Slope calculator — grade in %, ‰, mm/m and 1:N

A few gradients come up on almost every project. Accessibility ramps are commonly limited to 1:12 (about 8.3%) — the maximum in both the US ADA guidelines and much of European practice — with gentler slopes like 1:20 preferred and landings breaking up longer runs. Drainage falls on floors and pipework typically sit around 1–2%: wet-room floors are often laid at 1:50–1:100 toward the drain, and gravity drainage pipes at about 1% (1:100), which US plumbing codes express as 1/4 or 1/8 inch per foot depending on pipe size.

Outdoors, ground around a building is usually graded away from the facade at about 1:20 for the first few meters (roughly 6 inches over the first 10 feet) to keep water away from the foundation, and driveways stay comfortable up to about 12%. These are rules of thumb — the binding numbers come from your local building code and the project documents, so treat the calculator as a converter, not a code check.

Every form describes the same slope ratio: the rise divided by the horizontal run. Percent is that ratio × 100 and per-mille × 1,000 — and since a meter is 1,000 mm, per-mille and mm per meter are always the same figure. A ratio 1:N reads as 1 unit of rise per N units of run, so 1:50 is 2% and 20 mm/m. Inches per foot is the ratio × 12, which is why it matches the familiar x:12 roof-pitch convention. Degrees are the actual angle, found with the arctangent; for the shallow slopes that dominate on site, percent ÷ 1.75 is close enough.

On site, convert the requirement into a drop over the actual distance — that is how you work with a laser and a straightedge. A 1:50 fall in a shower zone 0.8 m from the drain means 16 mm of drop; a 12 m pipe trench at 1% must lose 120 mm; a 10 ft run at 1/4 in/ft drops 2.5 in. Check with a rotary laser at several points, or with a spirit level and a spacer block over short distances, and always measure the finished surface rather than the substrate.