Plumbing calculators & guides
Free plumbing tools and plain-English guides for New Zealand — each gives the answer fast and shows the working behind it.
Hot water cylinder size
What size water heater do I need? Pick people + fuel, get the size.
Open tool →Drain slope / fall
Total drop over a run, with the code-minimum slope cited for your area.
In the worksPipe volume / capacity
Litres or gallons held in a length of pipe, by diameter.
Open tool →GPM / flow rate
Measure your water flow rate — time a container filling, get gallons per minute.
Open tool →Plumbing unit converter
mm↔inch, bar↔psi, °C↔°F, L/min↔GPM, L↔gal, m²↔ft² — instant, both ways.
Open tool →About plumbing calculations
Plumbing work runs on a handful of recurring calculations: how much water a fixture or appliance needs, what volume a run of pipe actually holds, how much slope a horizontal drain needs to carry waste without leaving solids behind, and what size a system component should be for the household it serves. Get these numbers wrong and the symptoms show up later as slow drains, starved fixtures, or equipment that short-cycles — problems that cost far more to fix after the walls are closed.
The calculators here handle the arithmetic part of that work: flow rates, pipe volumes, slopes and sizing. Each one gives the answer first, then shows the working underneath, so you can sanity-check the result instead of trusting a black box. Where a plumbing code rule applies to a calculation, the tool names the code section and edition it used.
Using the results
Treat the results as the starting point for a decision, not the end of one. Plumbing codes differ between countries and are amended locally, so the number that satisfies the national model code may still need adjusting for your municipality. The honest workflow: calculate here, then confirm against the code edition your local authority actually enforces before you cut pipe or pour a slab.