Weir Flow Calculator & Measure Channel Flow
Gauges channels
Measure channel flow with a weir — enter the head over the crest (and notch angle or width) to get the discharge in L/s, m³/s and m³/h.
Enter your weir
Next: use this flow to size an irrigation set, schedule a water turn, or check a stream's yield; at 25 L/s you deliver about 91 m³ of water every hour.
V-notch suits small flows; rectangular suits larger; measure H well upstream of the crest (≈4× H back) where the water surface is level.
Weir flow — key facts
- V-notch Q
- ≈ 1.42·H^2.5 (90°)
- Rectangular Q
- 1.84·L·H^1.5
- Head H
- water height above crest
- Measure H
- ≈ 4H upstream of crest
- V-notch
- best for small flows
- Rectangular
- best for larger flows
- 1 m³/s
- = 1000 L/s
- Privacy
- Runs in your browser; nothing uploaded
A simple plate that measures your water
You can't manage water you can't measure — and a weir is the cheapest, most reliable way to gauge the flow in a channel, field drain or small stream. Set a notched plate across the flow, let the water spill over the notch, measure how high it backs up (the head), and a fixed formula gives the discharge. A small change in head means a big change in flow, which makes weirs sensitive and accurate.
This tool applies the standard V-notch and rectangular weir formulas to your head (and notch angle or crest width) and returns the discharge in L/s, m³/s, m³/h and L/min. Use a V-notch for small flows, a rectangular weir for larger, keep the crest sharp and clean, and measure the head a little upstream where the surface is level. Pair it with the Channel Flow, Borewell Yield and Irrigation Water tools to match supply to your crop.
Measure your flow
Turn a head reading into discharge in every useful unit.
V-notch or rectangular
Small flows or large — the right weir for the channel.
Plan your supply
Know the flow to match irrigation to the crop and area.
Cheap & accurate
A simple plate gives a reliable, repeatable flow gauge.
Frequently Asked Questions
How does a weir measure water flow?+
A weir is a notched plate set across a channel; water backs up and flows over the notch, and the depth of water above the crest (the 'head', H) relates to the discharge by a fixed formula. Measure H upstream and read the flow. It's one of the simplest, most reliable ways to gauge channel or stream flow.
What is the V-notch weir formula?+
For a triangular (V-notch) weir: Q = (8/15)·Cd·√(2g)·tan(θ/2)·H^2.5, where θ is the notch angle, H the head and Cd ≈ 0.6. For a 90° notch this is about Q = 1.42·H^2.5 (Q in m³/s, H in m). This tool applies it for any notch angle you set.
What is the rectangular weir formula?+
For a rectangular weir (suppressed): Q = 1.84·L·H^1.5 (the Francis formula), where L is the crest width and H the head, in metres, giving Q in m³/s. The tool uses this for rectangular weirs; for a contracted weir you can reduce the effective width slightly for end contractions.
V-notch or rectangular weir — which should I use?+
Use a V-notch for small flows and where accuracy at low flow matters — its triangular shape keeps a measurable head even when flow is small. Use a rectangular weir for larger flows where a wide crest passes the volume without too much head. The tool covers both.
Where do I measure the head?+
Measure the head H (the height of the water surface above the weir crest) a short distance upstream of the weir — about 4 times the head back — where the surface is level, not at the crest itself where it draws down. Accurate H is the key to an accurate flow reading.
What notch angle should a V-notch weir have?+
90° is the most common and well-documented; 60°, 45° and 30° notches pass less water for the same head and suit smaller flows. The tool lets you set the angle and applies the tan(θ/2) term, so you can use whatever notch you have.
Why is the head raised to a power?+
Because discharge over a weir grows faster than the head — as H^1.5 for rectangular and H^2.5 for V-notch — since both the flow depth and the velocity increase with head. That's why a small rise in head means a big rise in flow, and why weirs are sensitive, accurate gauges.
How accurate is a weir?+
Well-installed and read correctly, weirs are among the most accurate field flow gauges (a few percent). Keep the crest sharp and clean, ensure the nappe (overflowing sheet) is ventilated and falls freely, set the weir level and square to the flow, and measure H carefully upstream.
Can I use this to size irrigation supply?+
Yes — measure the flow your channel or stream delivers, then use it to plan how much area it can irrigate and for how long. Combine the discharge here with the Channel Flow, Irrigation Water and Borewell Yield tools to match supply to your crop's needs.
What units does it give?+
The flow in m³/s, litres per second (L/s), m³ per hour and litres per minute, so you can use whichever suits your planning. Enter the head (and notch angle or crest width) in metres and the tool converts the discharge into all four units.