Choose mean stream velocity, flow rate, width, or mean depth, then enter the three known values in common units.
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Stream Velocity Formula
The stream velocity calculator uses the continuity equation for a rectangular stream cross section. It assumes the flow area is width multiplied by mean depth. The output is cross-section mean velocity, not a local or surface velocity, and does not establish safe wading conditions.
Rearranged formulas used by the calculator:
- Q = volumetric flow rate
- W = width of the stream
- D = mean depth of the stream
- V = cross-section mean stream velocity
To calculate stream velocity, enter flow rate, width, and depth. The calculator divides flow rate by the stream cross-sectional area.
To calculate flow rate, enter width, depth, and velocity. The calculator multiplies all three values.
To calculate width or depth, enter the other three values. The calculator rearranges the same equation and solves for the missing dimension.
Unit conversions are handled before the calculation. Flow rate is converted to cubic meters per second, length to meters, and velocity to meters per second, then the result is converted back to the selected unit.
Common Stream Velocity and Flow Unit Conversions
| Quantity | Conversion | Equivalent base unit |
|---|---|---|
| Flow rate | 1 ft³/s | 0.0283168 m³/s |
| Flow rate | 1 US gal/min | 0.0000630899 m³/s |
| Length | 1 ft | 0.3048 m |
| Length | 1 yd | 0.9144 m |
| Velocity | 1 ft/s | 0.3048 m/s |
| Velocity | 1 mph | 0.44704 m/s |
Illustrative Stream Velocity Ranges
These descriptive ranges are not standardized classifications or safety limits. Stream conditions depend on depth, channel shape, turbulence and local currents.
| Stream condition | Approximate velocity | Illustrative description |
|---|---|---|
| Very slow flow | Less than 0.5 ft/s | Low movement, often in pools or flat sections |
| Moderate flow | 0.5 to 3 ft/s | An illustrative middle range; local conditions vary |
| Fast flow | 3 to 6 ft/s | Noticeably swift water with stronger current |
| Very fast flow | More than 6 ft/s | High-energy flow, often during steep or high-water conditions |
Example Problems
Example 1: Calculate stream velocity
You have a flow rate of 24 ft³/s, a stream width of 8 ft, and a depth of 2 ft.
The stream velocity is 1.5 ft/s.
Example 2: Calculate flow rate
You have a stream width of 5 m, a depth of 0.8 m, and a velocity of 1.2 m/s.
The volumetric flow rate is 4.8 m³/s.
FAQ
What measurements do you need to calculate stream velocity?
You need volumetric flow rate, stream width, and stream depth. The velocity is found by dividing flow rate by the cross-sectional area of the stream.
Why does the calculator use width times depth for area?
It treats the stream cross section as a rectangle. This is a simplified estimate. Natural streams often have uneven beds, so a field measurement may use several depth readings across the stream to get a better average area.
Can you use gallons per minute for a stream?
Yes. Here gallons means US liquid gallons; 1 US gal/min = 0.003785411784/60 m³/s, and 1 L/min = 0.001/60 m³/s. Gallons per minute is usually more common for pumps and smaller flows. For larger natural streams, cubic feet per second or cubic meters per second is usually easier to read and compare.
