Mattress warranties often cover mattresses that have sags deeper than 1 1/2 inches when no weight is applied to the mattress. Consumers can measure the mattress sag themselves before calling a service person to do an inspection of the mattress.
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Strip the mattress of sheets and any covers or pads. Visually inspect the mattress, looking for visible signs of sagging such as deep indents or bends in the mattress. Run your hands over the mattress to locate potential sags, but don't apply pressure.
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Hold the string across the mattress so part of the string goes over the possible sag. If you have two other people around, have them hold the string on each end so it is flush to the mattress and taut. You can also secure the string tightly with duct tape if no one is available. Most of the mattress should touch the string, while the sagging area should not be touching the string.
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Place the end of the measuring tape into the sag so that the metal end of the tape is flush against the mattress. The measuring tape handle and reel should be held up in the air above the mattress and string.
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Look at the measurement marking where the string and the measuring tape meet. This number will indicate the amount of mattress sag and give you an idea of whether the mattress warranty will cover the replacement costs. Repeat for any other sags you notice on the mattress.