Quick answer: a 1:50 gradient = 20 mm of fall per metre = 2%; a 1:100 gradient = 10 mm/m = 1%. Total fall = run ÷ ratio: an 8 m run at 1:50 needs 160 mm of fall. Canada's NPC requires 1:50 for pipe up to 3″ (DN 80) and 1:100 for 4″ (DN 100) and larger.
Metric drainage speaks in ratios: "lay it at 1 in 50." The notation is compact, the arithmetic is friendlier than fractions of an inch — and mixing it with imperial mid-job is where mistakes creep in. Here's the full translation layer.
Reading a gradient ratio
1:X means 1 unit of fall for every X units of run — units cancel, so it works in mm, metres or fathoms. The bigger X is, the flatter the pipe:
| Gradient | mm per metre | Percent | Closest in/ft |
|---|---|---|---|
| 1:40 | 25 mm/m | 2.5% | ≈ 0.30″/ft |
| 1:50 | 20 mm/m | 2% | ≈ ¼″/ft (2.08%) |
| 1:80 | 12.5 mm/m | 1.25% | ≈ 0.15″/ft |
| 1:100 | 10 mm/m | 1% | ≈ ⅛″/ft (1.04%) |
| 1:200 | 5 mm/m | 0.5% | ≈ 1/16″/ft (0.52%) |
Calculating total fall
Two equivalent formulas — use whichever matches how the spec is written:
- From a ratio: fall = run ÷ X. Example: 12 m at 1:100 → 12 ÷ 100 = 0.12 m = 120 mm.
- From mm/m: fall = run (m) × mm/m. Example: 6 m at 20 mm/m → 120 mm.
NPC 2020: Canada's metric slope rules
The National Plumbing Code of Canada writes its minimums as ratios:
| Pipe size | NPC 2020 minimum | Equivalent |
|---|---|---|
| 1½″ – 3″ (DN 40 – DN 80) | 1:50 | 20 mm/m · 2% · ≈¼″/ft |
| 4″ – 15″ (DN 100 – DN 375) | 1:100 | 10 mm/m · 1% · ≈⅛″/ft |
Provincial adaptations apply — verify with your local authority. For the U.S. codes, see IPC vs UPC vs NPC.
DN sizes vs imperial sizes
Mixed-unit sites (very much a Canadian specialty) juggle both labels for the same pipe:
| Imperial | DN | Imperial | DN |
|---|---|---|---|
| 1½″ | DN 40 | 5″ | DN 125 |
| 2″ | DN 50 | 6″ | DN 150 |
| 2½″ | DN 65 | 8″ | DN 200 |
| 3″ | DN 80 | 10″ | DN 250 |
| 4″ | DN 100 | 12″ / 15″ | DN 300 / DN 375 |
Work fully metric in the app
- Tap the units toggle to switch to Metric — inputs, sliders, presets and results all flip to mm/m and metres instantly.
- Pipe sizes display as DN designations (DN 40–DN 375) alongside their imperial equivalents.
- With NPC selected, minimums are shown in native ratio notation — 1:50 (2%) and 1:100 (1%) — and compliance is checked against them.
- Results show total fall in mm, slope in mm/m and percent — and switching back to imperial converts everything, so a U.S. spec sheet and a metric drawing can describe the same run.
Related questions
How much fall per metre does a drain need?
At 1:50 — 20 mm/m; at 1:100 — 10 mm/m. NPC: 1:50 up to DN 80 (3″), 1:100 for DN 100+ (4″+).
How do I get total fall from a ratio?
Fall = run ÷ X. 8 m at 1:50 → 160 mm.
What is 1:100 in inches per foot?
1% ≈ ⅛″/ft (1.04%). And 1:50 (2%) ≈ ¼″/ft (2.08%). Full table in how to calculate pipe slope.
Imperial and metric, one calculator
mm/m, ratios, percent, in/ft and DN sizes — converted live, checked against NPC, UPC and IPC.
