Guide

How to measure how fast you kick a soccer ball

You can measure how fast you kick a soccer ball with just an iPhone. Film the kick from a still ball with the phone held steady, and Strikr works out how fast the ball left your foot from the video, in km/h or mph. It needs no radar gun and no sensor in the ball.

What you need

• An iPhone, running iOS 17 or later.
• A size 5 ball. Strikr takes its scale from the ball, and assumes the 22 cm of a size 5. A smaller ball, like a size 4 or a futsal ball, reads fast.
• Something to prop the phone on, a tripod, or your bag. The phone has to stay still.
• Strikr, from the App Store.

Step by step

1. Set the ball down and let it settle

   Strike from a still ball: a set piece, a penalty or a shooting drill. Strikr measures the ball at rest before the strike, so a rolling ball or a moving camera won't measure well.

2. Prop the phone up, steady and level

   Side-on to the kick or behind the ball both work. Lean the phone on a bag if you have no tripod; Strikr's camera shows a level line that turns yellow when the phone is level.

3. Frame the whole flight

   Keep the ball big in the frame and in shot from the strike on. Film against trees, a fence or the net rather than open sky, and keep your feet in frame so Strikr can tell which foot struck the ball.

4. Film in Strikr, or import the clip

   Strikr's camera records in slow motion where your iPhone supports it. A clip from Photos works too: for a Slo-mo clip, allow access to your photo library so Strikr reads the original rather than a slowed copy.

5. Read the numbers

   Strikr finds the ball and the moment of the strike, then shows how fast the ball left your foot, its spin in rpm when the ball's markings can be read, and the flight traced over the replay with its height and distance.

6. Film every session the same way

   Same spot, same distance. Strikr charts your speed week by week and your left foot against your right, and your weeks compare best when the footage is alike.

How the speed is worked out

From the ball itself. A size 5 ball is 22 cm across, so the number of pixels it covers while it sits still tells Strikr how many pixels make a meter. As it flies, its size in each frame gives its distance from the camera, which is how a kick filmed from behind the ball, moving away rather than across, is still measured. The clip's own timestamps give the time between frames. Strikr fits the ball's path over the frames just after the strike and reports how fast it left your foot.

Spin is read differently: Strikr follows the ball's markings as it turns from one slow-motion frame to the next. A plain ball, or one too small or blurred to read, gets no spin number. Strikr says so instead of guessing.

What spoils a measurement

• The ball leaves the frame soon after the strike.
• The phone moves, pans to follow the ball, or is held at arm's length.
• Someone walks between the phone and the ball.
• The ball crosses open sky, where it is hard to follow.
• It's getting dark. Spin in particular needs daylight.
• The ball is a few pixels across: film closer, or zoom in.

When the footage isn't good enough to follow the ball, Strikr tells you what went wrong and how to film the next clip.

Questions