Plumbing Trade Math: Six Worked Examples
Updated 2026-10-21
These are original arithmetic examples for learning, not actual exam questions or code-prescribed design values. Write the formula, keep your units consistent, and check whether your result makes physical sense.
Slope: from a percentage to a fall
For a given 2% slope over a 6 m horizontal run: 6,000 mm × 0.02 = 120 mm fall. Convert the percentage to a decimal first. This is an exercise assumption, not a claim that 2% is the minimum permitted slope for every pipe.
A 45-degree offset
For a 300 mm offset with two 45-degree fittings, diagonal travel is 300 ÷ sin(45°), approximately 424 mm. This is geometric travel before fitting allowances; use the actual take-off dimensions to determine cut length.
Static water pressure
Using 9.81 kPa per metre of water head, an 8 m water column produces 8 × 9.81 = 78.48 kPa at its base. This is a static calculation. A flowing system also has friction and other losses.
Water volume in a pipe
Use actual inside diameter, not nominal size. For 50 mm inside diameter and 10 m length, radius = 0.025 m. Volume = π × 0.025² × 10 ≈ 0.019635 m³, or 19.6 L.
Flow rate and filling time
At a constant 15 L/min, filling an empty 180 L tank takes 180 ÷ 15 = 12 minutes, ignoring losses and changes in conditions. Units are part of the calculation: litres divided by litres per minute leaves minutes.
Diameter is not area
Circular area is π × diameter² ÷ 4. Doubling actual inside diameter makes the area four times as large. That does not, by itself, establish allowable flow or code-compliant pipe size.
A useful check after every calculation
State your assumptions, label the units and estimate the answer before reaching for a calculator. For code sizing, then find the applicable table and read every condition and note. The interactive drills in Study notes let you practise these six examples.
This guide is a study aid, not legal advice. Always confirm requirements in the current Ontario Building Code.
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