How do I compute slope for a 4" pipe?

Question

what's the slope for 4" pipe and 18" long?

Answer 1

If you  are asking what is a good slope for a 4-inch pipe so that it will be self cleansing then you need to look at the velocity during a normal day of flow. In Ohio we use the Ten State Standards when designing pipes for sewers. These are guidelines that should be used to meet building codes.  I have included a link below. I am guessing if it is a 4-inch pipe it is not a sewer but maybe a water line, in which case it will be under pressure. You can find information on all of this in the Ten State Standards.

If you want to calculate the full flow capacity of the pipe you can use Manning's equation. You will just need to know the roughness of the pipe material. For example PVC is about 0.011.

Manning's Eqn for Gravity Flow, NOT pressurized flow

Q=(1.49/n)*A*(R^(2/3))*S^1/2

Q = flow (cfs)

n= roughness factor

A=area of pipe = pi*r^2 (ft)^2

R=Hydraulic Radius (for full pipe flow this equals Diameter/4) (ft)

S=Slope (ft/ft)

You can try this for several slopes and see what kind of difference it makes.

Answer 2

Slope is the drop in elevation over the run length.  Your question does not give enough information for me to give a solid answer,