Determing GPM through PVC pipe (Gravity Drains)

Does anyone know the equation to find the flow rate, in GPM, of water through a vertical pvc pipe? No pump, just gravity.
depends on the accuracy required for your answer. first approximation: the kinetic energy (velocity) at the bottom equals the loss of potential energy from top to bottom. that neglects friction losses. Try googling 'bernoulli equation' and 'moody friction' for more details.

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