Quick answer: strip the bed, leave the mattress unloaded, span the sag with a rigid straight edge, then measure vertically from the straight edge to the deepest point of the indentation.
What you need
- a rigid straight edge, yardstick or level long enough to span the impression
- a ruler or tape measure with clear markings
- a phone or camera
- your mattress warranty so you know the required threshold
Step-by-step
- Remove sheets, protectors and toppers.
- Make sure nobody is sitting or lying on the mattress.
- Allow the surface to settle.
- Place the straight edge across the deepest part of the impression.
- Hold the ruler vertically at the deepest point.
- Measure from the mattress surface at the bottom of the depression to the underside of the straight edge.
- Take a wide photo showing location and a close photo showing the measurement.
What not to do
Do not press the ruler into the foam, measure with weight on the bed, or photograph only a close-up that makes it impossible to see where the indentation is located.
Also photograph the foundation
Take photos of the bed frame, center support and slats. Some claims fail because the support system does not meet the warranty requirements even when the mattress visibly sags.
Next step
Take more than one measurement
If the sag covers a wide area, take measurements at the deepest point and at one or two nearby points. That creates a clearer record of the shape and size of the depression. Keep the straight edge level and photograph the ruler markings so the depth is easy to verify later.
Keep the original photos
Do not rely only on screenshots or compressed images sent through a support chat. Save the original full-resolution photos and your order documents in one folder. If the manufacturer asks for additional evidence later, you will have a clean record of the first measurement and mattress condition.
Compare your measurement with the threshold in the brand warranty, then use our claim checklist.