Float-Method Flow & A Float & a Tape
Measures stream flow
Time a float over a known length to get surface velocity, apply a coefficient for mean velocity, and multiply by the cross-section area to read the flow (Q = A × V) in L/s and m³/h.
Gauge stream flow with a float
Next: this reach carries about 425 L/s (1,530 m³/h); average several float runs and pick a straight, uniform section for the most reliable reading.
Q = A × mean velocity. The float gives surface velocity (travel ÷ time); multiply by ~0.85 to estimate the depth-averaged mean velocity. A simple, gauge-free field method.
Float-method stream flow — key facts
- Flow
- Q = A × V
- Surface velocity
- length ÷ travel time
- Float coefficient
- ≈ 0.85 (0.8–0.9)
- Mean velocity
- surface × coefficient
- Area
- width × average depth
- Units
- L/s and m³/h
- Equipment
- float, tape, watch
- Privacy
- Runs in your browser; nothing uploaded
Measure your water with a stick and a watch
You do not need a flow meter to know how much water is moving down a stream or canal. The float method is as old as irrigation: pick a straight reach, drop in a float and time how far it travels, then measure the channel's width and depth. The float gives the surface velocity, a coefficient of about 0.85 turns that into the mean velocity, and velocity times the cross-section area gives the discharge. It is the most accessible way to put a real number on the water you are sharing or pumping.
This tool computes the surface velocity, mean velocity, cross-section area and the flow in both L/s and m³/h. Use it to size a pump to a stream, check a canal delivery against your share, or work out how long a channel will take to fill a tank. Pair it with the Channel Flow, Weir Flow and Pump Run Time tools to plan the whole water system.
No equipment needed
A float, a tape and a watch give the flow.
Size the pump right
Match a pump to the stream it draws from.
Check your water share
Put a real number on a canal delivery.
Plan filling times
Know how fast a channel fills a tank or field.
Frequently Asked Questions
What is the float (velocity-area) method?+
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.
How is stream flow (discharge) calculated?+
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).
Why multiply surface velocity by a coefficient?+
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.
How do I find the cross-section area?+
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.
What length of reach should I use?+
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.
How do I convert the flow into useful units?+
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.
Where is the float method most useful?+
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.
How accurate is the float method?+
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.
Can I use this for a pipe or weir instead?+
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.
Does it run in my browser?+
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.