What is ILD?
ILD (Indentation Load Deflection) is the international standard for objectively measuring mattress firmness. It measures the force in pounds required to compress the mattress by 25% of its height. Higher numbers mean a firmer mattress.
ILD Firmness Scale
ILD 14-18: Ultra Soft — Light body types, side sleeping
ILD 19-25: Soft — Side sleepers, petite builds
ILD 25-31: Medium — Most body types, back sleeping
ILD 31-37: Medium-Firm — Back sleepers, larger builds
ILD 37-44: Firm — Stomach sleepers, heavy builds
ILD 44+: Extra Firm — Over 220 lbs, strong lumbar support needed
Ideal ILD by Sleep Position
Side Sleepers (ILD 19-28): Shoulders and hips need to sink in enough for the spine to stay aligned. Too firm causes shoulder pain.
Back Sleepers (ILD 25-35): The mattress must support the natural S-curve of the spine. Too soft lets the lower back sag.
Stomach Sleepers (ILD 31-40): Firm support prevents the lower back from over-arching.
Combination Sleepers (ILD 25-31): Medium firmness works across multiple positions.
Ideal ILD by Body Weight
Under 120 lbs: ILD 19-25 — Lighter bodies get adequate support even from softer mattresses.
120-175 lbs: ILD 25-31 — The range that fits most people.
175-220 lbs: ILD 31-37 — Needs firmness to support body weight adequately.
Over 220 lbs: ILD 37-44+ — High support to prevent bottoming out is essential.
Common myths about firmness
"Firm mattresses are better for your back" — Not necessarily. Too firm can fail to support the spine's natural curve.
"Soft mattresses cause back pain" — For lighter people, a soft mattress may actually provide better support.
"Firmness is just personal preference" — Preference matters, but there are science-based guidelines based on body type and sleep position.
Cultural differences in firmness preference
Korea and Japan traditionally used firm sleeping surfaces (ondol floors, tatami) and tend to prefer firmer mattresses. Europe and the US generally prefer softer options. However, scientifically, individual body type and sleep position matter more than cultural preference.
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