How to Measure Mattress Sag: Step by Step Guide


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how to measure mattress sag

Does your mattress feel less comfortable than it used to be? Do you see signs of sagging or indentations on your bed, often in the areas you sleep? If so, then your mattress may be saggy.

There is nothing less comfortable than sleeping on a sagging mattress. Your body doesn’t receive proper support, you’re more likely to experience aches and pains, and you won’t feel well rested.

All mattresses eventually sag, or form body impressions to some degree over time. However, if your relatively new mattress has developed significant sagging (anything in excess of 1.5″ to 2″), there may be a structural defect, and you may be eligible to claim a replacement under warranty.

Not all dips on mattress surfaces are a point of concern!

Mattress sag is often confused with body impressions, as both involve indentations on the mattress surface. Body impressions are shallow depressions (usually 0.5 to 1.5 inches) that form on the mattress surface in the area where you often sleep. They are caused by the weight of your body compressing the top foam layer over time. However, body impressions are not a sign of sagging and are normal. In fact, body impressions are quite desirable and may provide the extra pressure relief that many people look for in a mattress.

Mattress sag, on the other hand, is a permanent and deeper depression. It is caused by the springs or other support materials in the mattress becoming damaged or weakened over time. This can be from normal wear and tear, or from a manufacturing defect.

Body impressions only become problematic when they exceed 1 to 1.5 inches in depth, as this is indicative of premature sagging.

Sometimes, the foam layers permanently soften around the body impression, making the mattress feel uneven, lumpy and uncomfortable. Such cases are troublesome as you can’t claim a warranty nor sleep comfortably. However, there are a few ways to firm up the softened area and make the mattress usable to some extent (more on that below).

But first, let’s try and measure the depth of indentation or sag.

How to Measure Mattress Sag

sagging mattress example

Mattress manufacturers may have their policies and procedures for measuring mattress sag. The general idea is to measure the depth of the deepest point of indentation in your mattress surface.

Here’s what you will need:

  • [Either one] String, yardstick, shower curtain rod, or any straight edge stick.
  • Measuring tape or a ruler.
  • Camera or a decent phone to take pictures (optional)

Step 1: Strip off all the bedding, including sheets, blanket, mattress protectorstopper, and pillows.

Step 2: Smooth the surface: Use your hands to remove wrinkles and lumps in the top of your mattress to expose the dip, and ensure an accurate measurement.

Step 3: Lay the string or any straight edge over the largest part of the dip horizontally (parallel to the headboard). Also, repeat this step by measuring the dip vertically (perpendicular to the headboard). In some cases, the dip may be larger when measuring vertically – that’s why taking both measurements is prudent.

Note: If you’re using a string, make sure to pull it taunt. You can use markers or painter’s tape to secure the string.

Step 4: Measure: Hold the measuring tape or ruler vertically from your mattress surface to the string. Make a note of the measurement at the deepest point of the sag. Do not apply pressure to the measuring device, as this may result in an inaccurate reading. The distance from the string to the bottom of the dip is how much your mattress is sagging.

Step 5: Take pictures (optional): If the mattress manufacturer requires pictures as part of the warranty claim process, take pictures of the sagging area from different angles. Include the ruler or measuring tape in the photo for reference.

Now that you know how to measure mattress sag, all you need is to determine if your mattress is within the warranty period.

Claiming the Warranty

Most mattresses come with a 7 to 10 year warranty, covering sags anywhere from 0.75″ to 2″. If your mattress is within warranty and the sag is higher than the threshold limit, you can file for a warranty claim. The mattress company may send someone to do a physical inspection, or ask you to send pictures or video as part of the claim process. You will also require the original invoice or proof of purchase.

Some brands might give you a brand new mattress as a replacement, or offer a pro-rata refund. If it’s a pocket spring mattress, some brands may repair it by replacing the faulty coils.

If the mattress is beyond warranty or the company denies your claim, don’t worry. There are still a few things you can do to make the mattress usable.

Tips to Limit/Fix Mattress Sag

Truth be told, you can’t completely fix the sag, or make the mattress as good as new, but there are a few ways to minimise the effect to get some extra time out of your bed.

1) Rotate your mattress: It’s a good practice to rotate your mattress by 180-degree every 3 to 6 months to even out the wear and tear. If your mattress is already sagging, you can try rotating your mattress and see if it makes any difference.

2) Switch sides with your partner: You can also try sleeping on your partner’s side and see if it’s any better. It’s similar to the rotating your mattress method, except you switch sides rather than getting into the hassle of rotating the mattress.

3) Add a topper: A mattress topper may cover the bumps and dents on the mattress surface enough to sufficiently provide an even surface for sleeping. Also, it adds an extra layer of comfort. Depending on your preference, you can choose from various toppers like latexmemory foam, feather, etc. You can check our top mattress topper picks here.

4) Extra pillows: As a temporary solution, you may try putting a thin pillow over the saggy areas or where you feel discomfort to even out the indentation.

5) Add a piece of plywood: If your mattress feels soft in certain areas or there are deep dents, it’s also possible your bed base isn’t providing proper support to the mattress. Beds with slats far apart or metal frames usually face this issue.

You may try sliding a piece of plywood (1/4″ to 1/2″) between the mattress and bed base. Note that the thicker the plywood, the more firm your mattress will feel.

Upgrade if nothing works.

If you can’t claim the warranty or our tips to minimise the sag didn’t help, it’s time to start thinking about replacing your mattress. Sleeping on a less than the ideal surface is not only bad for your back, but also disrupts your sleep quality.

Hopefully, this guide helped you understand how to measure mattress sag and what can be done about it. If you have any queries or suggestions, please reach out to us in the comments section below.

James Lee

James Lee

James is the content director at WhatASleep, a certified sleep science coach from Spencer Institute and our in-house sleep product expert. Over three years, he's personally tested hundreds of mattresses. He likes to keep his reviews simple, objective, and straightforward so that the readers don't have to go through decision fatigue which is often the case while shopping for sleep products.

He hails from Sydney and has a journalism degree from the University of Queensland and an MBA from Melbourne Business School.


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