# Weir Flow Calculator (Rectangular & V-Notch Weirs)

Compute the discharge over a sharp-crested thin-plate weir. Rectangular (suppressed, no side contraction): Q = (2/3)·C_d·√(2g)·b·H^1.5. V-notch (triangular): Q = (8/15)·C_d·√(2g)·tan(θ/2)·H^2.5. b is the crest width, H the head over the crest (or over the vertex for a V-notch), θ the notch angle. If C_d is left blank it defaults to 0.611 for rectangular weirs (Rehbock/Francis typical value) and 0.58 for V-notch weirs. Crest width in m/cm, head in m/cm/mm; output in m³/s, L/s, L/min and m³/h.

> Canonical page: https://elysiatools.com/en/tools/weir-flow-calculator

- **Category:** Math & Numbers

- **Keywords:** weir, weir flow, rectangular weir, v-notch weir, triangular weir, sharp-crested weir, thin-plate weir, discharge, head, flow rate, discharge coefficient, Francis formula, Rehbock, open channel flow, hydraulics

## 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.

## Inputs

- **Weir Type** (select): Rectangular (suppressed, no side contraction) takes crest width b and head H. V-Notch (triangular) takes notch angle θ and head H.
- **Crest Width b** (number): Crest width b of the rectangular weir. Enter in the selected Crest Width Unit. Required when Weir Type = Rectangular.
- **Crest Width Unit** (select)
- **Notch Angle θ (°)** (number): V-notch angle θ in degrees (0 < θ ≤ 180). Required when Weir Type = V-Notch.
- **Head H** (number): Head H over the crest (rectangular) or over the vertex (V-notch). Enter in the selected Head Unit.
- **Head Unit** (select)
- **Discharge Coefficient C_d** (number): Discharge coefficient C_d. Leave blank to use the default: 0.611 for rectangular weirs (Rehbock/Francis typical value), 0.58 for V-notch weirs.
- **Gravity g (m/s²)** (number): Gravitational acceleration g in m/s².
- **Decimal Places** (number)

## 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

- Select Rectangular Weir or V-Notch (Triangular) Weir.
- Enter the crest width for a rectangular weir, or the notch angle for a V-notch, along with the head and units.
- Optionally enter a discharge coefficient and gravitational acceleration; blank discharge coefficients use the type-specific defaults.
- The 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.

## Frequently asked questions

### 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.

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