How to measure your garage door opening
Short answer
Measure the finished opening, not the existing door. Width across the opening at its narrowest point, height from the finished floor to the underside of the header. Then three more numbers that decide what hardware fits: headroom, side room on each side, and backroom depth. Five measurements. If you land on 9 by 7 or 16 by 7 you are in stock pricing.
Why you measure the opening and not the door
A garage door is larger than the opening it covers, because it seals against the inside face of the jambs and header. Measure the door panel and order that size and you will order a door that is too small. Measure the hole.
Step 1: Width
Measure horizontally across the finished opening, from the inside face of the left jamb to the inside face of the right jamb. Take it three times: near the floor, at mid-height, and just under the header. Use the narrowest number and note the spread. A spread over about a quarter inch tells you the jambs are not plumb or parallel, which matters for how the door seals.
Step 2: Height
Measure from the finished floor to the underside of the header, three times: at the left jamb, the center, and the right jamb. Garage slabs are poured with a slope toward the opening for drainage and old slabs settle. If the center is an inch lower than the corners, the bottom seal has to close that gap. Record the largest height.
If you plan to add a concrete overlay or epoxy floor later, measure to the future floor height.
Step 3: Headroom
The vertical distance from the top of the opening to the lowest obstruction above it. Ceiling, joists, ductwork, pipes, light fixtures. This decides your spring and track system.
- About 12 inches or more: standard lift torsion hardware fits
- Roughly 6 to 12 inches: low headroom hardware. It works, it costs more, and the track layout changes
- Under about 6 inches: specialty low headroom hardware. Confirm with an installer before buying anything
Measure to the true lowest obstruction and let the installer solve it.
Step 4: Side room
The horizontal distance from the edge of the opening to the nearest wall, post or obstruction, on each side. Measure both separately, because they are frequently different. Roughly 3.75 inches minimum per side for standard track hardware; 5.5 inches is comfortable. If one side is tight because a wall returns immediately, say so up front.
Step 5: Backroom
The depth from the opening straight back into the garage along the ceiling, clear of obstructions. Rule of thumb: door height plus about 18 inches for the door alone, or door height plus roughly 4 feet 6 inches if you are installing an opener. Confirm against the opener's installation manual, because rail lengths vary by model.
Check that path for water heaters on a platform, storage shelves, an attic ladder or a low soffit. Those come up constantly and are easier to handle before the door is ordered.
Standard sizes
Single doors
8 ft × 7 ft
9 ft × 7 ft
9 ft × 8 ft
Double doors
16 ft × 7 ft
16 ft × 8 ft
18 ft × 7 ft
The 9 by 7 single and the 16 by 7 double are the two most common openings in Omaha housing. The 8-foot-high versions show up in newer construction and in homes built to take a truck or a lift. At these sizes you get stock pricing and a normal lead time.
What non-standard means
Cost. Expect a premium over stock, depending on how far off standard you are.
Lead time. A stock door is days. A custom width or height is typically weeks, because it is built to order. If you are working against a deadline, get the lead time in writing before you commit.
A third option is worth knowing. If your opening is only slightly non-standard, it is sometimes cheaper and faster to modify the opening than to order a custom door. Adding a jamb strip to narrow a 9 foot 2 inch opening to 9 feet even is a small carpentry job. Ask whether that is on the table.
If the opening is out of square
Measure both diagonals, corner to opposite corner. If they differ by more than about a quarter inch, the opening is out of square. Garage doors are rectangular and rigid. They do not conform to a parallelogram, and the result is a visible triangular gap where cold air and drifting snow get in.
- Up to about a half inch: usually corrected with perimeter weatherstrip and careful jamb shimming
- Half an inch to about an inch and a half: the jambs need shimming or re-setting before the door goes in. This is real work and it should appear on the estimate as a line item, not as a surprise on install day
- Larger, or a sagging header: stop. This is structural. A header that has deflected is not carrying its load properly, and a new door on a failing header will not close correctly in two years
Always give your installer the diagonal numbers. An installer who does not ask about square is one who will hang the door and leave.