By Craig Weeks
Founder, Chief Technical Designer · Updated November 05, 2026
Fire agencies across the country rate wildfire danger every day. One of the numbers they use is the Burning Index, usually shortened to BI. Firefighters know it well. Homeowners almost never hear about it, even though it describes the days their home is most at risk.
This guide explains it the way our team does, based on notes from Wetline founder Craig Weeks, a retired Division Chief with the Los Angeles County Fire Department.
What the Burning Index is
The Burning Index is part of the National Fire Danger Rating System. The US Forest Service defines it as "a number related to the contribution of fire behavior to the effort of containing a fire" (US Forest Service).
In plain English: it rates how hard a fire would be to control today. Agencies calculate it for each fire weather station from two things:
- How fast a fire would spread (the Spread Component)
- How much heat it would release (the Energy Release Component)
How to read the number
The Burning Index is tied to flame length. The Forest Service describes it as "a numeric value related to potential flame length in feet multiplied by 10." So a quick rule of thumb: divide the Burning Index by 10 for a rough idea of flame length in feet.
| Burning Index | Flames on the worst ground nearby (approximate) |
|---|---|
| 20 | About 2 feet |
| 40 | About 4 feet |
| 80 | About 8 feet |
| 120 | About 12 feet |
The scale has no upper limit. And the Burning Index is not exactly flame length; the two are calculated differently, so treat the table as a guide.
What pushes the Burning Index up
The same conditions that drive red flag days push the Burning Index higher:
- Sustained winds or strong gusts
- Low relative humidity
- Drought
- Dry vegetation, measured daily as fuel moisture
- Hot afternoon temperatures
- Thunderstorms or dry lightning
What the Burning Index can't tell you
It describes an area, not your house. The Burning Index represents the near upper limit for the worst fuel, weather and terrain within a fire danger rating area. Conditions that bad aren't expected everywhere in the area at once.
It doesn't predict whether a fire will start. It rates how hard a fire would be to fight if one does. A study of Los Angeles County fire records found the Burning Index is linked to fire occurrence but has limited value for predicting it (Schoenberg et al., 2007).
What counts as "high" depends on where you live. A number that is normal in one area can be unusual in another. Local fire agencies decide what levels mean extra staffing.
Why it matters for your home
A high Burning Index means a fire would be harder to stop, so you can't count on crews reaching every house in time. What decides whether your house catches is usually much smaller than the fire front: embers landing in flashy fuels, the dry grass, leaves and pine needles around your home. Embers cause up to 90 percent of home ignitions, according to the Insurance Institute for Business & Home Safety's study of the 2017 and 2018 California fires (IBHS).
You can't lower the Burning Index. You can take away the fuel it needs to reach you. Read our guide to flashy fuels.
What to do on a high Burning Index day
- Clear leaves and needles from gutters, roof valleys and decks.
- Move doormats, cushions, firewood and anything that burns away from the house.
- Close garage doors and windows; keep vents clear of debris.
- Water down dry grass near fence lines and the house if you can do it safely.
- Charge your phone, keep your car fueled, and know two ways out.
- If officials order you to leave, leave.
Want a firefighter's read on your home? Wetline's free assessment shows where flashy fuels collect around your house and what each hardening layer would change. Book a free assessment.
Frequently asked questions
Is the Burning Index the same as a red flag warning?
No. A red flag warning is issued by the National Weather Service when weather conditions are critical for fire. The Burning Index is a daily fire danger rating from the National Fire Danger Rating System. They often rise together, because the same wind, heat and dry air drive both.
Does a high Burning Index mean a fire will happen?
No. It rates how hard a fire would be to control if one starts, not whether one will start. Research in Los Angeles County found it has limited value as a predictor of fire occurrence.
Is the Burning Index the same as flame length?
Not exactly. It is related to potential flame length in feet multiplied by 10, but it is calculated differently from flame length predictions. Dividing it by 10 gives a rough guide.
Where can I see the Burning Index for my area?
Fire agencies publish Burning Index readings by weather station. Regional Predictive Services offices post station charts, for example Northern California Predictive Services.
What should I do when the Burning Index is high?
Clear flashy fuels from gutters, roofs and decks, move anything that burns away from the house, and be ready to leave. Always follow official evacuation orders.
