About
How we measure and check figures
A number is only as good as the way it was arrived at. Here is ours, plus the method for taking your own.
On this page (5 sections)
Short answer
Every figure is checked against a named source, cross-checked against a second where one exists, labelled by the kind of authority it carries, and given in both units. Where sources conflict, both are published.
Our verification process#
Step 1: find the primary source. A dimension is traced back to the document that originates it, not to another website quoting it. The number for a hallway width comes from the IRC, not from an article about the IRC.
Step 2: classify its authority. Code, planning guideline, ergonomic reference, or trade practice. This determines the language the page is allowed to use about it. Only codes get "required".
Step 3: cross-check. Where a second independent source exists, the figure is checked against it. Conflicts are published, not resolved by averaging.
Step 4: sanity-check against the body. Clearances are ultimately about people, so a figure that cannot be explained in terms of a body dimension gets a second look. A 36 in walkway is a 24 in walking sweep plus a margin. A 42 in work aisle is a person plus an open dishwasher door. If a number has no such explanation, it is usually either wrong or context-specific.
Step 5: convert carefully. Imperial to metric conversions are rounded to a precision that matches how the dimension is used on site.
Step 6: date it. The page records when the figure was last checked.
What we cannot do for you#
We cannot tell you what your local building code says. Model codes are amended by states, counties and municipalities, and the amendments are exactly where the differences that matter live. Anything structural, electrical, plumbed, gas-fired or subject to permit needs to be confirmed with the authority having jurisdiction or a licensed professional. Treat this site as the thing that tells you what to ask them.
How to take your own measurements#
Most planning errors on domestic projects trace back to a measurement, not to a decision.
Measure the room three times. Width at the front, middle and back; length at both ends and the centre. Use the smallest figure. Almost no domestic room is truly square, and a 12 ft wall in an older house is routinely out by half an inch to an inch end to end.
Measure to the finished face. Skirting boards take 0.5 to 1 in (13 to 25 mm) off each wall, architraves take more at a doorway, and a radiator can take 4 in (10 cm). A plan drawn to bare plaster is a plan that fails.
Record the fixed obstacles. Door swings, window sills and their heights, radiators, sockets and switches, vents, the position of the ceiling rose, and the direction of the floor joists if you are hanging anything heavy.
Measure the ceiling in the corners, not the middle. Older ceilings sag.
Photograph the tape in position. It sounds fussy. It saves a second trip.
The tolerances that matter#
| Element | Realistic tolerance | Why it matters |
|---|---|---|
| Room width, older house | 0.5 to 1 in (13 to 25 mm) over 12 ft | Built-in joinery and wall-to-wall furniture |
| Floor level | 0.25 to 0.75 in (6 to 19 mm) across a room | Curtain hems, tall units, appliance doors |
| Door opening, out of square | Up to 0.5 in (13 mm) | Delivery clearance for large items |
| Ceiling height variation | 0.5 to 1.5 in (13 to 38 mm) | Full-height cabinetry, tall wardrobes |
| Published furniture dimensions | 1 to 2 in (25 to 50 mm) | Cushion overhang and handles often excluded |
That last row is the one that catches people. A sofa listed at 84 in wide may measure 86 in across the arms once upholstered, and a fridge listed at 35.75 in deep may need 38 in with the handle and 42 in for the door to open past 90 degrees. Where a page on this site quotes an appliance or furniture figure, it says whether the figure includes handles and doors.
Reporting a figure you think is wrong#
Send the page, the figure and the source you believe is correct. See corrections for what happens next.