Drain Pipe Sizing Calculator

Inputs

Design flow

L/sL/minm³/sm³/hft³/sUS gpm

Use average, peak, or fixture-based flow.

Slope (percent)

Example: 1% means 1 m drop per 100 m.

Flow depth ratio (y/D)

1.00 = full flow; 0.80 is common for design checks.

Pipe material roughness (Manning n) PVC (typical n=0.009)HDPE (typical n=0.011)Concrete (typical n=0.013)Clay (typical n=0.015)Corrugated metal (typical n=0.024)Custom n value

Lower n means smoother pipe and higher capacity.

Custom n (only if selected)

Typical range: 0.009–0.024 for drains.

Peaking factor

Multiplies the base flow before sizing.

Reserve capacity (%)

Adds spare capacity for sediment and future loads.

Minimum velocity (optional, m/s)

Use to screen for self-cleansing conditions.

Maximum velocity (optional, m/s)

Use to screen for erosion and noise risk.

Calculate Reset

Tip: For partially full design checks, try y/D = 0.80 and add reserve.

Example Data Table

Formula Used

This calculator sizes gravity drains using the Manning equation: Q = (1/n) · A · R^(2/3) · S^(1/2)

• Q = flow rate (m³/s), converted from your selected units.
• n = Manning roughness coefficient (material dependent).
• A = flow area (m²) for the chosen depth ratio y/D.
• R = hydraulic radius (m) = A/P, where P is wetted perimeter.
• S = energy slope (m/m), entered here as percent and converted to decimal.

For full flow (y/D = 1.0) the diameter is solved directly. For partial flow, the calculator iteratively finds the smallest diameter meeting required capacity.

How to Use This Calculator

1. Enter the expected base flow and choose a unit.
2. Input the available slope in percent for the pipe run.
3. Select a roughness value or enter a custom Manning n.
4. Set the design depth ratio (e.g., 0.80 for partly full checks).
5. Add a peaking factor and reserve capacity to reflect uncertainty.
6. Optionally set velocity limits to flag low or high velocities.
7. Click Calculate to view results above the form.
8. Use the CSV or PDF buttons for reporting and submittals.

Drain Pipe Sizing Guide

1) Purpose and typical project data

Gravity drains carry runoff and groundwater without pumping. On many sites, branch design flows fall between 5 and 150 L/s, with common nominal diameters from 100 to 400 mm. Laterals are often shorter and steeper, while mains are flatter and longer. Slope, roughness, and depth ratio control the final selection.

2) Design flow, peaking, and reserve capacity

Begin with a base flow from catchments or demand. Apply a peaking factor (often 1.10 to 1.50) for short surges. Add reserve capacity (commonly 10% to 25%) for sediment and future tie-ins. The calculator combines these into one required capacity flow and sizes accordingly.

3) Slope and available head

Slope sets the energy slope S used in Manning’s equation. Increasing slope from 0.5% to 1.0% can significantly increase capacity. Where slope is limited, a larger diameter or smoother material may be more effective. Keep grades consistent through manholes and avoid unnecessary drops.

4) Roughness values by material

Roughness is represented by Manning n. Smooth plastics often use 0.009 to 0.011, concrete about 0.013, clay about 0.015, and corrugated metal near 0.024. Deposits and aging increase effective roughness, so reserve capacity remains valuable.

5) Partial flow and depth ratio (y/D)

Drains commonly run partly full to accommodate peaks and ventilation. Checking y/D from 0.70 to 0.90 is typical for gravity service. Full flow can still occur during surcharge or downstream restriction. This tool computes circular-segment area and wetted perimeter to size at your chosen depth ratio.

6) Velocity checks and sediment control

Velocity is a practical indicator for clogging and erosion risk. As a screening range, many projects aim for roughly 0.6 to 3.0 m/s. If velocity is low, consider cleanouts, sumps, or improved maintenance access. If high, confirm outlet protection, bedding, and inlet stability.

7) Selecting nominal diameters

After the required internal diameter is computed, it is rounded up to a standard nominal size. This supports constructability and procurement. The calculator then rechecks capacity and velocity at the chosen nominal diameter to show operational margin and flag concerns early.

8) Reporting and quality control

Export CSV for design logs and option comparisons, and PDF for submittals. For quality control, verify units, confirm slope measurement, and document assumptions for peaking, reserve, and roughness. Record the selected nominal size, computed velocity, and any warnings. Keep maintenance access and inspection points practical.

FAQs