Wildfire Risk Calculator

Wildfire risk comes out of several environmental factors interacting. No single one decides fire danger; the combination is what matters, and the dangerous combinations are specific rather than general.

Relative Humidity is one of the most important factors. Dry vegetation (low humidity) ignites far more easily than moist vegetation. Humidity below 20% is considered critical fire weather. Below 10% is extreme. Above 50% significantly reduces fire danger.

Wind Speed spreads fire and dries out vegetation rapidly. Higher wind speeds increase fire spread rate exponentially. Winds above 25 mph (40 km/h) are considered critical for fire spread. Gusty or erratic winds are especially dangerous.

Days Without Rain (Drought Duration) reflects how dry the soil and vegetation are. After 7–10 days without rain, surface fuels dry significantly. After 21+ days, deep fuels (roots, large logs) also become dangerously dry.

Vegetation Type affects ignition risk and fire intensity:

• Dry grass ignites instantly but burns out quickly.
• Shrubs burn hotter and longer than grass.
• Dense forest has enormous fuel loads and can produce catastrophic crown fires.

Temperature raises risk directly, because heat dries the fuels and the atmosphere and drives relative humidity down. It carries the smallest weight here (10 points of 100) for a reason: temperature mostly acts through humidity, and humidity is already counted. Scoring both heavily would double-count the same effect.

The combination that matters most is humidity under 20% with wind over 20 mph. Fire agencies issue red flag warnings on those two together rather than on either alone, because dry fuel with no wind smoulders and wind over damp fuel goes nowhere. Put them together and an ignition becomes a running fire.

This calculator uses a simplified scoring system inspired by the Fosberg Fire Weather Index (FFWI) and the National Fire Danger Rating System (NFDRS). The FFWI was developed by Richard Fosberg in 1978 and combines wind speed, humidity, and temperature into a single fire danger number. This tool gives a general estimate. Always check your local fire authority for the official rating and any restrictions in force.

Risk levels:

• Low (0–19): Fuels do not ignite easily. Fires will burn slowly with low intensity.
• Moderate (20–39): Fires start easily and spread at moderate rates.
• High (40–59): Fires start very easily and spread rapidly. Control is difficult.
• Very High (60–79): Fires start extremely easily, burn intensely, and spread very rapidly.
• Extreme (80–100): Any fire that starts will likely be uncontrollable. Evacuate if ordered.