FIRE BEHAVIOUR · GEOMETRIC SCENARIO
Wildfire Fire Growth Calculator
Turn constant head and backing spread rates into an idealized fire ellipse. See how distance, width, area and perimeter scale with time—and exactly which assumptions drive the result.
IDEALIZED FOOTPRINT
54.74 ha
after 60 minutes
Initial area, when entered, is added to the calculated ellipse area only; it does not alter the displayed axes.
READ THE SHAPE
The ignition point is not the centre.
When the head spreads faster than the backing edge, the geometric centre moves downwind. This calculator locates ignition from the two travelled distances rather than placing it at the middle of the ellipse.
Travel
Head and backing distance each equal their constant rate multiplied by time.
Length
Total major-axis length is the sum of the two distances from ignition.
Width
The selected length-to-breadth ratio determines maximum width.
Footprint
Ellipse area is exact; perimeter uses Ramanujan’s accurate approximation.
CONSTANT-CONDITION LIMIT
Useful for scenarios.
Wrong for certainty.
Ellipses are a common simplified representation of fire growth, especially under relatively uniform conditions. Operational simulators divide landscapes and time into many changing calculations; this page deliberately does not.
Not represented
- Wind shifts or gust fronts
- Slope and aspect changes
- Fuel transitions and barriers
- Spot fires and long-range embers
- Suppression or containment
- Topographic channeling
Sources and limitations
The ellipse is a transparent geometric scenario, not a substitute for FARSITE, FlamMap, official incident mapping or qualified fire-behaviour analysis.
USDA Forest Service: fire spread across landscapes · USDA Forest Service: mechanistic fire-pattern modelling · USDA Forest Service: FARSITE development
Quick answers
Why enter backing spread separately?
It locates the ignition point within the footprint and avoids assuming symmetric growth.
Where does flank spread appear?
It is implicit in the ellipse width selected through the length-to-breadth ratio.
Can the calculated area be compared with mapped acres?
Only as a simplified scenario under constant inputs—not as a reconstruction of an irregular perimeter.