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Schematic turbofan engines; the high-bypass engine (lower) has a large fan that routes much air around the turbine, the low-bypass engine (upper) has a smaller fan routing more air into the turbine.

In aeronautical engineering, and jet engine design in particular, bypass ratio is a common measurement that compares the amount of air deliberately blown past the engine to that moving through the core. For instance, an engine that blows two kilograms of air around the engine for every kilogram that passes through it is said to have a bypass ratio of 2 (or 2 to 1). Higher bypass ratios generally infer better specific fuel consumption as an increasing amount of thrust is being generated without burning more fuel.

Jet engines are generally able to create considerably more energy than they can use in moving air through the engine core. This is because the limiting factor is the temperature at the turbine face, and that is a function of the total amount of fuel burned. Increasing airflow, and thus thrust, would imply burning more fuel and generating higher temperatures. It is possible to increase the airflow by burning "too much" fuel or adding water in front of the turbine to cool it, but both methods lead to incomplete combustion and very poor fuel efficiency. This was nevertheless common for some time to produce added thrust on takeoff, which is why the exhaust plumes of older aircraft appear so smoky in films.

Rolls–Royce came up with a better use of the extra energy in their Conway turbofan engine, developed in the early 1950s. In the Conway, an otherwise normal axial-flow turbojet was equipped with an oversized first compressor stage (the one closest to the front of the engine), and centered inside a tubular nacelle. While the inner portions of the compressor worked "as normal" and provided air into the core of the engine, the outer portion blew air around the engine to provide extra thrust. In effect, the Conway was a much larger engine which only burned some of the air flowing through it, but did so in an efficient way. The Conway had a very small bypass ratio of only 0.3, but the improvement in fuel economy was notable, as a result, it and its derivatives like the Spey became some of the most popular jet engines in the world.

Through the 1960s the bypass ratios grew, making jetliners competitive in fuel terms with piston-powered planes for the first time. Most of the very-large engines in this class were pioneered in the United States by both Pratt & Whitney and General Electric, which for the first time was out-competing the United Kingdom in engine design. Rolls-Royce also started the development of the high-bypass turbofan, and although it caused considerable trouble at the time, the RB.211 would go on to become one of their most successful products.

Turbofans are typically broken into one of two categories—low–bypass ratio and high–bypass ratio. In a low–bypass turbofan, only a small amount of air passes through the fan ducts and the fan is of very small diameter. The fan in a high–bypass turbofan is much larger to force a large volume of air through the ducts. The low–bypass turbofan is more compact, but the high-bypass turbofan can produce much greater thrust, is more fuel efficient, and is much quieter.

Today almost all jet engines include some amount of bypass. For lower speed operations such as airliners modern engines use bypass ratios up to 17, while for higher speed operations such as fighter aircraft the ratios are much lower, around 1.5.