The froude number calculator works out hydraulic mean depth (hd) and froude number (fr) from acceleration due to gravity (g), area of cross-section (a), width of the channel (w) and flow velocity (u). Enter what you have and the relationship is applied straight away, so you get the answer without working through the algebra yourself.
What Is the Froude Number Calculator?
This froude number calculator is for the kind of problem where you know the physical quantities but not the number you need. It turns acceleration due to gravity (g), area of cross-section (a), width of the channel (w) and flow velocity (u) into hydraulic mean depth (hd) and froude number (fr).
Physics calculations go wrong in predictable ways, usually through mixing units or dropping a digit. Work through the result one step at a time and check that the figures are the size you would expect at each stage.
How the Froude Number Physics Works
The relationship is applied step by step, and each step is a standard piece of physics rather than a single opaque formula. That means you can check the intermediate values against what the theory predicts, which is usually faster than checking only the final answer.
Keep every value in the same unit system throughout the calculation. Mixing metres and feet part way through is the single most common reason a correct formula gives a wrong answer.
Units, Assumptions and the Limits of the Model
The calculation assumes ideal conditions: no air resistance, no heat loss, and dimensions that do not change while you work. Those assumptions are reasonable for most classroom and engineering estimates, and unreasonable for anything more precise.
Where a value is a rate, it has to be quoted for the same period as everything else. A per-hour figure cannot be combined with a per-day figure without converting one of them first.
A Worked Example
Working through an example, with acceleration due to gravity (g) set to 10, area of cross-section (a) set to 5, width of the channel (w) set to 2, the calculator returns hydraulic mean depth (hd) of 2.5, and froude number (fr) of 20. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out hydraulic mean depth (hd)?
Enter acceleration due to gravity (g), area of cross-section (a), width of the channel (w) and flow velocity (u). The calculator then works through the relationship and reports hydraulic mean depth (hd) and froude number (fr).
What does the froude number actually calculate?
It takes acceleration due to gravity (g), area of cross-section (a), width of the channel (w) and flow velocity (u) and converts them into hydraulic mean depth (hd) and froude number (fr) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter acceleration due to gravity (g)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Can I use this for a quick estimate as well as a precise answer?
Yes. The calculator is built for quick checks, so round your inputs to a sensible number of significant figures and the result will be just as useful. If you need more precision, keep the same method and add digits to the inputs rather than adjusting the method itself.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.