CLD or ILD - what they measure and why the numbers don't match

European data sheets usually state hardness as CLD in kilopascals, while American and Asian catalogues state ILD in newtons or pounds. Same material, two completely different figures.

CLD - pressure over the full surface

CLD (Compression Load Deflection, EN ISO 3386) compresses the sample with a plate larger than the sample - the whole surface carries the load. The stress at 40% compression is read and expressed in kPa. CLD is therefore a material property: it does not depend significantly on sample dimensions.

ILD - indenting a disc

ILD (Indentation Load Deflection, ASTM D3574) presses a 323 mm disc into a sample about 100 mm thick and measures force in N (or lbf) at 40% indentation. Because the disc does not cover the whole surface, the result also carries the tensioning of the surrounding material - ILD is a property of the sample, not just the material.

Why no conversion exists

• Different units: stress (kPa) versus force (N).
• ILD depends on sample thickness and area; CLD does not.
• The ILD/CLD ratio is not constant - it changes with foam type (HR foams have a differently shaped compression curve than conventional ones).

The rough conversion tables circulating online may serve for first orientation, but they are unusable for product specification.

What to do in practice

• Compare hardness only within the same method and standard.
• In a specification, state the method, compression percentage and tolerance (e.g. CLD 40%: 4.0 ± 15% kPa).
• If you have a reference sample of a competitor material, send it with your inquiry - measuring both with the same method is the most reliable comparison.