Thermal conductivity shows up in different units depending on the industry and region: SI-based engineering and physics use watts per meter-kelvin (W/(m·K)), US construction and HVAC use BTU per hour-foot-°F, and some older references use the CGS calorie-based unit. All of them measure the same underlying material property — how readily heat moves through a substance — just scaled differently. This article explains how each unit relates to the SI base unit and how thermal conductivity connects to the more familiar R-value.
W/(m·K) is the common thread
Every thermal conductivity unit in this converter reduces to a multiple of W/(m·K), the SI base unit. It represents the rate of heat flow through a 1-meter-thick slab of material, per square meter of cross-sectional area, for a 1-kelvin temperature difference across it. Once every unit is expressed in W/(m·K), converting between any pair — say, cal/(s·cm·°C) directly to BTU/(hr·ft·°F) — is a single multiply-then-divide step through that common base, the same pattern used by this converter's general Unit Converter.
Why US construction still uses BTU/(hr·ft·°F)
US building codes, HVAC load calculations, and most American insulation manufacturers publish thermal conductivity (and its inverse, R-value) in imperial units because that's what the historical body of construction standards and equipment specs is built around. A material rated at 1.7307 W/(m·K) and one rated at 1 BTU/(hr·ft·°F) are physically identical — the number just depends on which unit system the spec sheet was written in. Engineers working across international projects frequently need to convert between the two when reconciling US-manufactured materials against SI-based design calculations.
Thermal conductivity vs. R-value
Thermal conductivity (k-value) measures how well a material conducts heat per unit thickness; R-value measures how well a specific thickness of that material resists heat flow. They're inversely related and also depend on thickness: R = thickness ÷ k. A material with a low k-value (like fiberglass at 0.04 W/(m·K)) needs less thickness to achieve a given R-value than a material with a higher k-value. Use the R-Value Calculator once you know a material's thickness and k-value to find its insulating R-value, or the Specific Heat Calculator and Heat Transfer Calculator for related heat-flow calculations.