Drainage Slope Calculator: engineer's guide

What this calculator does

The calculator applies Manning's equation to verify velocity is within the self-cleansing range (2 to 10 fps) at the entered slope, pipe size, and flow.

Formulas used

• Manning velocity: V = (1.486/n) × R2/3 × S1/2 (imperial)
• Flow: Q = V × A

Variables

• n = Manning roughness
• R = hydraulic radius
• S = slope, ft/ft or m/m

Worked example

Scenario: 4 in PVC (n = 0.010) at half-full, 2% slope.

1. R for half-full circular = D/4 = 1 in = 0.083 ft
2. V = (1.486/0.010) × 0.083^0.667 × 0.02^0.5
3. V ≈ 4 fps — within 2 to 10 fps self-cleansing range
4. Flow at half-full ≈ 4 fps × 0.044 ft² = 0.176 cfs ≈ 79 GPM

Common mistakes to avoid

• Slope in mm per m without unit consistency
• Ignoring downstream backwater effect on slope
• Using clean-pipe roughness for aged cast iron

Frequently asked questions

Maximum slope?

Around 8% before water outruns solids and pipes desiccate.

What about vertical stacks?

Vertical stacks operate as annular flow, not full-bore — sizing rules are separate.

Standards and references

IPC 704, ASPE Data Book Ch. 4

Related tools on this site

• Drainage Pipe Sizing
• Water Demand Calculator

Open the calculator that pairs with this guide: Drainage Slope Calculator.

Verification note

This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.