Is Your Bed Sinking? How to Measure Mattress Indentation & Sagging
Waking up with an aching lower back or feeling like you are rolling into a middle crater every night? It might be time to test your bed for sagging.
While a minor mattress indentation is a completely natural part of the breaking-in process as comfort layers adapt to your body shape, deeper structural dips can ruin your sleep posture. Knowing the difference between normal wear and true mattress sagging is key to keeping your spine aligned — and checking if your mattress qualifies for a warranty replacement.
Body Impression vs. True Mattress Sagging: What's the Difference?
Before grabbing your measuring tape, it helps to understand what you are looking at:
- Normal Body Impression (less than 35 mm): Over time, top quilting and latex or foam layers settle slightly under body weight. A slight contour in your primary sleeping spot actually helps relieve pressure points
- True Mattress Sagging (35 mm or greater): A permanent, visible dip that remains even when nobody is on the bed. If the depression is deeper than 35 mm (the standard warranty threshold across most New Zealand bed brands), it indicates structural fatigue or base failure.
4 Steps to Measure Mattress Indentation at Home
You don't need fancy tools to test your bed — just a broomstick and a tape measure.
Lay a straight edge flat across the mattress and measure down to the deepest indentation point. Source: Peace Lily Australia website.
1. Strip the bed completely:
1-2 min.
Remove all pillows, sheets, and mattress protectors. The top cover of the mattress should be completely exposed and bare.
2. Ensure a level surface:
1 min.
If you suspect your bed base or slats are bowing, place the mattress directly on a flat, hard floor for the most accurate measurement.
3. Position a long straight edge:
1 min.
Lay a straight object — such as a broom handle, long spirit level, or straight piece of timber — flat across the bed, directly spanning the deepest point of sagging from edge to edge.
4. Measure down to the deepest point:
1 min.
Using a ruler or measuring tape, measure the vertical gap from the underside of the straight edge down to the surface of the mattress. While measuring, avoid the quilting pattern of the mattress as its deeper anyway. Do not press into the fabric, and measure from the flat surface rather than deep seam stitching.
If your measurement shows a dip equal to or greater than 35 mm, take clear photos showing the ruler reading next to the straight edge. This serves as primary documentation if you need to lodge a warranty claim with your mattress maker.
What Causes a Bed to Sag Early?
While natural wear happens over years of use, premature bed sagging is often triggered by external factors:
- Inadequate Bed Frame Support: Flexible curved slats or widely spaced slats (greater than 7 cm apart) allow the weight of heavy mattresses to push through the gaps, creating permanent dips.
- Lack of Rotation: Sleeping in the exact same spot every single night concentrates wear in one area.
- Moisture & Humidity: Unventilated bedrooms common in Kiwi homes during winter can trap body moisture inside mattress fabrics, weakening internal latex and foam layers.
Quick Tips to Prevent Sagging & Extend Bed Life
- Rotate Regularly: Rotate your mattress end-to-end once a month for the first six months, and every two to three months after that to distribute wear evenly.
- Use a Solid Base: Ensure your bed frame features strong, flat wooden slats with a center support rail for heavier hybrid or natural latex mattresses.
- Keep It Dry & Ventilated: Use a breathable, moisture-wicking mattress protector and air out your room regularly to prevent humidity build-up.
Looking for a bed built to last? Explore our mattresses — backed by our trust of over 40 years by 1 million+ customers, 4.9-star Google rating, and warranty of up to 10 years.