The simplest way is to take twine and have someone help you stretch it across the bed, side to side. Stretch it tight, touching the sides of the bed. If there is an impression, the middle of the stretched twine should be floating above the bed, highlighting the depth of the impression. You want to measure with a tape measure from the deepest part of the impression to the twine. Careful not to push the ruler or measuring tape down into the mattress, just rest it on the surface as not to exaggerate the impression. Hope this helps!

First you remove your sheets and protector. Then you can do as joe says or a yard stick. Then measure the deepest point as said by Joe. Every manufacturer allows for so much so be sure that you know the allowance for yours.

If you are by yourself and can't get the twine to lay taught across the mattress, try tying some weights on either end. Fishing weights are ideal, but anything will do. Also, when measuring the depression, most manufacturers will not accept measurements taken into stitching and/or buttons as those parts of the mattress are already about 1/2 an inch lower than the mattress when brand new. You have to measure into an open area between stitching. Hope this helps!!!

I use a piece of door molding I purchased from Home Depot (the really narrow one) as it is longer than a yard stick and doesn't have much weight to it. If you don't have a yard stick or the door molding you can use a broom handle. Don't just measure from edge to edge because if you sit on the edge of your bed, you may also have a dip there. Also measure from corner to corner, as no one sleeps on the corner of the bed and that may not have compressed over time.

Hope this helps.

To be clear, you will measure from the string or straight edge, down to the lowest point at the top of quilt...not into the stitching. Most warranties require a sustained depth of 1.5" or greater to prevail in a warranty claim. Check your mattress for stains of any kind and anywhere on the surface of the mattress. A stain will automatically void the warranty...make a moot point of the depth of your impression.