Weir Flow Calculator (Rectangular & V-Notch Weirs)

Weir Flow Calculator (Rectangular & V-Notch Weirs) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Math & Numbers
Input types: select, number
Output type: json
Sample coverage: 4
API ready: Yes

Overview

The Weir Flow Calculator estimates discharge over a sharp-crested thin-plate weir using rectangular or V-notch formulas. Enter the crest width or notch angle, head, gravity, and optional discharge coefficient to get flow in m³/s, L/s, L/min, and m³/h.

When to use

Estimate flow over a suppressed rectangular weir without side contraction.
Calculate discharge through a triangular V-notch weir from the notch angle and measured head.
Convert a weir flow result into m³/s, L/s, L/min, and m³/h for hydraulic calculations.

How it works

1Select Rectangular Weir or V-Notch (Triangular) Weir.
2Enter the crest width for a rectangular weir, or the notch angle for a V-notch, along with the head and units.
3Optionally enter a discharge coefficient and gravitational acceleration; blank discharge coefficients use the type-specific defaults.
4The calculator applies the selected weir formula and reports the discharge in four flow-rate units.

Use cases

Open-channel flow estimation for hydraulic engineering calculations.
Sizing and checking measurements from laboratory or field weir setups.
Comparing rectangular and V-notch discharge under different head conditions.

Examples

1. Estimate flow over a rectangular weir

Hydraulic engineer

Background

An engineer is checking the discharge from a suppressed rectangular weir with a 1 m crest width and a measured head of 0.2 m.

Problem

Calculate the flow rate using the typical rectangular discharge coefficient.

How to use

Select Rectangular Weir, enter a crest width of 1 m and a head of 0.2 m, and leave the discharge coefficient blank.

Weir type: Rectangular; crest width: 1 m; head: 0.2 m; C_d: default 0.611; gravity: 9.81 m/s².

Outcome

The estimated discharge is approximately 0.1614 m³/s, or 161.378 L/s.

2. Measure low flow with a 90° V-notch

Water-resources technician

Background

A technician uses a 90° triangular notch and records a head of 0.15 m above the vertex.

Problem

Determine the discharge through the V-notch using the typical V-notch coefficient.

How to use

Select V-Notch (Triangular) Weir, enter a 90° notch angle and a 0.15 m head, and leave the discharge coefficient blank.

FAQ

What inputs are required for a rectangular weir?

Enter the crest width, head, and their units. The discharge coefficient and gravity can be left at their defaults.

What inputs are required for a V-notch weir?

Enter the notch angle and head with their units. The default notch angle is 90°.

What discharge coefficient is used by default?

The default is 0.611 for rectangular weirs and 0.58 for V-notch weirs.

Which units can I use for the head?

Head can be entered in meters, centimeters, or millimeters.

What flow units are included in the result?

Results are provided in m³/s, L/s, L/min, and m³/h.

Weir type: V-notch; notch angle: 90°; head: 0.15 m; C_d: default 0.58; gravity: 9.81 m/s².

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Estimate flow over a rectangular weir

2. Measure low flow with a 90° V-notch

FAQ

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