Work out the fall of a drainage pipe, its invert levels and whether it meets the minimum slope. The calculator also gives the pipe's capacity and velocity with Manning's equation, so you can check it carries the flow and stays self-cleansing.
At 1% (1:100, 10 mm per metre) a 20 m pipe falls 200 mm. The invert is the lowest inside point of the pipe; levels on drainage drawings are given at the invert (IL).
| Pipe size | IPC Table 704.1 minimum |
|---|---|
| 2½" (DN65) and smaller | ¼ in/ft (2.08%, about 1:48) |
| 3" to 6" (DN80–DN150) | ⅛ in/ft (1.04%, about 1:96) |
| 8" (DN200) and larger | 1/16 in/ft (0.52%, about 1:192) |
Other codes use similar values: many European and Gulf specifications ask for about 1:40 to 1:60 for small branches and 1:80 to 1:100 for 100–150 mm drains. Always follow the local code.
Manning's equation, V = (1/n) × R2/3 × S1/2, gives the velocity for the hydraulic radius R at the flow depth. Sewers should reach at least 0.6 m/s (2 ft/s) regularly so solids do not settle, and stay below about 3 m/s to limit erosion and separation of liquids and solids. A half-full pipe has the same velocity as a full one; capacity is half of full bore.
Size drains by fixture units with the Fixture Unit Calculator and rainwater pipes with the Rainwater Pipe Calculator. For drawings see our drainage fitting sections DWG.
Under the IPC, 1/8 inch per foot (about 1%) for 3- to 6-inch pipe, so a 4-inch drain falls at least 1/8 inch for every foot. Many designers use 1/4 inch per foot where space allows.
Multiply the pipe length by the slope. At 1:100 (1%) a 20 m pipe falls 0.2 m; at 1/4 inch per foot a 40 ft pipe falls 10 inches.
25 mm per metre, which is 2.5%. 1 in 80 is 12.5 mm per metre (1.25%) and 1 in 100 is 10 mm per metre (1%).
Very steep pipes can let the water run ahead of solids. Above about 3 m/s, or slopes much over the code minimum on long runs, use drop manholes or vertical drops instead.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
No comments yet. Be the first to add one.