How do you calculate rainwater runoff to determine culvert size?

To calculate rainwater runoff for determining culvert size, you first need to assess the drainage area, including its size and land use characteristics, which affect runoff coefficients. Utilize the Rational Method, where runoff (Q) is calculated using the formula Q = CiA, with C being the runoff coefficient, i as the rainfall intensity (in inches per hour), and A as the area (in acres). After determining the peak runoff, consider factors such as culvert slope, material, and local regulations to select an appropriate culvert size that can handle the expected flow without causing flooding or erosion.