Speed GunHow to Calculate Speed from Slow-Motion Video
Measuring speed from video uses the same formula you learned in grade-school science: speed = distance ÷ time. In a video, time comes from counting frames. If a video captures 240 frames per second, each frame is 1/240 of a second.
Worked example
Say that in a video shot at 240fps, the ball takes 100 frames to travel from point A to point B, and the distance between A and B is 16.8m.
- Time elapsed = 100 ÷ 240 = about 0.417 s
- Speed = 16.8m ÷ 0.417 s = about 40.3m/s
- Converted to km/h: 40.3 × 3.6 = about 145 km/h
The slow-motion trap
Slow-motion video shot on a phone can be saved in one of two ways.
- Original high-speed video: It was actually recorded at 240 frames per second, and the file still carries that information. This is the most accurate.
- Video converted to play back slowly: If footage shot at 240fps was stretched out and saved at 30fps, the file looks like 30fps. Calculate with 30fps and you'll get a speed one-eighth of the real value.
So when measuring slow-motion footage, you must match the slowdown factor (how many times slower it plays). The web tool has a setting that corrects for this.
Why videos from social media are unreliable
- Re-encoding: When uploaded to YouTube, TikTok, and the like, frames are often dropped or merged, leaving a lower frame rate than the original recording.
- Editing: If a scene is cut or a section's speed is changed, the timing math goes wrong.
- Screen recording: Screen-recording another video often saves the same image across several frames.
Treat readings from videos like these as a "rough estimate." If you need an accurate record, shooting and measuring your own original footage is best.
When you don't know the distance
If you don't know the distance, you can use an object of known size in the frame as a ruler. For example, a baseball is about 7.3cm across and a tennis ball about 6.7cm. Count how many "ball diameters" the ball moves on screen and you can work out a rough speed. If the ball is moving toward or away from the camera, though, its size changes and the error grows.