WaterCalculator

Water Drainage Calculators

16 tools for moving water off a property and out of a building — pipe slope and sizing, fixture units and sewers, sump pumps and pits, roof and storm drainage, and yard and foundation drains. Each runs on the recognised standard formula, with the working shown.

John Mac, PE Curated and reviewed by John Mac, PE, Water Systems Specialist.

Slope & pipe sizing

Pipe Slope CalculatorDrain-pipe slope for proper gravity flow. Drain Pipe Size CalculatorSize a drain pipe for expected flow.

Fixture units & sewers

Drainage Fixture Unit CalculatorTotal drainage fixture units (DFU) for a system. Sewer Pipe Size CalculatorSize a building sewer for its fixture load.

Sump & pit

Sump Pump Sizing CalculatorSize a sump pump for inflow rate and head. Sump Pit Size CalculatorSize a sump pit for sensible pump cycling.

Roof & storm drainage

Gutter Size CalculatorSize a gutter for roof area and rainfall intensity. Downspout Size CalculatorSize a downspout for the gutter flow it carries. Roof Drain Size CalculatorSize a roof drain for a flat roof's drainage area. Catch Basin SizingSize a catch basin for stormwater inflow. Swale Sizing CalculatorSize a swale to carry stormwater runoff. Storm Drain Size CalculatorSize a storm drain pipe for peak runoff.

Yard & foundation

Trench Drain SizingSize a trench drain for surface runoff. Yard Drainage CalculatorDrainage capacity a yard needs to handle runoff. Foundation Drain CalculatorSize a foundation (footing) drain. French Drain SizingSize a french drain for subsurface water.

About our drainage calculators

These tools are built on the standards drainage design rests on — plumbing-code drainage fixture unit (DFU) tables for sizing pipes and sewers, Manning's equation for open-channel and swale flow, and rational-method rainfall intensity for gutters and storm drains. Every figure is verified against the live calculator, and each page lists its sources. For the full detail of how we build and check them, see our methodology.

Browse other categories

ScienceDensity, weight, heat and properties. FlowPipe flow, velocity, friction loss. PressureHead, PSI, depth and losses. UsageHousehold use, footprints, savings. CostBills, rates and running costs. TreatmentSoftening, filtration, chemistry. PumpsSizing, head, horsepower, NPSH.