THE SCIENCE OF SWING

A seam, a shine,
and a curve.

A swinging cricket ball is a lesson in air.

The seam and the state of the leather decide how air clings to each side of the ball. When the two sides differ, the ball is pushed sideways. Wind-tunnel research explains how, and the Laws of Cricket set limits on what fielders may do to help it.

67 MPH the sweet spot for swing

Pick a delivery

A red leather cricket ball held between a bowler's first two fingers with the seam upright, against a blurred green outfield — AI-generated image, not a photograph of a real person or place.

Seam angled, not upright
One side kept smooth
Speed changes everything
Air on two sides, two different stories

THE SHORT ANSWER

Two sides,
two kinds of air.

Swing comes from an imbalance.
The seam and the surface create it.

THE SEAM

A trip wire for air

Bowled with the seam at an angle, the seam trips the thin layer of air on one side into turbulence. That layer clings on longer, so the ball is pushed towards the seam side.

Pick a delivery

THE SHINE

Why one side is polished

Conventional swing needs a smooth, polished side so the air there stays smooth and lets go early. Roughness changes where the air separates, which is why the ball's condition matters so much.

Reverse swing

THE LAW

Sweat yes, saliva no

Under Law 41.3 a fielder may polish the ball on their clothing, but the only natural substance allowed is sweat. Saliva has been banned since the 2022 Code.

See the numbers

A QUICK DELIVERY CHECK

Will this one
swing?

Pick a delivery to see what wind-tunnel tests found. The bar shows how much sideways force the tests measured, compared with the best case.

Delivery check

Choose a deliverySeam straight at the batterNew ball, seam angled, about 34 mphNew ball, seam at 20°, about 67 mphNew quarter-seam ball, over 80 mphSame ball on a humid day

Seam straight at the batterLittle or none

With the seam at nominally zero angle there was no significant side force, except at high speeds when a local rough spot such as an embossment mark started to have an effect.

New ball, seam angled, about 34 mphSwing just starts

With the seam angled to the flow, the side force started to increase at about 15 m/s (34 mph). It keeps growing as the ball gets faster.

New ball, seam at 20°, about 67 mphMost conventional swing

The maximum side force came at about 30 m/s (67 mph) with the seam at 20°. Faster than that, the smooth side also turns turbulent and swing falls away.

New quarter-seam ball, over 80 mphReverse swing

A new four-piece ball showed significant reverse swing, towards the non-seam side, at speeds above about 80 mph, even with no tampering of the surface.

Same ball on a humid dayNo measured change

Balls kept at 75% humidity for 48 hours, and tested dry, humid and wet, showed no increase in side force. Mehta found no positive scientific evidence that humid days help swing.

Source: R. D. Mehta, Cricket Ball Aerodynamics: Myth Versus Science (NASA Ames, 2000).

Based on wind-tunnel and projection tests summarised by NASA Ames researcher Rabindra Mehta (2000). Real deliveries also depend on spin, wind and the individual ball.

TWO NUMBERS WORTH KNOWING

Not the weather,
the ball.

One figure on the best speed for swing,
one on how little the air changes in a day.

≈ 67mph

Bowling speed for the most conventional swing

In wind-tunnel tests the sideways force on a new ball peaked at about 30 m/s (67 mph) with the seam angled at 20°, reaching about 0.3 of the ball's weight before falling off.

PEAK SWING · NASA AMES

≤ 2%

Change in the air's flow properties over a day

Typical changes in temperature and pressure through a whole day alter the air's density and viscosity, and so the flow over the ball, by no more than about 2%. Humid air is in fact less dense than dry air.

THE AIR · NASA AMES

A 2022 University of Cambridge engineering thesis, using conditions measured in field tests, found that conventional swing on new balls is greatest when both sides are smooth, and reverse swing is largest when the seam side has a fully turbulent layer of air.

FOUR THINGS TO WATCH

See the science
from the boundary.

Next time you watch a match, these are the clues that the research says matter most.

Look at the seam in flight
A seam held steady at an angle is what trips the air on one side. A wobbling or upright seam gives little swing.
Watch the fielders shine it
Players rub one side on their trousers to keep it smooth. Law 41.3 allows this with sweat only, and it must waste no time.
Notice the ball's age
A new ball keeps its shine. As one side roughens, faster bowlers may start to get the ball to swing the other way.
Count the overs
In matches of more than one day, the fielding captain may take a new ball once 80 overs have been bowled with the old one.

A fielder in cricket whites, seen from the side without the face, rubbing a red ball on the thigh of their trousers — AI-generated image, not a photograph of a real person or place.

A used red cricket ball on short grass, one half polished and shiny and the other half scuffed and dull — AI-generated image, not a photograph of a real person or place.

REVERSE SWING

When the ball
goes the other way.

At higher speeds, or on a ball with a rough surface, the air turns turbulent on both sides. The seam then thickens the layer of air on its own side, which lets go earlier, so the ball swings towards the smooth, non-seam side instead.

That is why the Laws police the ball's condition. Under Law 41.3 it is an offence for any player to change the condition of the ball beyond what is allowed; if umpires find it has been unfairly changed, they award 5 penalty runs and the opposing captain may ask for a replacement ball with comparable wear.

Questions

Questions people ask

Does a cricket ball swing more on a cloudy or humid day?

Wind-tunnel tests have not shown it. Balls kept at 75% humidity and tested dry, humid and wet showed no increase in side force, and changes of up to 40% in relative humidity made no measured difference.

Why do bowlers shine only one side of the ball?

Conventional swing needs a contrast: a smooth side where the air stays smooth and separates early, and a seam side where it becomes turbulent and clings on. Keeping one side polished preserves that contrast.

Can players use saliva to shine the ball?

No. Since the 2022 Code of the Laws, saliva is treated like any other unfair way of changing the ball. MCC noted that during COVID-19 bans, sweat proved equally effective and swing barely changed.

How heavy is a cricket ball?

Under Law 4 of the 2026 edition, a new Size 1 ball weighs 156 to 163 g and measures 22.4 to 22.9 cm round. Sizes 2 and 3 are smaller and lighter.

Is reverse swing the same as ball tampering?

No. Reverse swing is a natural effect of speed and surface roughness, and tests show it even on a new four-piece ball above about 80 mph. Tampering means changing the ball's condition in ways the Laws do not allow.

A seam, a shine,
and a curve.