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This tool applies a standard formula from the relevant domain. The computation is deterministic — given the same inputs, the result is always identical. The formula accounts for the primary variables that determine the output.
DFU to pipe size: ≤3 DFU = 1-1/2″, ≤20 = 2″, ≤160 = 3″, ≤360 = 4″The calculation requires: Drainage Fixture Units, Pipe Slope. Each parameter contributes to the final result according to the formula above. Accuracy depends on the precision of your input measurements.
Updated: July 2026
A typical use case demonstrating the calculator with representative values.
→ Calculated result based on the formula and provided inputs
Enter your own values to get a personalized result for your specific situation.
→ Result calculated from your specific input values
Check the expected unit for each input field (shown as suffix). Converting between imperial and metric units before entering values prevents calculation errors.
Ensure denominator values (divisors) are non-zero. The calculator validates this, but understanding which fields cannot be zero helps interpret results correctly.
Size building sewer/drain pipe from drainage fixture unit count. Free online plumbing calculator with instant, accurate results. It applies the calculation formula (DFU to pipe size: ≤3 DFU = 1-1/2″, ≤20 = 2″, ≤160 = 3″, ≤360 = 4″). For example: 25 dfu, 2% slope — A typical use case demonstrating the calculator with representative values.