Enter the quantity of each fixture. Totals and pipe sizes update as you type.
| Fixture | Qty | WSFU | DFU | DU |
|---|
Formula & Engineering Reference
| Symbol | Variable | Unit |
|---|---|---|
| WSFU | Water supply fixture units (Σ qty × unit) | — |
| DFU | Drainage fixture units | — |
| DU | EN 12056 discharge units | — |
| GPM | Peak supply flow from the NSPC table | GPM → L/s |
| Vmax | Supply velocity limit | m/s |
| K | Drainage frequency factor | — |
| Qww | Wastewater flow in the stack | L/s |
The supply peak comes from interpolating the NSPC Table B.5.4 (1 GPM = 0.0630902 L/s), then the pipe is sized on the velocity limit. The drainage stack uses EN 12056-2: Qww = K·√ΣDU, matched against the Table 11 Qmax for the entry type. K runs from 0.5 (intermittent) to 1.2 (special).
Four WCs (gravity tank), four basins, two showers, and one kitchen sink, on flush tanks, with a 2 m/s supply limit, intermittent drainage (K = 0.5), and swept entries.
ΣWSFU = 4×2.5 + 4×1 + 2×2 + 1×1.5 = 19.5. From the NSPC table that interpolates to about 13.75 GPM = 0.87 L/s. Required ID = √(4 × 0.000867 / (π × 2)) = 23.5 mm, so the supply pipe is DN 25.
ΣDU = 4×1.8 + 4×0.3 + 2×0.4 + 1×0.6 = 9.8. Qww = 0.5 × √9.8 = 1.57 L/s, which a swept DN 70 stack carries. But because WCs are connected, the stack is stepped up to the DN 100 minimum.
Adding flows instead of fixture units. You cannot sum each fixture's peak flow — they never all run together. Sum the fixture units and convert once through the probability table; the diversity is built into that curve.
Using the wrong supply column. Flush valves and flush tanks have very different peak demands for the same WSFU. Pick the column that matches the actual WC type, or the supply main can be badly undersized.
Forgetting the WC minimum on drainage. The Qww formula may return a small DN, but any stack receiving a WC has a code minimum — typically DN 100. The calculator flags this; honour it.
Treating the supply size as final. The supply pipe here is sized on velocity alone. On a long run the pressure drop, not velocity, often governs, so confirm with a head loss calculation.
Ignoring the K factor. The drainage frequency factor changes the flow by more than a factor of two between a dwelling and a laboratory. Choosing it carelessly mis-sizes the whole stack.
Weighting numbers for a fixture's load, blending its flow and how often it runs. WSFU is for supply, DFU and the EN 12056 DU are for drainage.
Through a probability curve, not a constant. The total WSFU is looked up against the NSPC table to get the simultaneous peak GPM, which this tool interpolates and converts to L/s.
They take a big, brief surge straight from the supply, unlike tanks that refill slowly. For the same WSFU the peak flow is much higher, so the supply pipe must be larger.
The EN 12056-2 drainage flow: sum the discharge units, take the square root, multiply by the frequency factor K (0.5 intermittent to 1.2 special). The result sizes the stack.
It is a solid first size. Confirm supply pressure drop on long runs, and apply the drainage layout rules — WC minimum DN, branches, vents, developed length — before finalising.
Fixture Units & Pipe Sizing Guide
3 topics • Plumbing load & sizing referenceEvery plumbing system has to answer a deceptively hard question: how much water will actually be flowing at the busy moment? You cannot just add up the taps, because a building full of fixtures never runs them all at once. The fixture-unit method is the elegant old trick that solves this — it turns each fixture into a weighted number, sums those numbers, and reads off the realistic peak flow from a probability curve. From that flow, the pipes size themselves.
This calculator runs the whole chain for both sides of the system: supply and drainage. Count your fixtures, and it gives you the peak water flow and supply pipe size, plus the wastewater flow and drainage stack diameter. The guide explains why fixture units exist, how the supply and drainage sides differ, and where the quick answer hands off to detailed design.