Measure a door’s width, height, and thickness in three spots each, then use the smallest number you get. Replacing a door starts with getting the numbers right. How to measure a door the right way keeps you from ordering a size that won’t fit.
Southeast Iowa homes deal with old framing and shifting foundations, so small measuring mistakes show up fast here. This guide covers each step, from checking the door slab to measuring the rough opening. Follow it in order, and you’ll walk into your next home project with real numbers, not guesses.
TL/DR
- Measure width, height, and thickness in three spots each, then use the smallest number
- Check the width at the bottom, middle, and top of the door slab
- Check the height on the left, centre, and right, from threshold to top jamb
- Interior doors run about 1 3/8 inches thick; exterior doors run about 1 3/4 inches thick
- Measure the jamb and rough opening only for full-frame or prehung replacements
What are the Steps to Measure a Door for replacement?
To measure a door for replacement, you need to measure the Width of Your Door, measure the Height of Your Door, measure the door thickness, and measure the Jamb and rough opening.
How to Measure the Width of Your Door
Measure the width in three spots and use the smallest number. Close the door and stand facing it. Run your tape measure across the door slab from edge to edge.
Measure the Bottom
Start at the bottom corner of the door. Stretch the tape straight across to the other side. Write down the number before you move up.
Measure the Middle
Move the tape to the centre of the door. Run it straight across from one side to the other. This spot often gives a slightly different number than the bottom.
Measure the Top
Use your step ladder if the door is tall. Stretch the tape across the very top of the door. Compare this number to your first two.
Doors settle over time, so width often shifts from top to bottom. That’s why the smallest number keeps your new door from sticking or rubbing.
How to Measure the Height of Your Door
Measure the height from the threshold to the top jamb in three spots. Start where the door meets the floor or threshold. Run the tape straight up to the top of the frame.
Measure the Left Side
Place your tape at the bottom left corner. Run it straight up to the top of the frame. Write the number down right away.
Measure the Centre
Move to the middle of the door. Run the tape straight up from floor to frame. Older homes often show a different number here.
Measure the Right Side
Repeat the same motion on the right side. Run the tape from the threshold to the top jamb. Compare all three numbers before you finish.
Floors settle too, especially in older homes near Fort Madison and Mount Pleasant. Take the smallest height here as well, since it keeps the door from binding at the top.
How to Measure Door Thickness
Measure straight across the door’s edge to find its thickness.
Open the Door Fully
Swing the door all the way open. This gives you full access to the edge. A partly open door will throw off your number.
Measure Across the Edge
Place the tape measure flat against the door’s edge. Read the number where the edge ends. Most homes use a standard thickness, so this step goes fast.
Standard interior door slabs run about 1 3/8 inches thick. Most exterior doors run 1 3/4 inches thick. A door that’s too thin won’t seal right against weather stripping. A door that’s too thick won’t close in the frame at all.
| Door Type | Standard Thickness |
| Interior door | 1 3/8 inches |
| Exterior door | 1 3/4 inches |
| Double door panel | 1 3/4 inches |
How to Measure the Jamb and Rough Opening
Measure the jamb width and the rough opening only if you’re replacing the full frame.
Measure the jamb width
Run your tape from the inside edge on one side of the jamb to the inside edge on the other. This tells you the width of the frame itself. Write this number down before moving on.
Measure the Rough Opening
Measure the raw wall space before any trim or frame goes in. This space should run slightly larger than your finished frame. That extra room gives the installer space to shim and level the door unit.
Common Door Measuring Mistakes to Avoid
The most common mistake is measuring the door frame instead of the door slab.
- Measuring the door frame instead of the door slab
- Only measuring once instead of three times
- Guessing at thickness instead of using a tape measure
- Skipping the rough opening on a full frame job
- Ignoring uneven floors or settled framing
Catching these early saves you a second trip to the store.
Ready for Your New Door?
How to measure a door takes just a few careful steps, and now you know exactly what to check. The Window Source of Southeast Iowa offers free, no-pressure consultations across Burlington and the Quad Cities. Our installers confirm every measurement before we order your new door.
FAQs
What are common door sizes?
Standard interior doors run 24 to 36 inches wide and 80 inches tall. Exterior doors often measure 36 inches wide for a single door.
What if my measurements don’t match on both sides?
Small differences are normal in older homes. Always use the smallest number you recorded to keep your new door from binding.
Can I measure a door while it’s still on its hinges?
Yes, but close it fully first. An open or hanging door gives you inaccurate readings for your final order.
Do I need a pro to double-check my numbers?
Not required, but it helps. A quick professional check catches small errors before they turn into an expensive reorder.