In the calculation I = H × R, what unit must R be in?

• A. R must be in ft/s
• B. R must be in ft/min
• C. R must be in miles per hour
• D. R must be in knots

Answer: (A)

Think of I as fire intensity along the front, an energy-per-length-per-time quantity (Btu per foot per second). H is the heat released per unit area of fuel per unit time (Btu per square foot per second). R is how fast the front advances (feet per second). To make I come out in Btu/(ft·s) when multiplying H by R, the speed R must be in feet per second. Using feet per minute would require adjusting H to per-minute units (or introduce a 60 factor) to keep the units consistent. So the unit for R that keeps the calculation clean and consistent is feet per second.