How is fireline intensity calculated from heat release and spread?

• A. I = H × R, with H in Btu/ft^2 and R in ft/s (convert R from ft/min by dividing by 60)
• B. I = H + R
• C. I = H × R^2
• D. I = H × R, with H in Btu/ft and R in m/s

Answer: (A)

Fireline intensity comes from how much heat is released per area of fuel and how fast that fuel is being consumed along the line. The correct relationship is a product: I = H × R. Here, H is the heat released per unit area of burning fuel, measured in Btu per square foot, and R is the rate of spread of the fire front, measured in feet per second. Multiplying these gives the heat released per unit length of fireline per second (Btu per second per foot), which is the fireline intensity.

To keep the units consistent, use R in ft/s. If you have R in ft/min, convert by dividing by 60. The other options don’t fit because they either add instead of multiply, square the spread rate, or mix incompatible units (such as H in Btu/ft without the^2) or different unit systems, which would not yield a correct rate of heat release per unit length of fireline. For a quick check, if H = 9,000 Btu/ft^2 and R = 0.25 ft/s, then I = 9,000 × 0.25 = 2,250 Btu/ft-s.