Float-Method Flow & A Float & a Tape

It is the simplest way to measure flow in a stream or channel with no equipment. You mark a straight reach of known length, drop a float (a stick, orange or bottle) in and time how long it takes to travel that length. Distance ÷ time gives the surface velocity, which — adjusted to mean velocity and multiplied by the cross-section area — gives the flow.

Discharge Q = A × V, where A is the channel cross-section area (width × average depth) and V is the mean velocity. The float gives the surface velocity (length ÷ travel time); mean velocity is a little less, so you multiply by a coefficient of about 0.85. So Q = area × (surface velocity × 0.85).

Water at the surface moves faster than the average across the whole cross-section, because friction along the bed and banks slows the deeper and edge water. A float rides the fast surface, so its speed overstates the mean. The float coefficient — about 0.85 for typical channels (0.8 rough, 0.9 smooth/deep) — corrects for this.

Measure the water-surface width and the depth at several points across the channel, then take the average depth. Area = width × average depth. For an irregular channel, split it into segments, find the area of each (width × its mean depth) and add them up. This tool uses width × average depth for a quick estimate.

Pick a straight, uniform stretch with no big rocks, bends or pools — roughly 5 to 30 metres works well. A longer reach reduces timing error. Time the float two or three times and average the runs, discarding any where it snagged or drifted to the bank, for a more reliable surface velocity.

Flow in cubic metres per second (m³/s) is area (m²) × mean velocity (m/s). Multiply by 1000 for litres per second and by 3600 for cubic metres per hour. The tool shows both L/s and m³/h so you can size a pump, plan an irrigation turn or work out your share of a canal.

It is ideal in the field where you have no flow meter — sizing a pump to a stream, checking a canal delivery, planning water shares between farmers, or estimating how long a channel will take to fill a tank. It trades a little accuracy for needing only a tape, a float and a watch.

With a good straight reach, careful depth measurements and a sensible coefficient it is usually within 10–20% — plenty for practical irrigation decisions. Errors come from a poorly chosen coefficient, an irregular channel, wind pushing the float, or a short reach making timing imprecise. Average several runs to improve it.

No — this is for open channels and streams. For flow over a weir use the Weir Flow Calculator, and for a pipe or designed channel section use the Channel Flow Calculator. The float method is specifically for measuring an existing natural or earthen channel where you cannot install a structure.

Yes — everything is computed locally in your browser. Nothing you enter is uploaded or stored, so you can measure and plan in the field on a phone, even offline once the page has loaded.