How Many Ping Pong Balls
Fit in the Space? 🏓
The classic interview brain teaser. This turns a volume into a ball count using sphere packing.
About Ping Pong Ball Estimator
This tool answers the famous interview brain teaser: how many ping pong balls[1] fit in a given space. Pick a space like a car trunk, a room, or a bus, or enter a volume, and it estimates the number of balls, with a range.
A regulation table tennis ball is 40 millimeters across, so it takes up about 33 milliliters of space. Identical spheres never fill a container completely: dropped in at random they reach a packing density[2] near 64 percent, the random close packing[3] limit, below the 74 percent of a perfectly ordered sphere packing[4].
This is a textbook Fermi problem[5]: estimate the volume[6] of the space, multiply by the packing fraction, and divide by the volume of one ball. The point is a defensible order of magnitude, not an exact count, which is why the answer comes as a range.
The presets give rough volumes for everyday spaces, and you can enter your own in liters, gallons, or cubic measures. Everything runs in your browser and nothing is uploaded.
How to Use
- Pick a space, or enter your own volume.
- Press Estimate to count the ping pong balls.
- Read the headline number and the range.
- See how long they would stretch in a line and how much they would weigh.
- Use the method, not just the number, if you are answering this in an interview.
Frequently Asked Questions
How many ping pong balls fit in a car?
Very roughly, a car trunk of about 400 liters holds around 7,000 to 8,000 ping pong balls, and a whole car interior far more. Enter the space to get an estimate with a range.
How many ping pong balls fit in a room?
A small bedroom of about 30 cubic meters holds on the order of 500,000 ping pong balls. The exact figure depends on the room size and how the balls settle.
How does the estimator work?
It multiplies the space volume by a packing density of about 64 percent, the random close packing limit for spheres, then divides by the 33 milliliter volume of a 40 millimeter ball.
Why is the answer a range?
Because it is a Fermi estimate. The real number depends on the exact volume and packing, so an order of magnitude with a range is the honest answer.
What size ball does it assume?
A regulation 40 millimeter table tennis ball, which takes up about 33 milliliters including the gaps it creates.
Is my data private?
Yes. The estimate runs entirely in your browser and nothing is uploaded.
Sources (6)
About Matt George
I'm Matt George, a software engineer in the startup space. I run my own shop building software for early-stage founders, and I love making free tools and experiences that people actually use. Enjoy using Ping Pong Ball Estimator!