CFS Converter
Convert cubic feet per second into cubic metres per second, gallons per minute and other flow units.
Open tool : CFS ConverterRiver Flow Calculator
Estimate discharge in cubic feet per second from stream width, average depth and surface velocity.
Enter width, depth and velocity to estimate discharge.
How to use it
- Pick a straight uniform sectionAvoid bends, pools and pinch points. You want a stretch where the flow looks even across the channel.
- Measure the widthBank to bank at the water line.
- Average the depthTake readings at regular intervals across the channel and average them. More readings means a better number.
- Time a floatMark a known distance, float something buoyant down it, time the trip, and repeat several times. Distance divided by average time is your surface velocity.
The formula and its correction factor
Discharge is cross sectional area multiplied by average velocity. Width times average depth gives the area, and multiplying by velocity gives volume per second, which is your figure in cubic feet per second.
Surface water moves faster than the water lower in the column, where friction with the bed slows it down. The float method therefore applies a roughness coefficient to convert surface velocity into average velocity. Smoother beds sit nearer 0.9 and rough boulder beds nearer 0.8. This is the standard field survey approach.
If the river you are on has a stream gauge, the gauge is the number to trust. Use this for small ungauged water, or to build an instinct for what a given number of cubic feet per second actually looks like when you are standing in it.
Common questions
How is discharge calculated
Discharge is cross sectional area multiplied by average velocity. Width times average depth gives the area, and multiplying that by the water speed gives volume per second. The calculator applies a channel roughness factor because surface velocity is faster than the average through the whole column.
Is this a substitute for a stream gauge
No. If your river has a gauge, the gauge is the number to trust. This is for small ungauged water, for building intuition about what a given flow actually looks like, and for checking that a reading matches what you are seeing.
How do I measure surface velocity
The float method is the simple field version. Mark a known distance along a straight section, float something buoyant down it, and time the trip. Repeat it several times and average.