Fix your mattress · Updated September 2026
How to Fix a Sagging Mattress
First, the honest part: compressed foam does not un-compress, so no fix restores a sagged mattress to new. What you can do is move the sag away from your body, support the mattress properly, mask a shallow dip, or get the manufacturer to replace it. Which one applies depends on how deep the sag is and how old the mattress is.
Step one: confirm it is the mattress
Put the mattress on the floor for a night or two. If the sag disappears, your base was dipping and the mattress was following it — fix the base and the problem is solved for the price of a bunkie board or a set of slats.
If the sag is still there on the floor, it is in the mattress. Measure it with a straight edge, unloaded, so you know what you are dealing with.
Check the warranty before anything else
If the mattress is within its warranty period and the sag meets the depth threshold in the terms, a replacement costs you an email, not money. Photograph the measurement with the straight edge in place, find your proof of purchase, and check that your foundation meets the warranty's requirements — claims are commonly denied over non-compliant bases.
Rotate, and support the middle
Rotating 180 degrees moves the hollow from under your hips to the foot of the bed. The sag is still there, but you are no longer sleeping in it, and on a mattress used by one person this can buy a year or more.
If the frame has no centre support, add it. A sagging centre beam accelerates mattress sag on queen sizes and up, and it is a cheap fix.
The topper bridge
A firm, thick topper laid over a shallow sag levels the sleeping surface by filling the dip relative to your body. It genuinely works as a comfort fix for a shallow sag — but understand it as a bridge to replacement, because the mattress underneath keeps degrading and the topper gradually takes the shape of the dip.
What does not work
Plywood under the mattress firms the overall feel but does not fill a dip — the mattress still sags into its hollow on top of the board. Mattress helpers and inflatable under-mattress supports can level a shallow dip but shift and deflate. And flipping a one-sided modern mattress just puts the support core on top, which is worse.
If the sag is deep enough to roll you into it at night, no accessory fixes it. That is a replacement conversation.