Win the “guess how many jelly beans are in the jar” contest, or finally answer that classic interview question: how many ping pong balls fit in a room? Pick an object and a container, enter the size, and the calculator uses real packing density (how tightly things settle, gaps included) to estimate the count.
The formula
Packing densities used
| Object | Size used | Packing density |
|---|---|---|
| Spheres (balls, gumballs, marbles) | By diameter | ≈ 64% (random close packing) |
| Jelly beans | ≈ 2 × 1.4 × 1.4 cm | ≈ 68% |
| M&M’s | ≈ 1.3 × 1.3 × 0.6 cm | ≈ 67% (measured in a 2004 Science study) |
| Party balloons | 11″ balloon ≈ 28 cm across | ≈ 60% (squishy and irregular) |
A 2004 study published in Science found that M&M’s pack more densely than spheres, about 68% versus 64%. It is one of the few candy facts to appear in a top science journal.
Classic answers
- A 10 × 10 × 10 ft empty room holds about 540,000 ping pong balls.
- A 1-gallon jar holds roughly 1,250 jelly beans or around 4,700 M&M’s.
- A 12 × 12 × 8 ft bedroom holds roughly 1,700 inflated party balloons.
Frequently asked questions
How many jelly beans are in a gallon jar?
About 1,200–1,300 standard jelly beans fit in a US gallon (3.79 L) jar, based on a bean volume of about 2 cm³ and roughly 68% packing.
How many ping pong balls fit in a room?
A 10 ft × 10 ft × 10 ft empty room (28.3 m³) holds about 540,000 ping pong balls at a random packing density of 64%.
How do I win a guess-the-jar contest?
Measure or estimate the jar’s inside diameter and height, pick the candy type here, and guess close to the estimate. Round slightly down if the jar tapers or the top isn’t completely full.
How many balloons does it take to fill a room?
For a 12 × 12 × 8 ft room, about 1,700 inflated 11-inch balloons. Decorators often fill only the bottom third of a room, which needs about a third as many.
Related tools
Estimates for fun and contests. Real counts vary with candy brand, balloon inflation and how the container was filled.