Sinkage

Sinkage is measured by placing a known load on the surface and measuring the vertical displacement from the uncompressed height. We use a 50 lb steel plate at the hip position and record displacement in inches, then repeat with the mattress under a full-body load.

Typical figures: a plush memory foam bed sinks 3.5–5 inches at the hip under an average-weight sleeper; a medium-firm hybrid sinks 2–3 inches; a firm latex or high-density foam bed sinks 1–2 inches. More sinkage means more pressure relief and less mobility; less sinkage means easier repositioning and better spinal alignment for stomach sleepers.

Sinkage scales with body weight, which is why the same mattress produces completely different sinkage numbers for a 130 lb and a 250 lb sleeper. This is the mechanism behind the whole position-and-weight matrix: firmness ratings are calibrated for an average body, sinkage is what actually happens to yours.

Excessive sinkage at the hips relative to the shoulders is what causes the hammock effect and the resulting lower-back ache. Zoned mattresses fight this by using firmer material under the lumbar region so the hip sinks less than the shoulder.

Sinkage is how deep the body settles into the surface, and it is what most people actually mean when they describe a bed as soft or firm. We measure it with fixed weights at the shoulder, hip and lumbar positions, because a single average conceals whether a bed cradles evenly or lets the midsection drop.

Excessive hip sinkage relative to shoulders is the classic cause of morning lower-back pain, and it worsens as comfort layers age. If a mattress that once felt fine has begun producing back pain, measured sinkage almost always shows the hip figure has grown while the shoulder figure has not.