Close the door the way you normally do and note where you see daylight. Record the gap at several points around the frame. One number can hide an uneven fit.
Measure each edge separately
Write down readings for the top, hinge side and latch side. On each side, measure where the gap appears narrowest and widest. Note the position of each reading. Record the bottom separately, because the threshold or sweep may need attention of its own.
If a spot is hard to measure safely, photograph it and say where it is. Keep your fingers clear of the door edge.
Check how the door closes
Does the latch miss the strike plate? Do you have to lift the handle to lock the door? Mark any rubbing or sticking points. The U.S. Department of Energy’s weatherstripping guide advises addressing alignment and hardware issues before fitting a new seal.
Check the instructions for the seal
Use the recorded range when you compare products. If a seal lists a supported gap range, compare it with both your narrowest and widest readings. Bring a photo of the existing seal and its mounting surface if you can.
Send the readings and a photo of the closed door when you ask about door repair.