How to Measure for a Door Threshold & Door Bottom: The Complete Guide
Order the wrong size threshold, and you'll know within five minutes of trying to install it.
The door won't close. The screw holes don't line up. The gap under the door is either too tight for the sweep or too wide for the seal to reach. And if you ordered a custom-cut piece, there's often no return.
Measuring for a door threshold or door bottom isn't complicated - but it does require three specific measurements that most people either skip or take incorrectly. This guide covers exactly what to measure, how to measure it, and the mistakes that lead to costly returns.
Why Accurate Measurement Matters More Than You Think
A door threshold does three jobs at once: it seals the bottom of the opening, it bridges the transition between two floor surfaces, and it provides a mounting surface for the door sweep or door bottom to compress against.
If the width is off by even 1/8 inch, the threshold won't sit flush against both jambs. If the depth is wrong, the threshold won't cover the full opening - leaving a gap at the exterior or interior edge where water, drafts, and insects get through. If the gap height is mismeasured, the door sweep either drags on the floor or fails to make contact at all.
This is especially critical on commercial-grade thresholds and custom-length orders, where a wrong measurement often means a non-returnable product and a delayed jobsite.
Tools You'll Need Before You Start
- A metal tape measure (not a cloth or plastic one - they stretch)
- A pencil and paper, or a phone note
- A flashlight (for checking the gap under the door)
- A small level (optional, but helpful for checking floor transitions)
- A helper (optional, for holding the tape on wide openings)
Step 1: Measure the Door Width (Three Places)
This is the most important measurement, and it's where most people make their first mistake. Don't measure just once.
Open the door and measure the clear width of the opening from the inside face of one jamb to the inside face of the other jamb. Take this measurement in three places:
- Near the top of the opening
- In the middle of the opening
- Near the bottom (just above the existing threshold)
Record all three numbers. If they're not identical, use the smallest measurement. Door frames are rarely perfectly square, and a threshold that's even slightly too wide won't fit without cutting - which voids most warranties and finishes.
Most standard doors fall into these width ranges:
- 32 inches - common for interior closets and bathrooms
- 36 inches - the most common exterior and interior door width
- 42 inches - wider entries and some commercial doors
- 48 inches and above - double doors and oversized openings
Step 2: Measure the Threshold Depth
The depth of a threshold is the distance from the exterior face of the door opening to the interior face. This is not the same as the door thickness.
Place your tape measure at the outside edge of the door frame and extend it straight across to the inside edge. Most residential thresholds are 4 to 6 inches deep. Commercial thresholds and saddle thresholds often run 6 to 8 inches deep or more, especially where they need to cover a floor transition.
If you're replacing an existing threshold, measure the depth of the old one as a reference - but also measure the actual opening, because the old threshold may not have been sized correctly either.
Step 3: Measure the Gap Under the Door
This is the measurement that determines whether your door sweep or door bottom will actually seal - or whether it will drag.
Close the door completely. Shine a flashlight along the bottom edge and look from the other side. Then measure the distance from the bottom of the door to the top of the threshold (or the floor, if no threshold is installed yet).
Measure this gap at three points: left side, center, and right side. If the gap varies by more than 3/16 inch, the door may be sagging or the floor may be uneven - and you'll need an adjustable sweep or a threshold with adjustability built in.
Here's what the gap height tells you:
- Under 1/4 inch - a standard surface-mount sweep is usually sufficient
- 1/4 to 1/2 inch - most standard door bottoms and sweeps will work
- 1/2 to 3/4 inch - you need a drop-down or automatic door bottom, or a thicker sweep
- Over 3/4 inch - the gap is too large for most standard sweeps; a threshold riser or repair may be needed
This measurement is also where heavy-duty door bottoms earn their price difference - they have a deeper seal projection and can handle larger, more irregular gaps than a basic residential sweep.
Step 4: Check the Floor Transition Height
If your door separates two different floor surfaces - for example, tile inside and concrete outside, or carpet inside and hardwood in a hallway - measure the height difference between the two surfaces.
A saddle threshold is designed to bridge a small height difference (usually up to 1/2 inch). An offset threshold is designed for larger, uneven transitions. If the height difference is over 1/2 inch and the opening needs to be accessible, you'll need a ramped or beveled threshold to meet ADA requirements.
Step 5: Account for Door Swing and Clearance
Before you finalize your measurement, check which way the door swings. An in-swinging door closes against the interior side of the threshold. An out-swinging door closes against the exterior side.
This matters because the threshold profile needs to match the swing direction. A threshold designed for an in-swinging door may not seal properly on an out-swinging door, especially if the door has a drip cap or a bottom rail that projects outward.
Also check the door's clearance above the threshold: if the door has a bottom rail or a sweep already installed, measure the height from the bottom of that hardware to the floor. This tells you how much room you have for the new threshold without the door dragging.
Common Measurement Mistakes (And How to Avoid Them)
Mistake 1: Measuring only once
Door frames are rarely perfectly square. Always measure width in three places and use the smallest number.
Mistake 2: Confusing door thickness with threshold depth
Door thickness is typically 1-3/4 inches. Threshold depth is the full width of the opening from exterior to interior face - usually 4 to 8 inches.
Mistake 3: Measuring the gap with the door open
The gap only matters when the door is closed. Measure with the door fully closed and latched.
Mistake 4: Forgetting about existing hardware
If a sweep or door bottom is already installed, measure from the bottom of that hardware - not from the bare door bottom.
Mistake 5: Assuming the old threshold was the right size
Old thresholds are often undersized, warped, or installed incorrectly. Always measure the actual opening, not just the old piece.
Measurement Quick-Reference Table
| What You're Measuring | Where to Measure | What to Record |
|---|---|---|
| Door width | Jamb to jamb, at top, middle, and bottom | Smallest of the three numbers |
| Threshold depth | Exterior face to interior face of the opening | Full depth in inches |
| Gap under door | Bottom of door to top of threshold/floor, at left, center, right | Largest of the three numbers |
| Floor transition height | Difference between the two floor surfaces | Height difference in inches |
| Door swing direction | Visual check | In-swinging or out-swinging |
When to Call for a Custom Size
If your measurements fall outside standard sizing - typically anything over 95 inches in width - the threshold or door bottom will need to be custom-cut. Custom sizes are available up to 12 feet, but they require a phone call because oversized freight shipping is priced individually.
Don't guess on a custom order. Call our team with your measurements and we'll confirm the exact size, check availability, and provide an accurate shipping quote before you commit. You can see the full process on our custom orders page.
FAQ
Do I measure the door or the opening for a threshold?
Measure the opening (jamb to jamb), not the door slab. The threshold needs to span the full width of the opening, from one jamb to the other.
What if my door width measurements are all different?
Use the smallest measurement. A threshold that is slightly too narrow can often be shimmed or sealed, but one that is too wide cannot be installed without cutting - which may void the finish or warranty.
How do I measure for a door sweep if there's already a threshold installed?
Measure the gap from the bottom of the door to the top of the existing threshold, with the door closed. Then measure the door width in three places and use the smallest number.
Can I measure for a threshold without removing the old one?
Yes, but you'll also need to measure the height of the old threshold to know how much clearance you have. If the old threshold is thick, a new one of the same height may cause the door to drag.
What's the difference between measuring for a threshold and a door bottom?
A threshold requires three measurements: width, depth, and gap height. A door bottom or sweep only requires two: door width and gap height - because it mounts to the door, not the floor.
How do I measure a double door opening?
Measure the full width from jamb to jamb, and also measure the width of each individual door panel. You'll need both numbers to choose the right threshold and astragal setup.
Have more questions? Visit our full FAQ page.
Final Thoughts: Measure Three Times, Order Once
A door threshold or door bottom is a small piece of hardware, but it's the piece that determines whether the opening actually seals, closes smoothly, and meets code. The difference between a 36-inch and a 36-1/8-inch measurement can be the difference between a five-minute install and a return shipment.
Measure width in three places. Measure depth from exterior to interior face. Measure the gap with the door closed. Use the smallest width number and the largest gap number. And if anything falls outside standard sizing, call before you order.