Drainage fixture unit calculator
Enter how many fixtures drain to the line. You get drainage fixture units and the minimum horizontal drain diameter at 2% slope.
Drainage fixture units
24 DFU
- Total drainage fixture units24 DFU
- Minimum drain diameter (2% slope)110 mm Ø
- Capacity of that diameter264 DFU
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How it's calculated (visible formula)
DFU = Σ fixtures × units per fixture; diameter = smallest pipe whose capacity ≥ DFU; minimum 110 mm (4 in) with a toilet
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- List every fixture draining to the line (lavatories, toilets, showers…).
- Assign each fixture its drainage units.
- Add up the units on that run.
- Find the smallest pipe whose capacity covers them at the line's slope.
- If toilets connect, go to at least 3–4 in (110 mm).
Fixture units used (CTE, private use)
| Fixture | Units | Min. trap |
|---|---|---|
| Lavatory | 1 | 1¼ in |
| Bidet | 2 | 1¼ in |
| Shower | 2 | 1½ in |
| Bathtub | 3 | 1½ in |
| Toilet (tank) | 4 | 4 in |
| Kitchen sink | 3 | 1½ in |
| Dishwasher / washer | 3 | 1½ in |
Worked example: House with 2 lavatories, 2 toilets, shower, tub, kitchen sink, dishwasher and washer
- 2×1 + 2×4 + 2 + 3 + 3 + 3 + 3 = 24 units
- With toilets: 4 in (110 mm) minimum
→ A 4 in (110 mm) building drain.
Common mistakes
- Downsizing a drain that serves toilets below 3–4 in.
- Using private-use values for public buildings.
- Forgetting the vent.
Pro tips
- Size each branch for the fixtures it actually carries, not the whole house.
- Vent the stack through the roof to protect trap seals.
- Add cleanouts at changes of direction and at the base of stacks.
- Avoid 90° bends on horizontal drains — use two 45s or a long sweep.
From calculation to quote
Add labor, overhead and your margin, and generate the professional PDF.
FAQs
- How many fixture units is a toilet?
- 3–4 DFU in the IPC/UPC for a private 1.6 gpf toilet; this tool uses the Spanish CTE values (4).
- What size drain for a toilet?
- 3 in minimum in the US codes; 110 mm in Europe.
- What slope for a drain?
- ¼ in per foot (2%) for 3 in and smaller.
- What size drain for a shower?
- 2 in is the common minimum in US codes.
- Can the washer share the kitchen sink drain?
- With a properly sized branch, yes — add both fixtures' units.
- What happens if a drain is undersized?
- Clogs, gurgling and sewer odor from siphoned traps.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.