thermal resistance at your chosen thickness in both metric and US units. That last pair is the practical output most people are actually after, because insulation is specified by resistance rather than conductivity.

How to Use It

1. Enter the conductivity you have. Datasheets usually quote W/(m·K) in metric regions and BTU·in/(h·ft²·°F) in the United States.
2. Choose your from and to units from the list, which is ordered from SI outward through the calorie and BTU families.
3. Enter the material thickness in millimetres. Resistance scales linearly with thickness, so this single number decides the R-value.
4. Read the resistance panels for RSI in m²·K/W and the US R-value in h·ft²·°F/BTU, which are the numbers building specifications are written in.
5. Use Swap to reverse the direction and confirm the factor has been applied the right way round.

The Formula and How It Is Calculated

Every unit is held as a multiplier to watts per metre kelvin, so conversion is result = value × factor(from) ÷ factor(to). The BTU factors come out of three exact definitions applied together. The International Table BTU is exactly 1055.05585262 joules, an hour is 3600 seconds, and a foot is exactly 0.3048 metres. A temperature interval of one degree Fahrenheit is exactly five ninths of a kelvin, so dividing by degrees Fahrenheit means multiplying by 1.8 when moving to kelvin.

Putting those together gives one BTU per hour foot degree Fahrenheit as 1.730735 W/(m·K). The inch-based variant, BTU inch per hour square foot degree Fahrenheit, is simply that divided by twelve, giving 0.1442279 W/(m·K). On the calorie side, one calorie per second centimetre degree Celsius is exactly 418.4 W/(m·K) using the thermochemical calorie, and one kilocalorie per hour metre degree Celsius is exactly 1.163 W/(m·K) using the International Table calorie. These defined values appear in NIST Special Publication 811 and the unit structure follows the BIPM SI Brochure.

Conductivity, Conductance and Resistance Are Three Quantities

Thermal conductivity k is a material property. It does not depend on how much of the material you have. Thermal resistance R is a property of a specific layer, equal to thickness divided by conductivity, and it does depend on how thick that layer is. Thermal transmittance U, sometimes called thermal conductance, is the reciprocal of the total resistance of an assembly including surface films.

The practical implication is that quoting a k-value tells you nothing about the performance of a wall until you say how thick it is, and quoting an R-value tells you nothing about the material until you say what it is. Two materials with the same R-value can differ by a factor of three in thickness, which matters when cavity depth is fixed. This is also why building regulations are written in U-values or R-values rather than conductivity: they describe the assembly, not the substance. Thicknesses convert in the length converter and areas in the area converter.

Why BTU Inch Per Hour Square Foot Degree Fahrenheit Exists

That unit looks absurd until you see the reasoning behind it. Insulation in North America is sold in inches of thickness and installed over areas measured in square feet, so a unit that puts inches in the numerator and square feet in the denominator lets a specifier read conductivity directly against the way material is bought.

It is exactly one twelfth of the BTU per hour foot degree Fahrenheit form, and confusing the two is a common and expensive error because the difference is a factor of twelve rather than something obviously wrong. A figure of 0.25 in inch form is ordinary insulation. The same 0.25 in foot form would be a poor structural material rather than an insulator. Always check whether an inch appears in the unit before converting, and if a datasheet writes only "BTU/hr ft F", confirm which form it means.

US R-Value and Metric RSI Are Different Numbers

Both describe thermal resistance, but in different unit systems, and they are routinely quoted without a label. The metric RSI is in square metre kelvin per watt. The US R-value is in hour square foot degree Fahrenheit per BTU. One US R-value unit equals 0.1761101838 RSI, and one RSI equals 5.678263 US R-value units.

So R-19 fibreglass batt, a standard North American product, is RSI 3.35. A European specification calling for RSI 5 corresponds to about R-28. Reading an R-value as an RSI understates the insulation by a factor of nearly six; reading it the other way overstates it by the same factor. Canadian documentation makes this worse by using both, sometimes on the same label, distinguished only by the RSI prefix. Whenever you see a bare resistance number, establish the unit system before doing anything with it. The US Department of Energy insulation guidance is written in the R-value convention throughout.

Conductivity Is Not a Fixed Property

Handbook k-values are measured under specific conditions and shift substantially outside them. Temperature is the main driver. For most metals conductivity falls slowly as temperature rises. For gases and for insulating foams, which conduct largely through the gas trapped in their cells, conductivity rises with temperature. Published insulation figures are typically stated at a mean temperature of 10 or 24 °C and will be higher in a hot roof space.

Moisture matters even more. Water conducts heat roughly twenty times better than still air, so a fibrous insulant that takes up a few percent of moisture by volume can lose a large fraction of its thermal performance while looking unchanged. Density also matters, and not monotonically: loose-fill insulation improves as density rises until convection is suppressed, then worsens as solid conduction takes over, giving a minimum conductivity at an optimum density. Density conversions live in the density converter and temperature conditions in the temperature converter.

Declared Conductivity Versus What a Wall Actually Does

A laboratory k-value describes a uniform sample under steady-state conditions with perfectly parallel heat flow. Real construction is nothing like that. Timber studs, steel fixings, joists and window frames all create thermal bridges that bypass the insulation, and steel in particular conducts several hundred times better than the insulation it interrupts.

The result is that an assembly built from R-19 batts rarely performs at R-19. Studs at 400 mm centres in a timber wall can reduce the effective resistance by 10 to 20%, and a steel-framed equivalent can lose far more. This is why thermal modelling uses an effective or whole-assembly resistance rather than summing layer values, and why converting a conductivity accurately is only the first step in a real calculation. Heat flow rates convert in the power converter and total heat in the energy converter.

Full Thermal Conductivity Conversion Table

Thermal resistance equivalents, for converting the R-value output:

Common Mistakes to Avoid

• Reading a US R-value as an RSI value — the two differ by a factor of 5.678, so R-19 is RSI 3.35 and not RSI 19.
• Mixing the foot and inch BTU forms — they differ by exactly twelve, which is large enough to be catastrophic and small enough to look plausible.
• Quoting a k-value without a thickness — conductivity is a material property and says nothing about a layer until thickness is stated.
• Using a dry handbook value for a damp material — moisture uptake raises conductivity sharply because water conducts about twenty times better than still air.
• Summing layer resistances and calling it the wall performance — thermal bridges through studs and fixings reduce the effective value, often substantially.

Related Free Tools From Arb Digital

Thermal calculations draw on several unit families. Convert heat flow in the power converter, total heat in the energy converter, temperature conditions in the temperature converter, material thickness in the length converter, and wall or roof areas in the area converter. Heat capacity is handled by the specific heat converter, material density by the density converter, and everything else by the general unit converter.

Frequently Asked Questions

This converter is a unit reference only. Building compliance, insulation specification, and process thermal design should be based on declared values from the manufacturer and a calculation method recognised by the applicable standard.