What's inside
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What a door reveal is
A door reveal is the visible gap between the edge of a door and its frame. On a typical interior door, the side and top gaps are often around 2–3 mm, but the right size depends on the door, frame, hinges, floor covering and job specification. Treat that range as a starting point, not a universal standard. Check the manufacturer’s instructions or the dimensions called for on the job.
The aim is not just to get one measurement right. It is to make the gap reasonably even along each edge, while leaving enough clearance for the door to swing without rubbing. A folding rule can do this reliably if you use the same reference points and method each time.
Choose a consistent reference
Measure the clearance between the door edge and the frame stop or rebate, depending on how the door is fitted. Do not measure some points from the stop and others from the frame face: those surfaces are not interchangeable. For a simple, repeatable check, measure the gap at the face where you can see both edges clearly.
Before measuring, close the door gently and check that it sits in its normal position against the stops. A door held away from the stop can make the gaps look larger on one side. If the latch is pulling the door hard against the frame, note that too; the reveal may change when the latch is released.
Prepare the folding rule
Use a folding rule with clear, undamaged graduations. A metric rule marked in millimetres makes small gaps easier to compare than one marked only in centimetres. A metric folding rule is a sensible choice if you regularly fit doors or mark out joinery. A basic rule is fine for occasional checks; paying more for a premium finish will not compensate for loose joints or worn markings.
Open only the sections you need, and make sure the joints are fully straight and not sloppy. Check the rule against a known length, such as a reliable steel tape, if it has been dropped or has seen heavy site use. A damaged hinge can let the rule flex, which makes a short measurement harder to repeat.
Measure the gap at repeatable points
Close the door, then measure at the top, middle and bottom of each side. On a tall door, add a point roughly halfway between the middle and each end if the frame or door looks bowed. For the head, check near both corners and in the centre. Write down each reading rather than relying on memory.
Hold the rule square to the door edge and frame. Use the same graduations each time—for example, line the zero mark up with the door edge and read the frame edge. Do not press the rule into a soft seal or let it sit at an angle. If the gap is smaller than a graduation, record it as approximate rather than pretending the rule gives a precise fraction of a millimetre.
Folding rules are more useful here as a consistent comparator than as a precision feeler gauge. If two readings are both close to 3 mm, the practical question is whether they are similar and whether the door swings freely, not whether one is exactly 2.8 mm.
Use a spacer for repeatability
For a quick visual check, a small offcut cut to the intended reveal can serve as a spacer. If the target is 3 mm, make a clean 3 mm strip and try it at each measurement point. It should pass through without force and without a noticeably loose gap. A spacer is helpful when comparing several points, but it only works if the offcut is straight and its thickness is checked.
A purpose-made door gap or feeler gauge can make very small gaps easier to compare. It is not essential for fitting a normal interior door, and a cheap gauge may be adequate for occasional work. Avoid forcing metal blades between painted surfaces: they can scratch the finish or wedge against a tight spot.
Compare the checking methods
| Method | Useful for | Limitations |
|---|---|---|
| Folding rule | Reading and recording gaps at several points | Hard to judge gaps smaller than its graduations; needs careful alignment |
| Cut spacer | Quickly comparing a gap with a chosen target | Only as accurate as its thickness and cut |
| Feeler gauge | Comparing small clearances more closely | Can mark finishes or get stuck if forced |
For most door work, use the folding rule to map the reveal and a spacer to confirm consistency. A feeler gauge is worth adding if you regularly work to tight tolerances or need to investigate a small rub.
Interpret uneven readings before adjusting
If the top and bottom gaps differ, do not assume the door edge needs planing. First check that the frame is plumb and square, the hinges are seated properly, and the door is not twisted. A door that touches at one corner may be responding to a frame that is out of square. Removing wood before checking the cause can leave an oversized reveal when the real problem is corrected.
Look for patterns in your notes. A gap that narrows steadily from top to bottom suggests a different issue from one tight spot in the middle. If the reveal changes when you lift the handle or push the door gently, check for hinge movement, loose screws or a sagging door. Planing is a last adjustment, not a substitute for diagnosing the fit.
Common measuring mistakes
One frequent error is measuring with the door open at a different angle each time. The apparent gap changes with the viewing angle, especially near the hinge side. Keep the door in the same closed position and look straight at the joint.
Another is measuring only at eye level. A frame can be bowed, and a door edge can vary along its length. Three points per edge are a minimum useful check. Finally, do not confuse a reveal with the clearance at the floor: floor clearance depends on the floor finish and door swing, so measure and assess it separately.
Record the readings, compare them with the job requirements, and make adjustments only after checking the frame, hinges and swing. That simple routine makes a folding rule a dependable site tool without asking it to do the work of a precision gauge.