Solve Your Drainage Problem
Figure out how much water your site needs to handle — whether you're fixing a soggy yard, sizing pipes for a project, or designing stormwater systems. No engineering degree required, but accurate enough for the pros.
53 free calculators used by homeowners, contractors, and engineers — based on NRCS, FHWA, and ASCE standards
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Calculator Categories
Find the right tool for your project
Hydrology
Runoff calculations, hydrographs, and watershed analysis
Hydraulics
Channel flow, pipe sizing, and open channel analysis
Stormwater BMP
Green infrastructure, detention, and water quality BMPs
Structures
Inlets, catch basins, culverts, and outlet structures
Featured Calculators
Most popular tools used by drainage professionals
Culvert Calculator
Size culverts and check headwater with FHWA HDS-5 inlet & outlet control
Storm Drain Pipe Sizing
Size gravity storm drain pipes for capacity and velocity with Manning's equation
Drainage Fall Calculator
Check pipe gradients and falls against UK Building Regulations Part H
Rational Method
Calculate peak discharge using Q = CiA for small watersheds
Manning's Pipe Flow
Calculate pipe flow capacity and velocity using Manning's equation
SCS Curve Number
Calculate runoff depth using the NRCS curve number method
French Drain Sizing
Size subsurface drainage systems for site conditions
Detention Pond Sizing
Size detention ponds using Modified Rational method
How Drainage Calculations Work
Most site drainage design comes down to two questions: how much water arrives, and can your pipes and channels carry it away?
Peak runoff — the Rational Method. For small drainage areas, engineers estimate peak stormwater flow with the rational method formula Q = C · i · A, where Q is peak discharge, C is the dimensionless runoff coefficient (how much rain runs off versus soaks in), i is rainfall intensity for your design storm, and A is the contributing area. It answers “how much water do I need to handle?” Run the numbers with the Rational Method calculator, or read the step-by-step rational method guide with worked examples.
Pipe and channel capacity — Manning’s equation. Once you know the flow, Manning’s equation tells you whether a pipe or channel can carry it: V = (k/n) R2/3 S1/2, where V is velocity, n is the roughness coefficient for the pipe or channel material, R is hydraulic radius, S is slope, and k is a unit conversion factor. Multiply velocity by cross-sectional area to get capacity Q. Size a gravity pipe or channel with the Manning’s pipe flow calculator, or learn the method in the Manning’s equation guide.
A typical drainage calculation chains the two together: estimate peak runoff from the site, then size the pipe, culvert, or channel to carry that flow within an acceptable velocity and depth. Every calculator on this site shows the formula, inputs, and standard references (NRCS, FHWA HDS-5, HEC-22) behind its result, so you can check the work.
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About DrainageCalculators.com
DrainageCalculators.com provides free, professional-grade drainage calculators for engineers, architects, and stormwater professionals. Our tools implement industry-standard methodologies from NRCS, ASCE, HEC, and other authoritative sources. Whether you're designing a residential French drain or a commercial detention pond, our calculators help you get accurate results quickly.