Are you getting ready to replace an old, outdated door, but feel completely paralyzed the moment you pull out your tape measure? You are not alone. Ordering the wrong door size is one of the most common—and expensive—DIY mistakes you can make, and it can easily ruin your entire weekend renovation plans.
A lot of confusion comes from simply not knowing what to measure. Should you measure the old door? The wooden frame? The hole in the wall?
Don’t worry! As professional door installers, we have broken down the process into the simplest, most foolproof method possible. If you are replacing an existing door slab, you don’t need a contractor. With a tape measure and these 3 easy steps, anyone can get the perfect measurements in under five minutes.
Before We Start: Tools and The Golden Rule
Before you start taking numbers down, make sure you are prepared. Having the right tools is the first step to an accurate measurement.
- A Steel Tape Measure: You must use a rigid metal tape measure. Never use a flexible fabric tailor’s tape, as it can stretch and bend, leading to disastrous inaccuracies.
- Pen and Paper: Or simply open the notes app on your smartphone to record the numbers immediately.
The Carpenter’s Golden Rule
“Measure twice, order once.” Always double-check your numbers before making a purchase. Note: This specific guide is for measuring an existing door slab (just the door itself) to replace it with a new one, not for measuring a rough opening for a pre-hung door system.
Step 1: Measure the Door Width
This is the most important measurement. If your new door is too wide, it won’t close; if it is too narrow, it will rattle and let drafts in.
How to do it: Stretch your tape measure horizontally across the door from the left edge to the right edge. Record the number in inches.
Expert Pro Tip: Measure in 3 Places
Older homes settle, and wood can warp over time. Your door might not be a perfect rectangle anymore. Always measure the width in three different places: the top, the middle, and the bottom.
Which number should you use? If the three numbers are different, use the widest measurement. It is always easier to plane or sand down a new wooden door by a fraction of an inch to make it fit. You cannot add wood back to a door that is cut too small!
Step 2: Measure the Door Height
Next, you need to ensure your door clears the floor without leaving a massive gap at the top.
How to do it: Run your tape measure vertically from the very top edge of the door down to the very bottom edge.
Just like the width, it is best practice to measure the height on the left side, the center, and the right side. Record the longest measurement.
Floor Clearance Warning: Take a moment to look at your floors. Are you planning to install thick new carpet, hardwood, or tile in the near future? If the floor height is going to increase, you may need to order a slightly shorter door to ensure it doesn’t drag on the new flooring.
Step 3: Measure the Door Thickness
This is the step that DIY beginners most often forget, yet it dictates whether your door will fit properly in the frame’s stop and whether your doorknobs will install correctly.
How to do it: Open the door slightly. Measure the thickness of the door edge (from the interior-facing side to the exterior-facing side).
Don’t panic if the number seems highly specific. The vast majority of doors follow standard industry thicknesses:
- Interior Doors (bedrooms, bathrooms, closets) are almost always 1 ⅜ inches thick.
- Exterior Doors (front doors, back doors) are typically thicker to provide security and insulation, measuring 1 ¾ inches thick.
Why it matters: When you buy new hardware, the latch mechanisms and the screws provided are sized specifically for standard door thicknesses. If you get this wrong, your lockset simply won’t work.
Bonus Step: Note the Hinge and Knob Locations
If you are ordering a “blank slab” (a door with no holes cut into it), you will need to transfer your hardware locations to the new door. While your tape measure is out, take these two extra measurements:
- Hinges: Hook the tape measure at the top of the door and measure down to the top edge of each of the three hinges.
- Bore Hole (Doorknob): Measure from the top of the door down to the exact center of the doorknob hole (usually around 36 inches). Then, measure from the edge of the door to the center of the hole (the “backset,” typically 2 ⅜ or 2 ¾ inches).
Frequently Asked Questions (FAQ)
Should I measure the door or the frame?
If you are just replacing the physical door (buying a slab door) because it is damaged or ugly, only measure the door itself. If the frame is rotting and you need to replace the entire system (buying a pre-hung door), you will need to remove the casing trim and measure the rough opening of the studs in the wall.
Do I round up or down when measuring a door?
Neither! Write down the exact measurement to the nearest 1/16th of an inch. Do not round up to the nearest whole inch. Give the precise numbers to your door manufacturer or hardware store, and let their experts determine the best standard size or custom cut for you.
Ready to Upgrade Your Doors?
Now that you have your measurements in the standard Width x Height x Thickness (W x H x T) format, you are ready to shop with confidence!
Take those numbers and start browsing our premium collection of interior and exterior doors. If you have any questions about your measurements or need a custom size, our team of experts is just a phone call away.
