Thermal Conductivity Converter
Same value in every unit
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Getting a k-value right before you convert it
Thermal conductivity, written k or lambda, is the heat flow through one square metre of a slab one metre thick for each kelvin of temperature difference across it. Lower means better insulation. The number on its own means nothing until you know which unit it was quoted in, and that is where most of the errors start.
The two BTU units differ by a factor of twelve
BTU per hour-foot-degF and BTU-inch per hour-sqft-degF look almost identical in print and are constantly mixed up. The second measures across one inch rather than one foot, so it is exactly twelve times smaller: 1.7307347 W/(m·K) against 0.1442279 W/(m·K). When a US datasheet quotes conductivity rather than an R-value, it is nearly always the inch form, because insulation there is specified per inch of thickness. If a converted figure lands twelve times too high or too low, that is the reason.
Because conductivity is expressed per unit of temperature difference, kelvin and degrees Celsius are interchangeable here, as are Fahrenheit and Rankine. Only the size of the degree matters, not where the scale starts. Older papers still use cal/(s·cm·degC), a very large unit at 418.4 W/(m·K), so values written in it appear as small decimals. That 418.4 follows the thermochemical calorie while the 1.163 for kcal/(h·m·degC) follows the International Table calorie; the two definitions differ by under 0.1 per cent.
k is a material property, R is not
k describes the material. R describes a specific piece of it: in SI, R is thickness divided by k, in m²·K/W. Double the thickness and R doubles while k does not move at all. US R-values use a different unit again, ft²·degF·h/BTU, where 1 m²·K/W is about 5.68 US R, so an R-13 batt is roughly 2.3 m²·K/W. Converting k will never hand you an R-value by itself; you need the thickness as well.
Approximate values to sanity-check against
All figures approximate, near room temperature, in W/(m·K):
- Copper around 400; pure aluminium around 240, alloys lower
- Carbon steel around 50; stainless steel around 15
- Glass and dense concrete roughly 1 to 1.5
- Water around 0.6
- Softwood across the grain roughly 0.12 to 0.15
- Mineral wool and expanded polystyrene roughly 0.033 to 0.040
- Still air around 0.026, which is why porous insulation works at all
Insulation figures shift with density, moisture and mean test temperature. Treat a manufacturer's declared lambda as the real number, and the list above only as a check that your conversion landed in the right decade.
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Frequently Asked Questions
It depends entirely on what the material is meant to do. For insulation you want the lowest k you can get, which is why mineral wool sits around 0.035 W/(m·K). For a heat sink, a pan base or a heat exchanger you want the highest, which is why copper at roughly 400 W/(m·K) is hard to beat. Same quantity, opposite goal, so always check which direction the specification is pushing before you compare two numbers.
k is a property of the material and does not change with thickness; R belongs to a particular layer and does. In SI, R is thickness in metres divided by k, measured in m²·K/W. A 100 mm board with k = 0.035 W/(m·K) gives R of about 2.9 m²·K/W, and 200 mm of the same board gives about 5.7. That is why a conductivity converter can give you k but never an R-value on its own.
Multiply by 0.1442279. A US datasheet quoting 0.24 BTU·in/(h·ft²·degF) is therefore about 0.035 W/(m·K). To go the other way, divide by the same factor. When an American insulation or building-product spec states conductivity rather than an R-value, this is almost always the unit it uses, because thickness in those specs is given in inches rather than feet.
You have almost certainly swapped the two BTU units. BTU per hour-foot-degF is about 1.7307 W/(m·K), while BTU-inch per hour-sqft-degF is about 0.14423 W/(m·K), and since a foot is twelve inches the two differ by exactly twelve. Check which one the source document used before converting. The inch form dominates American product data; the foot form turns up more in engineering texts and heat transfer work.
It means 35 mW/(m·K), which is the same as 0.035 W/(m·K). Insulation is often graded this way because writing conductivity in milliwatts avoids a run of leading zeros and makes small differences easy to compare at a glance: a lambda 32 board outperforms a lambda 35 board of the same thickness. Divide by 1000 to get back to W/(m·K).
Yes, for essentially every material, which is why a quoted k always belongs to a stated test condition. Insulation declarations name a mean temperature, commonly 10 °C in Europe and about 24 °C (75 °F) in the US, and the value drifts if the material is used well away from that. Moisture matters more still: damp insulation conducts far more heat than dry, so a wet wall will underperform its datasheet regardless of how carefully you convert the units.