Wildfire Risk Index Calculator
Turn temperature, relative humidity and wind speed into a numeric wildfire danger rating using three widely used fire weather indices.
🔥 What is a Wildfire Risk Index?
A wildfire risk index is a number computed from routine weather observations that summarises how favourable current conditions are for a fire to ignite, spread and resist control. Fire agencies rely on these indices because the three variables that drive most day-to-day variation in fire behaviour, air temperature, relative humidity and wind speed, are measured continuously at almost every weather station, so a danger rating can be produced anywhere, every hour, at essentially zero cost.
This calculator implements three well documented indices. The Fosberg Fire Weather Index (FFWI), published by Michael Fosberg in 1978, estimates the equilibrium moisture content of fine dead fuels from temperature and humidity, converts that into a moisture damping coefficient, and then scales it by wind speed. The Chandler Burning Index (CBI) drops the wind term entirely and produces a fuel-dryness style rating from temperature and humidity alone, which makes it usable with historical records that contain no wind data. The Angstrom index is an even simpler screening formula, developed in Scandinavia and widely used in Mediterranean Europe, that runs inversely: the smaller the number, the more likely a fire becomes.
Practical uses cover a wide range. Land managers use fire weather indices to decide whether to issue a burn ban, cancel a prescribed burn, or pre-position crews and aircraft. Utilities use them as one input to public safety power shutoff decisions on high wind days. Researchers use long index time series computed from reanalysis data to test whether fire weather seasons are lengthening under climate change. Insurers and property owners use them to understand seasonal exposure. Hikers, farmers and campers use them as a quick sanity check before using machinery or lighting a fire.
A common misconception is that a high index guarantees a fire, or that a low index means safety. Fire weather indices describe the weather only. They know nothing about how much fuel is on the ground, whether that fuel is continuous, how steep the slope is, how long the region has been in drought, or whether an ignition source is present. A grass paddock at FFWI 60 with no ignition source will not burn, while a forest at FFWI 20 with heavy drought-cured fuel and a lightning strike absolutely can. Another misconception is that different indices should agree numerically. They are on entirely different scales, so compare danger classes, not raw numbers, when switching between them.
This calculator makes the comparison easy. It reports the index value, the danger class it falls into, every arithmetic step behind the number, and a sensitivity chart showing how the index would change if wind or humidity moved, with your own conditions marked on the curve.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Hot, Dry and Windy Afternoon (Fosberg)
Temperature 32°C, relative humidity 15%, sustained wind 30 km/h
Example 2 — Mild, Humid, Light Wind (Fosberg)
Temperature 18°C, relative humidity 60%, sustained wind 10 km/h
Example 3 — Warm and Dry, No Wind Data (Chandler)
Temperature 25°C, relative humidity 25%, wind speed unknown
Example 4 — Quick Screening with the Angstrom Index
Temperature 25°C, relative humidity 25%, the same air as Example 3
❓ Frequently Asked Questions
🔗 Related Calculators
What is a wildfire risk index?
A wildfire risk index converts routine weather observations into a single number that describes how favourable current conditions are for fire ignition and spread. This calculator supports three of the most widely used indices: the Fosberg Fire Weather Index, which uses temperature, relative humidity and wind speed; the Chandler Burning Index, which uses temperature and humidity only; and the Angstrom index, a simple screening formula used across Europe and Australia.
What formula does the Fosberg Fire Weather Index use?
FFWI equals the moisture damping coefficient times the square root of one plus the squared wind speed in mph, all divided by 0.3002. The damping coefficient comes from the equilibrium moisture content, which is a piecewise function of relative humidity and temperature in Fahrenheit. The result is capped at 100.
What is a dangerous FFWI value?
As a common operational guide, FFWI below 15 is Low, 15 to 30 is Moderate, 30 to 50 is High, 50 to 75 is Very High, and 75 or above is Extreme. Many fire weather offices treat sustained values above 50 as critical fire weather, especially when they coincide with dry fuels and low overnight humidity recovery.
How is the Chandler Burning Index calculated?
CBI equals ((110 minus 1.373 times RH) minus 0.54 times (10.20 minus T)) multiplied by (124 times 10 raised to the power of minus 0.0142 times RH), all divided by 60, with T in degrees Celsius and RH in percent. Below 50 is Low, 50 to 75 Moderate, 75 to 90 High, 90 to 97.5 Very High, and 97.5 or above Extreme.
What is the Angstrom index and why is lower worse?
The Angstrom index is I = RH divided by 20, plus (27 minus T) divided by 10, with T in degrees Celsius. Both terms shrink as air gets hotter and drier, so the index falls as fire danger rises. Above 4.0 fire occurrence is unlikely, 3.0 to 4.0 is unfavourable, 2.5 to 3.0 is favourable, 2.0 to 2.5 means fires are likely, and 2.0 or below means fires are very likely.
Why does wind matter so much in the Fosberg index?
Wind supplies oxygen to the flame front, tilts flames forward into unburnt fuel, and carries embers ahead of the fire to start spot fires. In the FFWI formula the wind term enters as the square root of one plus wind speed squared, so the index grows almost linearly with wind speed once the wind is above a few mph. Doubling the wind roughly doubles the index for the same temperature and humidity.
Can I use these indices to predict whether a fire will actually start?
No. These are fire weather indices, not ignition probabilities. They describe how readily a fire would spread if one started, and they say nothing about ignition sources such as lightning, powerlines, machinery or arson, nor about fuel load, fuel continuity, slope, or how long the landscape has been in drought.
What units should I enter?
Enter temperature in degrees Celsius, relative humidity in percent, and wind speed in kilometres per hour. The calculator converts internally to degrees Fahrenheit and miles per hour for the Fosberg formula, which was published in imperial units, so you never need to convert by hand.
Which wind speed should I use, gusts or sustained wind?
The Fosberg index was designed around sustained wind at 6 metres or 20 feet above ground, typically the 10 minute average reported by a fire weather station. Using peak gusts will produce a noticeably higher index. For an upper-bound worst case, run it once with sustained wind and once with the forecast gust and compare the two danger classes.
Why is my Chandler Burning Index above 100?
The Chandler formula has no upper cap. In very hot and very dry air, for example 38 degrees Celsius at 10 percent relative humidity, it returns values well above 100. Anything at or above 97.5 is already in the Extreme band, so treat any three-digit value as extreme fire weather rather than reading extra meaning into the exact number.
How do these indices relate to official fire danger ratings?
Official systems such as the Australian Fire Danger Rating System, the Canadian Forest Fire Weather Index System, and the US National Fire Danger Rating System add fuel moisture models, drought codes, and multi-day weather history on top of the instantaneous weather. The indices here are simpler single-timestep formulas that correlate well with those systems but are not substitutes for them.
Does relative humidity or temperature matter more?
Relative humidity dominates in all three formulas. In the Chandler index the humidity term appears twice, once linearly and once inside a power of ten, so dropping humidity from 40 percent to 15 percent raises the index far more than a 10 degree Celsius temperature increase does. Humidity is effectively a proxy for how dry the fine fuels have become.