read the result
Invert the full-pipe capacity equation.
For a full circular pipe, the area is A = piD2/4 and the hydraulic radius is R = D/4. With target flow, slope, and roughness known, the diameter can be solved directly from the Manning relationship.
Governing relationship
Q = (1/n) A R2/3 S1/2, so D = [Q n 45/3/(pi sqrt(S))]3/8. The calculator then reports the full-flow area, velocity, and capacity corresponding to the estimated diameter.
A hydraulic diameter is not a final pipe selection.
- Compare the result with available internal diameters, joints, tolerances, cover, bedding, access, and project standards.
- Gravity pipes are often partially full; check partial-flow capacity, inlet control, outlet control, tailwater, and head losses.
- Check self-cleansing velocity, surcharge, air, sediment, debris, maintenance, and the full hydraulic grade line separately.