Wildfire Fuel Moisture Calculator | Forest Fire Prediction Tool (Metric) | Firefighter Field Guide

Interactive Fuel Moisture Calculator

Field-Ready Tool for Forest Fire Behavior Assessment, Fine Dead Fuel Moisture content Estimation | Metric Units (°C, meters)

🔥 Wildfire Fine Dead Fuel Moisture Estimation

Get instant fuel moisture predictions - follow the same process as the manual tables below, based on Fosberg - Rothermel

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FUEL MOISTURE CONTENT

Calculation Steps:

⚠️ Firefighting Implications:

What is Fuel Moisture and Why Does It Matter for Firefighting?

Fuel moisture content is the single most critical factor determining wildfire behavior. It directly controls fire spread rate, flame intensity, and suppression difficulty. This tool helps firefighters and fire behavior analysts predict fuel moisture from basic weather data, enabling better tactical decisions on the fireline.

Fuel moisture is expressed as a percentage of the fuel's dry weight. Lower moisture means faster fire spread and higher intensity. Understanding fuel moisture helps you:

  • Predict fire behavior - Estimate rate of spread and flame length
  • Plan suppression tactics - Direct vs. indirect attack decisions
  • Assess firefighter safety - Identify extreme fire behavior risk
  • Time prescribed burns - Optimize fuel reduction operations
  • Coordinate resources - Request appropriate suppression assets
⚠️ CRITICAL SAFETY NOTE: These calculations assume NO recent precipitation. After rain or heavy dew, allow several hours of drying before using these estimates. Fuels drying from saturation (desorption) can be 3-5% wetter than fuels gaining moisture from dry conditions (adsorption). Add 3-5% correction if fuels were recently wetted.

Fuel moisture values typically range from 1-2% in extreme drought (very dangerous fire conditions) to over 200% for living vegetation. For firefighting operations, the critical threshold is around 8-12% - below this, fires spread rapidly and resist control.

GLOSSARY - Key Terms for Wildfire Fuel Moisture Assessment

Fuel Types & Classifications

Dead Fuel: Non-living plant material (dried grass, fallen leaves, dead branches) that responds quickly to atmospheric conditions. Moisture content changes within hours.

Live Fuel: Living vegetation (green grass, shrub foliage, tree needles). Moisture content changes slowly over weeks/months based on plant physiology and season.

Fine Fuels (1-hour fuels): Dead material ≤6 mm diameter (grass, pine needles, small twigs). These fuels reach equilibrium moisture in about 1 hour and control fire spread rate.

10-hour Fuels: Dead twigs and branches 6-25 mm diameter. Reach moisture equilibrium in ~10 hours.

100-hour Fuels: Dead branches 25-75 mm diameter. Reach moisture equilibrium in ~100 hours.

Weather & Environmental Terms

Dry Bulb Temperature: Standard air temperature measured by thermometer (in this guide: °Celsius).

Relative Humidity (RH): Amount of moisture in air expressed as percentage of maximum possible at that temperature. Critical fire weather variable.

Aspect: Compass direction a slope faces (N, E, S, W). Affects solar heating and fuel drying. South-facing slopes (Northern Hemisphere) are typically warmest and driest.

Slope Steepness: Terrain gradient expressed as percentage. Example: 40% slope = 40m rise over 100m horizontal distance = ~22° angle.

Temperature Inversion: Atmospheric condition where cold air is trapped in valleys below warmer air, creating very high fuel moisture in valley bottoms.

Canopy Closure / Shading: Percentage of sky blocked by overhead vegetation. <50% = exposed (sunny), ≥50% = shaded. Affects fuel temperature and drying rate.

Fire Behavior Terms

Rate of Spread (ROS): Speed at which fire advances, typically measured in meters/minute. Directly influenced by fuel moisture.

Flame Length: Distance from flame tip to midpoint of flame base. Key indicator for suppression tactics. >3.5m typically exceeds direct attack capability.

Fire Intensity: Energy release rate of fire, measured in kilowatts per meter of fire front. Determines suppression difficulty.

LIVE FUEL MOISTURE ESTIMATION

Living vegetation moisture is critical for fires in shrublands, grasslands, and forest understory. Live fuel moisture varies seasonally based on plant physiology, recent rainfall, and temperature trends.

Live Fuel Moisture Quick Reference Table

Visual Indicators of Vegetation Stage Moisture Content
Fresh new growth, early spring green-up, high turgor pressure 300%
Maturing foliage, mid-season, full leaf development, still growing 200%
Mature summer foliage, growth complete 100%
Early dormancy, leaves beginning color change, some leaf drop starting 50%
Completely dormant or dead, brown/yellow color, brittle <30% (treat as dead fuel)
Field Example: You're assessing fire danger in Mediterranean scrubland in early October. Shrub leaves show early autumn color change with 10-20% leaves dropped. Some foliage still appears summer-green. Based on the table, estimate live fuel moisture between 50-100%. Use 75% as working value for fire behavior prediction.

HOW TO USE THE FUEL MOISTURE TABLES - FIELD PROCEDURES

STEP-BY-STEP DAYTIME PROCEDURE (08:00h-19:59h)

  1. Gather weather data: Current temperature (°C) and relative humidity (%) from weather station or forecast
  2. Get reference moisture: Use Table A - find intersection of temperature and humidity
  3. Select correction table: Choose based on season (Summer, Spring/Autumn, or Winter)
  4. Determine site characteristics:
    • Current time (hour)
    • Fuel shading: Exposed (<50% canopy) or Shaded (≥50% canopy)
    • Aspect: N, E, S, or W quadrant
    • Slope: 0-30% or 31%+ steepness
  5. Find correction value: Use characteristics to look up correction in appropriate table
  6. Calculate final moisture: Reference moisture + Correction = Field fuel moisture (%)

STEP-BY-STEP NIGHTTIME PROCEDURE (20:00h-07:59h)

  1. Gather weather data: Temperature (°C) and humidity (%)
  2. Get reference moisture: Use Table E
  3. Check for inversion: Is cold air trapped in valleys? (calm, clear night, valley location)
  4. If NO inversion: Use Table E value directly - you're done!
  5. If STRONG inversion exists: Use Table F to adjust for aspect differences

TABLE A - REFERENCE FUEL MOISTURE (DAYTIME)

For use during 08:00h-19:59h

Use this table to get your base fuel moisture from temperature and relative humidity

Temperature
(°C)		 RELATIVE HUMIDITY (PERCENT)
0-5101520253035404550 556065707580859095100
-12 to -1 1223455678 8899101112121314
-1 to 9 1223455677 7899101011121313
10 to 20 1223455667 788991011121213
21 to 31 1122345567 788891010111213
32 to 43 1122344567 788891010111213
43+ 1122344567 788891010111212
NEXT STEP: Go to seasonal correction table based on month

SEASONAL CORRECTION TABLES - SUMMER (May, June, July)

Add these corrections to your reference moisture from Table A

EXPOSED SITES - Less than 50% canopy shading

Aspect
Slope		 08:00h10:00h12:00h14:00h16:00h18:00h
N 0-30%+3+100+1+3
N 31%++4+2+1+1+2+4
E 0-30%+2+100+1+3
E 31%++200+1+3+5
S 0-30%+3+100+1+2
S 31%++3+1+1+1+1+3
W 0-30%+3+1000+2
W 31%++5+3+100+1

SHADED SITES - Greater than or equal to 50% canopy shading

Aspect 08:00h10:00h12:00h14:00h16:00h18:00h
N+5+4+3+3+4+5
E+4+4+3+4+4+5
S+4+4+3+3+4+5
W+5+4+3+3+4+4

SEASONAL CORRECTION TABLES - SPRING/AUTUMN

(February, March, April, August, September, October)

Add these corrections to your reference moisture from Table A

EXPOSED SITES - Less than 50% canopy shading

Aspect
Slope		 08:00h10:00h12:00h14:00h16:00h18:00h
N 0-30%+4+2+1+1+2+4
N 31%++4+4+3+3+3+4
E 0-30%+4+2+1+1+2+4
E 31%++3+1+1+2+4+5
S 0-30%+4+2+1+1+2+4
S 31%++4+2+1+1+2+4
W 0-30%+4+2+1+1+2+4
W 31%++5+4+2+1+1+3

SHADED SITES - Greater than or equal to 50% canopy shading

Aspect 08:00h10:00h12:00h14:00h16:00h18:00h
N+5+5+4+4+5+5
E+5+4+4+4+5+5
S+5+4+4+4+4+5
W+5+5+4+4+4+5

SEASONAL CORRECTION TABLES - WINTER

(November, December, January)

Add these corrections to your reference moisture from Table A

EXPOSED SITES - Less than 50% canopy shading

Aspect
Slope		 08:00h10:00h12:00h14:00h16:00h18:00h
N 0-30%+5+4+3+3+4+5
N 31%++5+5+5+5+5+5
E 0-30%+5+4+3+3+4+5
E 31%++5+3+2+4+5+5
S 0-30%+5+4+3+2+4+5
S 31%++5+3+1+1+3+5
W 0-30%+5+4+3+3+4+5
W 31%++5+5+4+2+3+5

SHADED SITES - Greater than or equal to 50% canopy shading

Aspect 08:00h10:00h12:00h14:00h16:00h18:00h
N+5+5+5+5+5+5
E+5+5+5+5+5+5
S+5+5+5+5+5+5
W+5+5+5+5+5+5

TABLE E - REFERENCE FUEL MOISTURE (NIGHTTIME)

For use during 20:00h-07:59h

Use this table to get nighttime base fuel moisture from temperature and relative humidity

Temperature
(°C)		 RELATIVE HUMIDITY (PERCENT)
0-5101520253035404550 556065707580859095100
-12 to -1 12455678910 11121214151719222525+
-1 to 9 1234567899 11111213141618212425+
10 to 20 1234566889 10111112141617202325+
21 to 31 1234456789 10101112131517202325+
32 to 43 1233456789 9101011131416192225+
43+ 1223456688 991011121416192124+
NEXT STEP: If no temperature inversion, use this value. If strong inversion exists, continue to Table F

TABLE F - NIGHTTIME INVERSION CORRECTIONS

Use only when cold air is trapped in valleys (20:00h-07:59h)

Aspect 20:00h22:00h00:00h02:00h04:00h06:00h
N or E +1+1+2+1+1+2
S or W 000000

WORKED EXAMPLES FROM THE FIRELINE

Example 1: Mediterranean Hillside Fire - Summer Afternoon

Situation: Active wildfire on south-facing slope, 14:00h in August

  • Current conditions: 27°C, 20% RH
  • Terrain: 35% slope, south aspect, open grassland
  • Sky: Clear

Calculation:

  1. Table A: 27°C + 20% RH = 2% reference moisture
  2. Spring/Autumn Table: South aspect, 31%+ slope, 14:00h, Exposed = +1% correction
  3. Final: 2% + 1% = 3% fuel moisture

Tactical Implications: 3% fuel moisture = EXTREME fire behavior expected. Direct attack likely impossible. Consider indirect tactics, safety zones, and evacuation triggers. Request aerial resources immediately.

Example 2: Night Operations Assessment

Situation: Planning night shift operations, 23:00h

  • Conditions: 16°C, 50% RH
  • Terrain: North aspect, moderate slope
  • Weather: Clear calm night, no temperature inversion evident

Calculation:

  1. Table E: 16°C + 50% RH = 9% fuel moisture
  2. No inversion = Use Table E value directly
  3. Final: 9% fuel moisture

Tactical Implications: Overnight moisture recovery to 9% = Good conditions for direct attack and mop-up operations. Fire spread rate significantly reduced from afternoon conditions.

🔥 QUICK FIELD REFERENCE

Step 1: Choose DAY or NIGHT

  • DAY = 08:00h - 19:59h
  • NIGHT = 20:00h - 07:59h

DAYTIME PROCEDURE:

A. Get BASE moisture (Table A)

  1. Temperature (°C) + Humidity (%) → Table A
  2. Write down reference moisture %

B. Apply CORRECTIONS (Seasonal Tables)

Select based on:

  • Season: Summer / Spring-Autumn / Winter
  • Time: 08:00h, 10:00h, 12:00h, 14:00h, 16:00h, 18:00h
  • Shading: Exposed (<50%) or Shaded (≥50%)
  • Aspect: N, E, S, W
  • Slope: 0-30% or 31%+

C. Calculate Final Value

Reference + Correction = FUEL MOISTURE

NIGHTTIME PROCEDURE:

  1. Temperature + Humidity → Table E
  2. No inversion? Done!
  3. Inversion? Add Table F correction

Decision Flow:

DAY (08:00-19:59h)? 
  ↓
Table A + Seasonal Correction
  ↓
FINAL FUEL MOISTURE (%)

Scientific References

Rothermel, R.C. 1983. How to predict the spread and intensity of forest and range fires. General Technical Report INT-143. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 161 p.

Fosberg, M.A. 1971. Moisture content calculations for the 100-hour timelag fuel in fire danger rating. Research Note RM-199. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 7 p.

About This Tool

This fuel moisture calculator is a metric adaptation of the Fosberg-Rothermel system, developed for international firefighting use. The original research was conducted by fire behavior scientists working on wildland fire prediction methods. These tables have been validated globally across diverse ecosystems including Mediterranean forests, boreal regions, grasslands, and shrublands.

Applicability: These tables are applicable worldwide for wildfire fuel moisture assessment in forests, shrublands, grasslands, and mixed fuel types. While developed in North America, the physical principles apply universally. Local validation and adjustment may be needed for unique fuel types or extreme climatic conditions.

Metric conversion prepared 2025 for global firefighting community. Maintained and distributed for operational wildfire management and firefighter safety applications.

Disclaimer: This tool provides field estimates for operational firefighting. For critical decisions involving significant values at risk or firefighter safety, consult with qualified fire behavior analysts and local fire management authorities. Always prioritize firefighter safety over property protection.

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