Plumbing Calculators
Drain Slope Calculator
Calculate required drain pipe slope and total fall over a run length using standard plumbing code minimums that vary by pipe diameter.
Pipe Flow Calculator
Calculate flow rate in GPM from pipe diameter and water velocity using the continuity equation, with a reference table of capacity by common pipe sizes.
Pipe Pressure Drop Calculator
Calculate friction pressure drop through a pipe using the Hazen-Williams equation, with C-factors for PVC, copper, and cast iron or steel pipe.
Pipe Size Calculator
Determine the minimum pipe diameter needed to deliver a required flow rate without exceeding a target maximum velocity, to prevent noise and water hammer.
Pipe Volume Calculator
Calculate the water volume held inside a pipe from its diameter and length, in gallons or liters, using either imperial or metric unit inputs.
Septic Tank Size Calculator
Look up the standard minimum septic tank capacity by bedroom count, based on common state and IAPMO/UPC plumbing code sizing tables for new systems.
Sump Pump Sizing Calculator
Convert a basin fill-rate test into groundwater inflow rate and a recommended sump pump horsepower tier for accurate, property-specific sizing.
Water Heater Size Calculator
Find the recommended storage water heater tank capacity for a household based on standard sizing guidance by number of occupants and usage patterns.
Water Pressure Calculator
Convert between water column height and static pressure in either direction using the standard 0.433 psi per foot hydrostatic constant for water.
Water Tank Size Calculator
Size a storage or cistern tank from household size, daily per-person water use, and required storage reserve days for wells and off-grid systems.
Sizing Pipes and Fixtures So Water Actually Flows Right
Plumbing sizing mistakes tend to show up as either annoyingly weak water pressure or, at the other extreme, an undersized septic system that fails prematurely — both stem from skipping the flow and capacity math at design time. Ten calculators here cover pipe sizing, water heater and septic capacity, and pressure calculations used in both residential and light commercial plumbing.
Popular Plumbing Calculators
Ten tools span pipe sizing, water systems, and drainage:
- Pipe Size Calculator — selects pipe diameter based on required flow rate and acceptable pressure loss.
- Water Heater Size Calculator — sizes tank capacity or tankless flow rate to household hot water demand.
- Septic Tank Size Calculator — sizes a septic tank based on household size and expected wastewater volume.
- Water Pressure Calculator — estimates pressure at a fixture based on supply pressure, elevation change, and pipe losses.
- Sump Pump Sizing Calculator — sizes a sump pump's capacity to expected water inflow rate.
Why Pipe Size Affects Pressure More Than Most People Expect
Reducing pipe diameter increases flow velocity and friction loss dramatically, not just slightly — because flow resistance in a pipe scales with roughly the fourth power of the radius, a pipe that looks only modestly smaller can produce a much larger pressure drop than intuition suggests. This is exactly why running a long hot water line in undersized pipe is such a common source of disappointing pressure at a distant fixture, even when the water heater and main supply pressure are both perfectly adequate.
Frequently Asked Questions
Why does septic tank size depend on bedrooms rather than exact occupancy?
Most codes size septic systems by bedroom count as a proxy for maximum potential occupancy, since actual occupancy can change over a home's life while the septic system is a fixed, expensive-to-resize installation.
Is a bigger water heater always better?
Not necessarily — oversizing wastes energy keeping unused hot water heated (for tank units) or can be unnecessary cost for a tankless system, so sizing should match actual peak demand rather than maximizing capacity.
Explore More
Working on the HVAC side of the same build? See the HVAC Calculators, or check broader construction material takeoffs in the Construction Calculators.