Plumbing calculators

Rough-in math for plumbers

Three calculators for rough-in and planning. The pipe offset calculator gives travel, run and cut length for simple and rolling offsets at the standard fitting angles. The drain slope calculator gives the total fall for a run at a chosen slope. The water heater size calculator matches tank capacity or tankless flow to how a household actually uses hot water at its busiest hour.

For offsets, travel equals the offset divided by the sine of the fitting angle, so a pair of 45° fittings needs a travel of about 1.414 times the offset. Cut length is the travel minus the fitting take-offs, which vary by manufacturer and pipe size, so measure your fittings or check the catalog. Allow extra pipe for the socket depth at every joint and a little more for cutting mistakes.

The plumbing codes set a minimum fall per foot that depends on pipe size. Under the International Plumbing Code, horizontal drains 2-1/2 inches and smaller need at least 1/4 inch per foot, and 3- to 6-inch pipe at least 1/8 inch per foot; the pipe slope chart lists the full table. Jurisdictions on the Uniform Plumbing Code differ slightly, so check which code applies.

Gas piping, water heater venting and any change to the drain, waste and vent layout normally need a permit and inspection, and in many places a licensed plumber. Every tank water heater also needs a temperature and pressure relief valve with a properly run discharge pipe.

How each calculator is specified, tested and corrected is described on our methodology page.