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.
50 free calculators used by homeowners, contractors, and engineers — based on NRCS, FHWA, and ASCE standards
Who Are You?
Get started with the right tools for your experience level
Not sure which calculator you need?
Answer 2-3 quick questions and we'll recommend the right tool for your situation.
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
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
Rain Garden Design
Size rain gardens for stormwater management and infiltration
Detention Pond Sizing
Size detention ponds using Modified Rational method
Time of Concentration
Calculate Tc using Kirpich, FAA, and NRCS methods
Catch Basin Sizing
Size storm drainage inlets for roadway and site drainage
More Popular Tools
Ready to Design Your Drainage System?
Our calculators use industry-standard methods from NRCS, HEC, ASCE, and other authoritative sources. All tools are free to use with no registration required.
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.