Bypass Ratio Calculator

Bypass air mass flow

Core air mass flow

Bypass ratio

Enter any two of bypass air flow, core air flow, and bypass ratio. The calculator solves for the third.

Please input values into the fields

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Show the air-split pie

Split total intake air into the bypass stream and the core stream.

Show the engine comparison chart

Plot your bypass ratio next to representative turbofans.

Show the reference engine table

A table of real engines and their bypass ratios.

Representative engines and their bypass ratios
Engine	Aircraft	Bypass ratio	Type
Pratt & Whitney F119	F-22 Raptor	0.3:1	Low-bypass military
General Electric CF34	Embraer E-175	5:1	Regional jet
CFM56-7B	Boeing 737 NG	5.5:1	Narrow-body
Rolls-Royce Trent 1000	Boeing 787	11:1	Wide-body
CFM LEAP-1A	Airbus A320neo	11:1	Modern efficient
Pratt & Whitney GTF	Airbus A220	12:1	Geared turbofan

The bypass ratio is a figure used to distinguish the engines of combat aircraft from those mounted under the wings of wide-body commercial airliners. In turbofan engines it compares the volume of air that bypasses the core with the volume of air passing through the core.

This calculator computes the ratio from either mass flow rate and also allows calculations in reverse. If you enter either the ratio or one of the flow rates, then the other value can be determined.

What is the bypass ratio?

A turbofan engine has a large fan at the front which draws in air. Part of this air goes into the core where it is compressed, mixed with fuel and burnt before exiting at the rear. The rest of the air bypasses the core and does not take part in the combustion process. The bypass ratio is the ratio of the volume of air flowing through the outer path to that flowing through the core.

\text{BPR} = \frac{\dot{m}_{bypass}}{\dot{m}_{core}}

Both flow rates are measured using the same method - mass per unit time - so this ratio is dimensionless and expressed as a number or with a colon, e.g. 9 or 9:1.

How to use this calculator:

Enter any two of the three values and the calculator will calculate the remaining value. The bypass mass flow is the flow rate of air that goes around the core, the core mass flow is the flow rate of air through the core, and the ratio is the result of dividing the former by the latter.

Leave unknown inputs blank. To determine an engine's ratio, enter both flow rates. If you have a published rate, you can use it with a known flow rate to solve for the size of the other airflow and read off the required value.

Formula for bypass ratio:

For example, the CFM LEAP engine used in the 737 MAX and A320neo has a bypass ratio of about 9:1.

If there are 90 kg of air flowing around the core per second and 10 kg passing through the core, then the ratio is as follows:

\text{BPR} = \frac{\dot{m}_{bypass}}{\dot{m}_{core}} = \frac{90}{10} = 9

The amount of air flowing around the core is nine times larger than the amount of air flowing through the core. The following table shows each term and its example value.

Symbol	Meaning	Example
BPR	Bypass ratio (no units)	9
m_bypass	Bypass air mass flow	90 kg/s
m_core	Core air mass flow	10 kg/s

Differences in bypass ratio according to engine type:

The required ratio varies widely from one aircraft to another. A pure turbojet engine directs all the air into the core, so its bypass ratio is zero. Low-bypass turbofan engines generally have a bypass ratio between 0 and 2 and are often used in military jets where it's more important to make the engine smaller than to optimize fuel consumption. High-bypass turbofans, which start at about 5, power the passenger jets we fly on. Modern geared fans have bypass ratios of over 12.

Bypass ratio	Engine type	Where you see it
0	Turbojet	Early jets, missiles
0 to 2	Low-bypass turbofan	Fighters, fast military jets
2 to 5	Medium-bypass turbofan	Older airliners, regional jets
5 to 10	High-bypass turbofan	Wide-body airliners (787, A350)
10 and up	Very-high-bypass or geared	A320neo, A220

Why bypass ratio is important.

There are two particularly important reasons for this. At the subsonic speeds at which commercial airliners fly, it is more efficient to throw a large volume of air slowly than to throw a small volume fast. The greater the bypass ratio, the lower the fuel consumption per unit thrust produced. This explains why high-bypass engines have better thrust-to-fuel ratios than turbojets.

The second reason is noise. The bypass airflow surrounds the hot jet exhaust like a shroud and reduces the violent mixing that creates jet noise. A high bypass ratio is one of the main reasons modern airliners are much quieter than jets from the 1960s. The disadvantage is size. Large fans increase frontal area and drag, which is why supersonic combat aircraft still use low bypass ratios.

Review area:

Note some rough values. Fighter jets have bypass ratios of about 0.3 to 2, regional jets are around 3 to 5, wide-body airliners are in the range of 5 to 9, and modern civil engines are in the region of 10 to 12. If your answer is significantly outside this range please check your inputs. Both mass flows must be positive numbers, and the core flow cannot be zero. Also no ratio can be negative.

Frequently asked questions

What is high bypass ratio?

A bypass ratio of more than about five is usually referred to as a high-bypass ratio. Modern passenger jet engines have a bypass ratio in the range 9–12, and some of the latest geared turbofan engines are even higher.

Can the bypass ratio be zero?

Yes it can. A bypass ratio of zero means a pure jet engine where all the air goes through the core and no air bypasses the core. Early jets and many missiles used this principle.

Why don't all engines use a high bypass ratio?

Large fans increase the frontal area and drag, which is disadvantageous at supersonic speeds. The appropriate ratio depends on the particular mission; passenger aircraft require efficiency while fighter jets need speed and a slim shape.

Does the bypass ratio change in flight?

There are no big changes to it. The geometry of the engine dictates this. Actual mass flow will vary with altitude, temperature and throttle position but the ratio between both values remains close to nominal.

What effect does the bypass ratio have on fuel consumption?

A high bypass ratio allows more air to be ejected at a lower exhaust velocity, which increases efficiency when traveling subsonic. As a result the thrust-to-fuel ratio is decreased so that an engine with a higher bypass ratio will consume less fuel per unit of thrust produced than one with a lower bypass ratio.

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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

NASA Glenn Research Center: Turbofan Engine
How a turbofan splits air into bypass and core streams.
NASA Glenn Research Center: Engine Thrust and Bypass Ratio
Thrust, fuel consumption, and the role of bypass ratio.