Cost Index Speed Calculator
The cost index (CI) is the fundamental optimization parameter for FMS-equipped aircraft cruise performance. It quantifies the relative importance of time cost versus fuel cost for a specific flight. By setting an appropriate CI, the FMS selects a cruise Mach number that minimizes total trip cost (fuel cost plus time cost). A CI of zero gives maximum range (minimum fuel); maximum CI gives minimum time at maximum allowable speed. This calculator computes CI from time cost and fuel cost, and also estimates the total trip cost at different speed options: maximum range cruise, long-range cruise (LRC), and high-speed cruise. Enter your aircraft's operating costs and fuel price to find the optimal CI for your operation.
Cost index formula
Fuel Cost per lb = Fuel Price ($/gal) / Fuel Density (lb/gal)
Cost Index = Time Cost ($/hr) / Fuel Cost ($/lb)
Trip Time (hr) = Trip Distance (nm) / Ground Speed (knots)
Trip Fuel Cost ($) = Trip Time x Fuel Flow (GPH) x Fuel Price ($/gal)
Total Trip Cost ($) = Time Cost x Trip Time + Trip Fuel Cost
CI units depend on the FMS manufacturer. Boeing FMS typically uses kg/min; Airbus uses kg/min or lb/min. The dimensionless ratio ($/hr / $/lb) = lb/hr, but the FMS maps this to an internal speed schedule. This calculator computes the raw CI ratio and the total trip cost for comparison between two cruise speeds.
Cost index interpretation
- CI = 0: Fly at maximum range cruise speed. Minimize fuel only.
- Low CI (e.g., 10-50): Economy-focused. Slightly faster than MRC but still efficient.
- Medium CI (e.g., 100-300): Balanced time and fuel. Typical airline CI range.
- High CI (e.g., 500+): Schedule-priority. Approaching maximum cruise speed.
- Maximum CI: Fly at VMO/MMO. Minimum time regardless of fuel cost.
Cost index calculator: frequently asked questions
What is cost index in aviation?
Cost index (CI) is the ratio of the cost of time to the cost of fuel. It tells the Flight Management System (FMS) how much to value time savings versus fuel savings when selecting cruise speed. A high CI flies faster (more fuel, less time); a low CI flies slower (less fuel, more time).
How is cost index calculated?
CI = Time Cost ($/hr) / Fuel Cost ($/lb). For example, if operating cost is $3,000/hr and fuel costs $2.00/lb, CI = 3,000 / 2.00 = 1,500 kg/min (or lb units depending on the FMS unit system). Different manufacturers use different units for CI.
What does a CI of 0 mean?
CI = 0 means maximum range cruise speed. Time has no value; only fuel cost matters. The aircraft flies at the slowest efficient speed to minimize fuel burn. This gives maximum endurance and maximum range.
What does a maximum CI mean?
Maximum CI means minimum time cruise speed (VMO or MMO limited). The aircraft flies as fast as structurally allowed. Time has infinite value relative to fuel. This is used when schedule recovery is critical and fuel cost is secondary.
Does CI affect only cruise speed?
CI affects the FMS-computed economical cruise speed (ECON speed), but it also influences step climb decisions, climb speed, and descent speed optimization on FMS-equipped aircraft. The concept applies whenever the tradeoff between time and fuel is relevant.
Official sources
- ICAO Doc 9976 Flight Planning and Fuel Management Manual: icao.int.
- FAA Aeronautical Information Manual (AIM): faa.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.