TSFC Calculator

Thrust specific fuel consumption (TSFC) tells you how much fuel an engine burns to produce one unit of thrust over one unit of time. Lower numbers mean a more efficient engine. A modern high-bypass turbofan runs around 0.5 lb/(lbf·h), so it burns about half a pound of fuel per hour for every pound-force of thrust it produces. An older turbojet can hit twice that figure, which is part of why airlines stopped flying them on long routes decades ago.

What is TSFC?

TSFC is fuel mass flow rate divided by thrust. In SI units that comes out to kg/(N·s), which produces awkward small decimals, so engineers usually rescale it to kg/(kN·h) by multiplying by 3,600,000. In imperial units the standard is lb/(lbf·h), where pound mass over pound force cancels into inverse hours. The relationship with propulsive efficiency is direct: the less kinetic energy you dump out the back as fast-moving exhaust, the lower your TSFC. That's the whole reason high-bypass turbofans took over commercial aviation. They move huge volumes of air at modest velocities instead of small volumes very fast, and they waste less energy doing it.

How to use this calculator

Enter the engine's fuel mass flow rate and its thrust output. Read TSFC in your chosen units. For example, an engine burning 0.8 kg/s of fuel while producing 40,000 N of thrust has a TSFC of 0.00002 kg/(N·s), or 72 kg/(kN·h) once you rescale (0.00002 × 1,000 × 3,600). Reverse the process by entering any two known values; the missing field fills in automatically.

Understanding the formula

The formula itself is short:

\text{TSFC} = \frac{\dot{m}_{fuel}}{F}

Where $\dot{m}_{fuel}$ is fuel mass flow in kg/s and F is thrust in newtons.

Run a realistic case: a regional jet engine producing 50 kN of thrust while burning 0.5 kg/s of fuel. Divide fuel flow by thrust to get 0.5 ÷ 50,000 = 0.00001 kg/(N·s). Multiply by 1,000 to go from newtons to kilonewtons and by 3,600 to go from seconds to hours, and you land on 36 kg/(kN·h).

\begin{aligned}\text{Thrust} &= 50 \text{ kN} = 50{,}000 \text{ N} \\\text{Fuel Burn Rate} &= 0.5 \text{ kg/s} \\\\\text{Specific Fuel Consumption (base)} &= \frac{0.5 \text{ kg/s}}{50{,}000 \text{ N}} \\&= 0.00001 \text{ kg/(N}\cdot\text{s)} \\\\\text{Convert to kg/(kN}\cdot\text{h)} &= 0.00001 \times 1{,}000 \text{ N/kN} \times 3{,}600 \text{ s/h} \\&= 36 \text{ kg/(kN}\cdot\text{h)}\end{aligned}

That's how much fuel the engine eats per hour for every kilonewton of thrust it makes.

Applications in engine selection

On long-haul routes, fuel is the single largest operating cost for an airline, so engine selection comes down mostly to TSFC. A 5% improvement compounds into millions per year across a large fleet. Fighter aircraft trade efficiency for specific thrust: a higher TSFC is the price you pay for the thrust density needed to go supersonic. Cargo operators, who spend most of their time at subsonic cruise, push the trade-off in the opposite direction and live with heavier engines if it gets them better fuel numbers.

Tips for interpretation

Modern high-bypass turbofans sit around 0.5 to 0.6 lb/(lbf·h), or roughly 15 to 20 g/(kN·s). Older turbojets land closer to 1.0 to 1.5 lb/(lbf·h). Lower numbers always mean better fuel efficiency. Any published TSFC value is tied to a specific operating point, almost always cruise at the engine's design conditions, so the same engine will look worse at takeoff power or partway through the climb.

Frequently asked questions

Why are there so many different TSFC units?

Different industries chose different conventions. Aeronautical engineers like lb/(lbf·h) because the mass and force units cancel down to inverse hours, which is a clean number to think in. SI's kg/(N·s) is mathematically tidier but produces awkward small decimals, so it usually gets scaled up to kg/(kN·h).

How does TSFC relate to specific fuel consumption (SFC) for piston engines?

Piston engines use brake-specific fuel consumption (BSFC), which divides fuel flow by shaft power output instead of thrust, typically in g/(kW·h). It's the same underlying idea, fuel burned per unit of useful work, but jets produce thrust and pistons produce shaft power, so the two metrics don't compare directly.

Does TSFC account for fuel heating value?

It doesn't. TSFC is a mass-flow metric, not an energy one. Two fuels with the same TSFC can carry very different amounts of chemical energy per kilogram, so any cross-fuel comparison has to factor in the heating value. Thrust-specific energy consumption (TSEC) is the version that does this.

Can TSFC improve during flight?

Yes. As an aircraft burns fuel, it gets lighter, and the thrust required to hold cruise drops. The engine throttles back to a more efficient operating point, and TSFC improves. This is why long-haul flights are most efficient in the last hour of cruise.