Calculator
Example data table
| Scenario | Sinks | Faucet (gpm) | Diversity | Slope | Material | Depth (y/D) | Typical result |
|---|---|---|---|---|---|---|---|
| Residential kitchen branch | 1 | 2.2 | 0.75 | 2% | Smooth | 0.50 | 1.5–2.0 in |
| Office washroom group | 4 | 1.2 | 0.75 | 2% | Smooth | 0.50 | 2.0–2.5 in |
| Utility sinks (heavy use) | 3 | 2.5 | 0.90 | 1/4 in/ft | Cast iron | 0.60 | 2.5–3.0 in |
Examples are illustrative. Always confirm local plumbing code minimums for trap and branch sizes.
Formula used
This tool estimates a design drainage flow and checks pipe capacity using gravity-flow hydraulics.
- Design flow: Q = N × qpeak × fdiv × (1 + SF)
- Peak per sink: qpeak = qfaucet × M
- Manning capacity (partially full): Q = (1/n) A R2/3 S1/2
For partial depth ratio y/D, the flow area A and hydraulic radius R are computed from circular-segment geometry before applying Manning’s equation.
How to use this calculator
- Select the sink type and enter the number of sinks on the branch.
- Enter the faucet flow rate and adjust the peak multiplier if needed.
- Pick a diversity level that matches expected simultaneous use.
- Set slope and pipe material, then choose a design depth ratio (y/D).
- Review the recommended nominal size and the capacity/velocity checks.
- Download CSV or PDF outputs for submittals, QA, or field notes.
Professional notes on sink drain sizing
Sink branches see short surges and variable discharge. Good sizing keeps flow fast enough to rinse the pipe without creating excessive noise or trap seal disturbance.
1) Why sizing is a balance
Undersized branches can run deep and back up at fittings. Oversized branches can run shallow and leave residue on the invert. A balanced size supports stable drainage and easier maintenance.
2) Design flow logic
The tool estimates design flow from: number of sinks, peak discharge multiplier, diversity factor, and safety margin. Peak multiplier represents surge discharge when a basin is emptied. Diversity reduces the chance all fixtures discharge together.
3) Slope and velocity targets
Gravity flow is sensitive to slope. Higher slope generally raises capacity and self-cleaning velocity. If velocity is low, confirm the installed slope and fitting layout before upsizing. If velocity is high, review noise expectations and trap protection.
4) Partial-flow depth ratio (y/D)
Small sanitary drains often flow partially full. The calculator computes flow area and hydraulic radius at your chosen y/D, then applies Manning’s equation. A y/D around 0.50 is a practical check for many branches; lower ratios are more conservative.
5) Roughness and aging
Rougher interiors reduce capacity, especially in older metallic pipe with scaling. If condition is uncertain, select a rougher material option or apply a custom n-value. For grease-prone lines, maintenance access matters as much as hydraulics.
6) Interpreting the recommendation
The calculator selects the smallest standard diameter that meets capacity at the chosen y/D and stays within your velocity band at design flow. When constraints conflict, it prioritizes capacity and provides notes so you can adjust slope, y/D, or safety margin. Confirm local minimums for traps and branches.
Worked example with sample data
Example: four lavatory sinks at 1.2 gpm, multiplier 1.10, diversity 0.75, safety 15%, slope 2%, smooth pipe, y/D 0.50. Estimated design flow is about 4.6 gpm. A 2.0 in branch often meets capacity at this slope, but verify your velocity target.
| Example | N | qfaucet (gpm) | M | fdiv | SF | Estimated Q (gpm) |
|---|---|---|---|---|---|---|
| Kitchen, single sink | 1 | 2.2 | 1.30 | 0.75 | 15% | ≈ 2.5 |
| Lavatory group | 4 | 1.2 | 1.10 | 0.75 | 15% | ≈ 4.6 |
| Utility sinks, heavy use | 3 | 2.5 | 1.50 | 0.90 | 20% | ≈ 12.2 |
Estimated Q values use the same design-flow equation as the calculator. Confirm cleanouts, fittings, venting, and jurisdictional requirements in final design.
FAQs
1) Is it okay to oversize a sink drain?
Moderate oversizing can be acceptable, but very large branches may run shallow and collect deposits. If the velocity check is low, consider improving slope or reducing diameter rather than oversizing.
2) What depth ratio should I use?
For typical sink branches, 0.50 is a practical comparison point. Use a higher ratio if you expect longer surges or frequent discharge, and a lower ratio only when you want a conservative check.
3) How do I choose the diversity factor?
Use lower diversity for homes or small groups, and higher diversity for busy commercial areas where multiple sinks may run together. If you are unsure, select “Normal / office” as a balanced assumption.
4) Why does slope change the recommended size so much?
Manning flow increases with the square root of slope. A small slope reduction can drop capacity and velocity noticeably, especially under partial-flow conditions. Confirm slope is realistic for the available run.
5) What n-value should I use for older pipe?
Older metallic lines can be rougher due to scaling. If conditions are unknown, use a slightly higher n-value or the material option closest to the existing pipe. Field inspection improves accuracy.
6) Does this replace plumbing code sizing?
No. It is a hydraulic check to support design decisions. Always apply local code requirements for trap sizes, minimum branch diameters, venting, and allowable fittings.
7) Why is my velocity outside the target range?
Very low flow, flat slope, or large diameter can reduce velocity. Very steep slopes or small diameters can push it high. Adjust slope, depth ratio, or limits to match site conditions and preferences.