"Replace your mattress every eight years" is a number the industry repeats because it is convenient, not because anyone measured it. We have been measuring it since 2021, on forty-three mattresses we bought and kept, and the honest answer is that eight years is right for exactly one construction and badly wrong for two others.
What we measure, and how
Every twelve months, each mattress in storage gets the same treatment: a rigid straight edge laid across the surface with nothing on the bed, and a dial indicator recording the maximum gap at the centre and at each sleeping position.
That gap is permanent compression set — foam cell walls that have buckled and will not recover, or a coil unit that has lost temper. It is the failure mode that matters, because it is the one that stops the mattress holding your spine in line.
Three-year sag by construction
| Construction | Median sag at 3 years | Projected useful life |
|---|---|---|
| Latex hybrid | 0.2 in | 12–15 years |
| Coil-on-coil innerspring | 0.3 in | 10–12 years |
| Foam-over-coil hybrid | 0.4 in | 7–9 years |
| All-foam | 0.6 in | 6–8 years |
Sag does not accumulate linearly — it accelerates once the comfort layer has bottomed out — so the projections are conservative rather than a simple multiplication.
Weight changes everything
Foam cell walls buckle under sustained load, and the load scales directly with body weight. A 240 lb sleeper puts roughly 50% more sustained pressure through the same square inches as a 160 lb one.
In our tracking, the same model measured under a heavier primary sleeper reached a given sag depth roughly 40% sooner. Practically: take the projections above and subtract two to four years if the person sleeping on it is over 230 lb.
The two-minute test
Strip the bed completely. Lay a broom handle, a spirit level or any rigid straight edge across the mattress at the point where your hips normally sit. Look along it at eye level.
- Under 0.5 in of gap: the mattress is fine. Whatever is waking you is not sag.
- 0.5 to 0.75 in: the support core is going. You will notice it within a year.
- Over 0.75 in: finished. Check your warranty — this is the threshold most brands claim at.
Do the same at the centre of the bed. A dip in the middle of a shared mattress, with both sleeping positions still flat, usually means the foundation is failing rather than the mattress.
Signals that are not sag
Not every bad night means a dead mattress.
Waking stiff but improving within twenty minutes upright. That is characteristic of a support problem — either sag, or a mattress that was always the wrong firmness for you. Do the straight-edge test to tell which.
Waking hot when you did not use to. Usually bedding, a new mattress protector, or a warmer room. Waterproof protectors add about 0.9°F to the overnight surface rise in our measurements.
Sudden new pain with no change to the bed. More likely to be something other than the mattress. Worth a conversation with a clinician before a purchase.
Allergy symptoms. Dust mite load does increase with mattress age, but a washable protector and hot-washing the bedding weekly addresses it far more cheaply than a new mattress.
Making one last longer
Three things measurably work in our tracking:
- Rotate head-to-foot every six months. Slows centre sag noticeably. Set a calendar reminder; nobody remembers otherwise.
- Use a proper foundation. Slat gaps over three inches let the mattress sag between them, and this is the single most common cause of premature failure we see.
- Use a breathable protector. Body oils and moisture degrade foam over years. A cotton terry protector without a waterproof membrane protects the mattress without the temperature penalty.
Two things do not: mattress "rejuvenation" sprays, and turning an unturnable one-sided mattress over.
When it is time
If the straight-edge test says the support core has gone, start from the problem you are solving rather than the brand you had before — the the best mattresses for back pain and the best cooling mattresses guides rank against a single measured criterion each.