Weir Flow Calculator & Measure Channel 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.