Cruise Mach Trade Calculator (Time vs Fuel)
M0.76 or M0.80? Compute minutes saved on your leg against the steep fuel-burn slope near MMO — the cost-index decision in raw form.
The 1.2%-per-0.01M specific-range penalty is a representative transonic drag-rise figure — actual FMC/QRH tables for your airframe govern. The lesson survives the approximation: minutes near MMO are bought at steepening prices.
Formula
⚠️ For flight planning and education only — verify with your POH/AFM, certified instruments and official sources. Not for primary navigation or airworthiness decisions.
Disclaimer: This tool is for general informational and estimation purposes only and is not professional financial, tax, accounting or legal advice. All figures are estimates — verify with a qualified professional before making decisions. Read the full disclaimer.
M0.76 or M0.80? Compute minutes saved on your leg against the steep fuel-burn slope near MMO — the cost-index decision in raw form.
About Cruise Mach Trade Calculator (Time vs Fuel)
Every airline flight quietly runs an auction between the clock and the fuel gauge, mediated by a number called cost index. This calculator exposes the raw trade: pick two cruise Machs, a leg length and a fuel flow, and see the minutes saved against the kilograms spent — including the transonic drag rise that makes each extra hundredth of Mach pricier than the last. It's why your A320 cruises at 0.78 and not the 0.82 it could.
How to use Cruise Mach Trade Calculator (Time vs Fuel)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula Δt = D/a × (1/M₁ − 1/M₂); fuel: flow scales ≈ M-ratio × (1 + 1.2%/0.01M drag-rise) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Cruise Mach Trade Calculator (Time vs Fuel)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula Δt = D/a × (1/M₁ − 1/M₂); fuel: flow scales ≈ M-ratio × (1 + 1.2%/0.01M drag-rise) with sources cited on the page
- ✓The 1.2%-per-0.01M specific-range penalty is a representative transonic drag-rise figure — actual FMC/QRH tables for your airframe govern. The lesson survives the approximation: minutes near MMO are bought at steepening prices.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
What is cost index, in plain terms?+
A dial trading fuel cost against time cost: CI 0 flies maximum-range speed (fuel is everything), CI 999 flies near MMO (time is everything). The FMS converts the airline's chosen ratio into an economical Mach for each altitude and weight. This calculator is cost index with the curtain pulled back — minutes priced in kilograms.
Why does each extra 0.01 Mach cost more than the last?+
Transonic drag rise: approaching the wing's drag-divergence Mach, wave drag grows steeply (and nonlinearly), so specific range degrades ~1–1.5% per 0.01M near typical cruise speeds. Far from MMO the same 0.01M is nearly free. The convexity is why optimum cruise sits in a narrow band rather than at the placard.
How much time does 0.04 Mach actually buy?+
On a 2,000-nm leg at FL370, accelerating M0.76→0.80 saves about 13 minutes — and costs several hundred kilograms of fuel at typical single-aisle flows. Whether that's worth it depends on crew costs, curfews and connections: precisely the inputs airlines encode in their cost index, route by route.
Is flying slower always cheaper then?+
Only down to maximum-range speed; below it, drag rises again (induced drag dominates) and you burn more fuel AND more time — the back side of the power curve in cruise form. LRC (long-range cruise) sits deliberately 1% faster than the absolute optimum because the speed costs almost nothing there. There's a bottom to the dial.
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