Engineering calculator reviewed for preliminary design use · Last updated: March 2026
Calculate Froude number Fr = v/√(gL). Fr < 1 indicates subcritical (tranquil) flow; Fr > 1 indicates supercritical flow. Apply to open channel flow, tank drainage, and two-phase flow design.
What this calculator is used for
The Froude number is a dimensionless parameter that compares inertial forces to gravitational
forces in a flowing fluid. It is essential for analyzing open-channel flows, free-surface
hydraulics, and vertical or inclined two-phase systems where gravity effects are significant.
Typical engineering use cases
Open channel and spillway design in hydraulic engineering
Flow regime assessment in vertical gas-liquid systems
Slug flow risk screening in multiphase pipelines
Free-surface stability evaluation in tanks and channels
Ship and marine structure design
Governing equation and methodology
The Froude number is defined as:
Fr = v / √(g · L)
Where L is a characteristic length such as hydraulic depth or pipe diameter.
Engineering assumptions and limitations
Gravity-dominated flow conditions
Appropriate selection of characteristic length
Does not predict detailed multiphase flow patterns
Practical design notes
When Fr < 1, gravity effects dominate and flow disturbances can propagate upstream
(subcritical flow). When Fr > 1, inertia dominates and the flow is supercritical.
For complete flow characterization, evaluate Froude number together with Reynolds number.
Fr = v/√(gL) compares inertia to gravity. Fr < 1 means gravity dominates (subcritical / tranquil flow). Fr > 1 means inertia dominates (supercritical / rapid flow). Fr = 1 is the critical condition.
Where is Froude number used in practice?
Open channel design, weir flow, tank drainage, ship hull design, mixing vessel surface vortex evaluation, and two-phase flow regime determination. In process engineering, it helps assess free-surface flow behavior.
These results are preliminary estimates for screening-level design use. They do not replace detailed engineering, code compliance verification, or vendor-certified calculations.