THE HONEST ANSWER. A topper cannot fix a sagging mattress, because a topper has no structure of its own. It is a flexible slab that takes the shape of whatever is underneath it. Lay a topper over a hollow and the topper develops the same hollow — you have added padding to the dip, not removed the dip. Sagging is a support failure; toppers change surface feel. They are solving different problems.
THE ONE THING A TOPPER CAN DO. A firm, dense topper — latex or high-density foam, not soft memory foam — can bridge a shallow depression, roughly up to half an inch, well enough to make the surface feel flatter. It does not restore support, but it can make a mildly worn mattress tolerable while you save for a replacement, and that is a legitimate reason to buy one. Be clear with yourself that you are buying time, not a repair.
WHEN IT MAKES THINGS WORSE. A soft or plush topper over a sagging mattress deepens the problem: you now sink through the topper into the hollow beneath, and the extra loft makes it harder to roll out of. Thicker is not better here — 4" of soft foam over a collapsed core is the worst combination available.
CHECK THE BASE BEFORE YOU BUY ANYTHING. If the hollow is caused by a broken slat, a missing centre support or a soft box spring, a topper is treating a symptom while the cause keeps getting worse. Look underneath first. Fixing a base is usually cheaper than a topper, it is permanent, and it protects the warranty — brands exclude sag caused by an unsupportive foundation, so leaving a bad base in place can also cost you a valid claim.
CHECK THE WARRANTY BEFORE YOU BUY ANYTHING. Measure the hollow with a straight edge and no weight on the mattress. If it exceeds your brand's threshold — 0.75" at Tempur-Pedic, 1" at Purple and Casper, 1.5" at Nectar, DreamCloud, Helix, Brooklyn Bedding and Avocado — you may be entitled to a repair, re-cover or replacement. Buying a topper instead is paying to hide a claim you already own.
WHERE TOPPERS GENUINELY EARN THEIR MONEY. Not on sagging. A topper is the right buy when the mattress is structurally sound but too firm, when you want to add cooling or a different surface material, when you are protecting a guest or dorm mattress you did not choose, or when you want to soften a bed you otherwise like. Those are real jobs and toppers do them well.
THE ARITHMETIC. A decent topper is a meaningful fraction of the cost of a budget mattress. If the mattress has a measurable, unrecoverable sag and no warranty coverage left, spending that on a topper generally buys a few months of slightly better sleep on a bed that is still failing. Putting the same money toward the replacement is usually the better decision.