How to Measure a Garage Door: The Right Way Before You Buy or Replace
By the TrustyGarageDoor Editorial Team —
written and reviewed with working garage door service professionals.
Published June 15, 2026
Ordering a garage door with wrong measurements is an expensive mistake — custom doors aren’t returnable, and a door that’s 2 inches too wide won’t fit in a frame that’s been there for 40 years. Measuring correctly before you get a quote (or place an order) takes about 15 minutes and five measurements.
Here’s exactly what to measure and why each number matters.
The five measurements you need
1. Opening width
Measure the width of the actual opening — the clear span between the inside faces of the door frame, at the narrowest point. Measure at the top and bottom (frames can be out of square). Use the smaller number.
Standard widths: 8 ft (single car), 9 ft (single car, wide), 16 ft (double car), 18 ft (double car, extra wide).
If your measurement doesn’t match a standard size, you’ll need a custom-width door. Custom doors add $200–$600 to the price and typically add 4–6 weeks to delivery.
2. Opening height
Measure from the floor to the bottom of the header — the horizontal framing member above the opening. Again, measure at multiple points; floors are sometimes uneven and frames sag.
Standard heights: 7 ft and 8 ft. Some older homes have 6 ft 6 in openings; some commercial-style residential garages run 9 or 10 ft.
Important: If your floor has a slope at the threshold, measure from the high point of the floor, not the low point. The door seals to the high side; if you order to the low side, you’ll have a gap.
3. Headroom
Headroom is the distance from the top of the opening to the ceiling (or to the first obstruction above — a beam, a pipe, the garage door opener track bracket).
Standard torsion spring systems require at least 10 inches of headroom. Low-clearance hardware is available for 4–6 inches of headroom, but it costs more and has limitations.
Measure from the top of the opening to the ceiling at the center (where the opener carriage typically runs) and at the sides. Note the minimum clearance point.
If you have less than 10 inches: tell your supplier before ordering. Some door and track configurations won’t work and need to be specified in advance.
4. Sideroom
Sideroom is the distance from the edge of the opening to the nearest side wall or obstruction on each side. Standard track systems need 3.5 inches of sideroom minimum, sometimes 5 inches for heavier doors.
Measure left and right separately. In older garages, the opening is sometimes very close to a side wall, which limits your hardware options.
5. Backroom (depth)
Backroom is how far back the garage goes from the inside of the door opening. The door, when fully open, is stored horizontally along the ceiling. The backroom must be at least as long as the door is tall, plus about 18 inches for the horizontal track and opener.
For a 7-foot door: you need roughly 7 ft + 18 in = 8 ft 6 in of backroom minimum.
For a 8-foot door: approximately 9 ft 6 in minimum.
Measure from the front wall (inside, at the door opening) straight back to the rear wall. If you have a car in the garage, measure to the furthest point the car extends toward the back wall — you need clearance for the door to open even when a vehicle is parked inside.
Standard vs. non-standard sizes
| Width | Height | Notes |
|---|---|---|
| 8 ft | 7 ft | Single car, most common in homes built 1950–1990 |
| 9 ft | 7 ft | Single car, modern standard |
| 9 ft | 8 ft | Single car, SUV-friendly |
| 16 ft | 7 ft | Double car, older standard |
| 16 ft | 8 ft | Double car, most common new construction |
| 18 ft | 7 ft | Double car, wider — rare but standard |
| 18 ft | 8 ft | Double car, premium |
If your measurements fall within ¼ inch of a standard size, the standard door fits (most installations have a small gap between door edge and frame, covered by weatherstripping). If your measurements are 1 inch or more outside standard sizes, plan for custom.
What about the door panel size vs. the opening size?
There’s a difference between the opening size (what you measured) and the door size (what you order). A door is typically the same size as or slightly larger than the opening — it overlaps the frame slightly. A 16 ft x 7 ft door goes in a 16 ft x 7 ft opening, with the frame and weatherstripping sealing the perimeter.
Don’t order a door that’s smaller than the opening to “leave room” — you’ll have gaps on all sides.
Measuring for a replacement vs. a first-time install
Replacement: If you’re replacing an existing door, also measure the current door itself — it tells you what the installer already determined fits. Confirm your frame measurements match. If the old door was a non-standard size, the replacement needs to be the same non-standard size unless you’re also modifying the frame.
New install: If you’re converting a car port or adding a garage door to a framed rough opening, coordinate with your contractor. Rough openings are typically framed 1 inch wider and ½ inch taller than the planned door size, leaving room for the frame and hardware.
Before you get a quote
Bring these five numbers when talking to a supplier or technician:
- Opening width (exact, at narrowest)
- Opening height (exact, at lowest point)
- Headroom (minimum clearance)
- Sideroom (left and right)
- Backroom depth
A good supplier can spec the right door and hardware configuration from these five numbers. If a salesperson gives you a quote without asking about headroom or sideroom, they’re guessing on the hardware — ask.
Ready to replace a door and want a vetted installer who measures correctly before quoting? Contact TrustyGarageDoor and we’ll connect you with a local pro who handles measurement, specification, and installation together.