Short answer: cushions sag because foam loses about 15% of its height in the first 18 months of daily sitting, and the loss is permanent — foam is not a spring, it is a sponge with a memory that fades. Density decides how fast. Foam under 30 kg/m³ collapses visibly within 18–24 months; high-resilience foam at 35–40 kg/m³ holds its shape for 8–10 years. If your two-year-old sofa has a valley where you sit, the foam was the cheap part of the build.
Shop the pieces in this guide
There are exactly three reasons a cushion flattens, and they are worth telling apart because the fixes are different. Foam fatigue (the cushion itself has given up). Core failure (the seat platform under it — springs or webbing — has broken, so the cushion has nothing to sit on). And cover stretch (the fabric has loosened, so a perfectly good foam core looks baggy). One of these is a €30 fix, one is a €200 fix, one is a new sofa.
Diagnose it in two minutes
| Symptom | Cause | Fix | Cost tier |
|---|---|---|---|
| Cushion springs back slowly when you press it | Foam fatigue | Replace foam core | Low |
| Cushion feels soft but the whole seat dips | Broken spring / webbing | Upholsterer repair | Medium |
| Fabric bunches and wrinkles but foam feels firm | Cover stretch | Re-staple or steam | Low |
| Dip is in one spot only, rest is fine | Point loading (one seat) | Rotate + flip | Free |
| Cushion slides forward constantly | Wrong size / smooth cover | Gripper pad | Free |
| Whole sofa creaks and rocks | Frame joint failure | Professional | High |
Press the cushion with a flat hand and time the recovery. Under 3 seconds, the foam is tired. Push the cushion aside and press the deck: if you can feel a gap where a spring should be, the problem is underneath.
The density numbers that actually matter
Foam density is quoted in kg/m³ and it is the single best predictor of lifespan. 25 kg/m³ is budget seating that will feel flat in two years. 30 kg/m³ is the entry point for a sofa you keep five years. 35–40 kg/m³ high-resilience (HR) foam is what good sofas use, and it lasts 8–10 years of daily use. Above 45 kg/m³ you are into contract furniture. A sofa's spec sheet will rarely volunteer this number — ask, or check the weight of the sofa: heavier generally means denser foam and a real frame.
The fixes, ranked by effort
Rotate and flip every three months. Cushions fatigue unevenly because humans sit in the same 40 cm square forever; rotating seat cushions left-to-right and flipping them front-to-back distributes the load across four faces instead of one, roughly tripling the useful life. This is free and almost nobody does it.
Add a firm topper. A 5 cm layer of 35 kg/m³ foam cut to size and slipped under the existing cushion buys 2–3 years for €40. Not elegant, invisible under the cover, effective.
Replace the foam. A local upholsterer will cut new HR foam to your cushion dimensions for €25–50 per cushion. Measure length, width and thickness inside the cover, subtract 1 cm per dimension for compression, and specify 35 kg/m³.
Repair the deck. If the springs are gone, this is a workshop job at €150–300 for a three-seater — worth it on a sofa with a solid frame, not worth it on one that cost €400 new.
What to check when buying the next one
Sit for ten minutes, not ten seconds, and sit in the middle rather than the edge — the middle is where the span is longest and the weakness shows. Then look for a sinuous spring deck rather than elastic webbing (webbing stretches and dies at about year four), a kiln-dried hardwood frame rather than particleboard, and cushions with a removable cover so the foam can be replaced without reupholstering. A sofa you can re-foam is a sofa you keep for fifteen years; a sealed one is landfill with a delivery date.
Keep reading
More dimension guides are in the collection, and the full index is on the furnolio journal.
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