Fixture Unit Calculator

Calculator

System type Water supply (WSFU) Drainage (DFU)

Choose the table that matches your design task.

Project label (optional)

Included in the PDF report.

Assumptions

Add a reminder for your notes.

---

Fixture list

Add rows, pick fixtures, set quantities, and adjust units if needed.

Add row Load example

Submit Reset

Example data table

This sample shows how fixture quantities convert into a summed total.

Formula used

The calculator totals fixture units using a weighted sum:

• Quantity is how many identical fixtures are present.
• Units is the selected fixture unit value for that fixture.
• The total supports load comparison and preliminary sizing checks.

How to use this calculator

1. Select system type that matches your design (water supply or drainage).
2. Click Add row, choose a fixture, and enter the quantity.
3. Review the auto-filled units each; override if your code differs.
4. Press Submit to see totals and the detailed breakdown.
5. Use Download CSV or Download PDF for records.

Professional article

Why fixture units matter

Fixture units provide a standardized way to estimate probable plumbing demand. Instead of adding raw flow rates, designers assign each fixture a unit value that reflects typical usage patterns. Summed units help compare branches, risers, and building services consistently.

Understanding WSFU vs DFU

Water Supply Fixture Units (WSFU) focus on supply-side demand and the likelihood of simultaneous draws. Drainage Fixture Units (DFU) describe waste and vent loading. A restroom group might total 10 DFU while the same fixtures could total about 5–8 WSFU, depending on fixture types.

Selecting a fixture unit table

Codes and standards publish different unit tables by occupancy and fixture design. Always confirm whether your jurisdiction uses private, public, or mixed-use ratings. This calculator lets you override the default unit values so you can match locally adopted tables without changing the workflow.

Handling mixed-use buildings

Mixed-use projects often contain fixtures with different usage intensity, such as office lavatories, restaurant sinks, and janitor service sinks. When compiling totals, keep floors or tenant spaces grouped, then create separate subtotals for each branch. This improves traceability during reviews and inspections.

Diversity and simultaneous demand

Fixture unit methods already incorporate diversity, meaning all fixtures are not assumed to operate at once. For example, doubling fixtures does not usually double probable flow. A small suite rising from 12 to 24 WSFU may increase calculated demand significantly, but typically less than a two‑times flow increase.

Translating units into pipe sizing

After totaling units, designers consult code tables to map units to probable flow and then confirm pipe diameters with velocity, pressure drop, and available head. A 20 WSFU branch might be acceptable on 1 inch in many layouts, but long runs and low pressure can require larger sizes.

Documentation and submittal tips

Reviewers frequently request a fixture schedule, unit totals by branch, and references to the adopted code section. Exporting a CSV supports transparent calculations, while a PDF report is useful for plan sets and contractor coordination. Add project labels and assumptions so the record is self-explanatory.

Common pitfalls to avoid

Common mistakes include mixing WSFU and DFU in the same total, using public values for private fixtures, and ignoring specialty fixtures such as hose bibbs or floor drain primers. Use consistent categories, verify each fixture type, and re-check totals whenever the fixture count changes.

FAQs

1) What is a fixture unit?

A fixture unit is a weighted value assigned to a plumbing fixture that represents its probable demand or loading. Totals help compare system demand without adding individual flow rates for every fixture.

2) Should I use WSFU or DFU?

Use WSFU when sizing water supply piping and DFU when sizing drainage and vent systems. They measure different design effects, so do not combine them in one total.

3) Can I edit the unit values?

Yes. The “Units each” field can be overridden to match your local code table, fixture model, or occupancy classification. Keep notes in assumptions so the change is documented.

4) Why is pipe sizing shown as a hint only?

Pipe sizing depends on code tables, length, pressure, elevation, velocity limits, and fixture distribution. The hint is a quick sanity check, not a substitute for detailed design calculations.

5) How many rows can I add?

You can add as many fixture rows as needed. For clarity, group fixtures by branch or area and use separate runs if you need totals for multiple zones or risers.

6) What should I include in the report?

Include the system type, each fixture name, quantity, unit value, and the total. Add a project label and assumptions so reviewers understand occupancy and the selected code basis.

7) Why export both CSV and PDF?

CSV supports editing, auditing, and sharing with estimators. PDF provides a clean snapshot for plan submissions and field coordination. Use both to keep calculations transparent across stakeholders.