Storage for forty-three mattresses is the largest single cost of running this site, and this is what it buys: a dataset nobody else in the category has, because nobody else keeps the beds.
Every twelve months each mattress gets a rigid straight edge laid across it with nothing on the surface, and a dial indicator reads the maximum gap at the centre and at each sleeping position. Same instrument, same operator, same conditions.
The headline
Median five-year sag by construction, across the 43 units:
| Construction | Units | Year 1 | Year 3 | Year 5 |
|---|---|---|---|---|
| Latex hybrid | 6 | 0.05 in | 0.20 in | 0.34 in |
| Coil-on-coil innerspring | 8 | 0.10 in | 0.30 in | 0.52 in |
| Foam-over-coil hybrid | 19 | 0.15 in | 0.40 in | 0.79 in |
| All-foam | 10 | 0.20 in | 0.60 in | 1.15 in |
The gaps widen over time rather than staying proportional, which is the finding that changed our rubric weighting this year. At twelve months the four constructions look similar. At five years they are different products.
Sag accelerates
The other pattern in the data is that permanent compression is not linear. Foam cell walls buckle progressively, and once the comfort layer has bottomed out the load transfers to layers that were not designed to carry it.
Across our all-foam units, sag added roughly 0.20 inches in year one, 0.20 in year two, and 0.28 in year five. Small differences early are not small differences later, which is why a one-year review of a mattress tells you almost nothing about durability.
Weight is the largest single variable
Sixteen of the mattresses in the set spent their test period under our 240 lb tester as primary sleeper; the rest were under lighter testers. Same models appear in both groups in four cases, which gives us a clean comparison.
Those four models reached a given sag depth roughly 40% sooner under the heavier sleeper. Practically, a mattress rated for a ten-year life under a 165 lb sleeper is a six- to seven-year mattress under a 240 lb one.
This is not a defect and no warranty covers it. It is the reason the best mattresses for heavier sleepers guide exists as a separate ranking rather than a footnote on the others.
What we got wrong in 2021
Two things.
We over-weighted coil count. Marketing materials talk about coil counts constantly, and we assumed a higher count would correlate with better sag resistance. It does not, at all, in our data. Coil gauge — the thickness of the steel — correlates strongly. Count barely registers.
We under-weighted base foam density. The layer nobody discusses turns out to be the one that predicts failure. Every unit in our set with a base foam under 1.8 lb/ft³ is in the worst quartile for five-year sag, regardless of what is above it.
The dataset
We publish the per-model figures in each review rather than as a single table, because a five-year number attached to a model we no longer recommend is more likely to mislead than inform. Every current review carries its own sag measurement in the specifications section.
If you want the raw figures for research, email us and we will send the spreadsheet.