How This Calculation Works
The plumbing pipe calculator sizes your main supply line by counting the total fixture units in your home. Each fixture (toilet, sink, shower, etc.) is assigned a fixture unit value based on its flow demand.
The total fixture units determine the minimum pipe diameter needed. More fixtures require larger pipes to maintain adequate water pressure throughout the house.
Common Mistakes to Avoid
- Undersizing the main line. Too small a pipe causes low pressure when multiple fixtures run simultaneously.
- Ignoring pipe length. Long runs experience more pressure drop — size up if your main line is over 30 m (100 ft).
- Forgetting future fixtures. Plan for additions like a second bathroom or outdoor faucets.
Worked Example
Scenario: A home with 2 toilets, 3 sinks, 2 showers, a dishwasher, and a washing machine.
Step 1 — Fixture units: 2×3 + 3×1.5 + 2×2 + 1.5 + 2 = 6 + 4.5 + 4 + 1.5 + 2 = 18 fixture units.
Step 2 — Recommended pipe: 18 fixture units → ¾-inch main supply line is sufficient.