Calculate stream velocity, flow rate, width, or depth by entering any three values and solving the missing variable 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 depth.
Q = W * D * V
Rearranged formulas used by the calculator:
V = Q / (W * D)
W = Q / (D * V)
D = Q / (W * V)
- Q = volumetric flow rate
- W = width of the stream
- D = depth of the stream
- V = 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 gpm | 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 |
Typical Stream Velocity Ranges
| Stream condition | Approximate velocity | What it usually means |
|---|---|---|
| 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 | Common for many small streams and channels |
| 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.
V = Q / (W * D)
V = 24 / (8 * 2) = 1.5 ft/s
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.
Q = W * D * V
Q = 5 * 0.8 * 1.2 = 4.8 m^3/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, but 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.
