Helium is sold by the cylinder and consumed by the cubic foot, and the two only meet through balloon size. An 11-inch latex takes about half a cubic foot; a 36-inch takes more than ten. Counting balloons without weighting them by size is how installers end up short at the venue.
480 eleven-inch latex at 0.5 cu ft each is 240 cu ft — five 55 cu ft tanks, not four.
The example
Three installs, each with a balloon count, the cubic feet a balloon of that size holds, and the capacity of the tank being rented.
| A | B | C | D | E | |
|---|---|---|---|---|---|
| 1 | Install | Balloons | Cu ft each | Tank cu ft | Tanks |
| 2 | Balloon arch | 480 | 0.5 | 55 | 5 |
| 3 | Ceiling cluster | 60 | 1 | 55 | 2 |
| 4 | Spiral column | 150 | 0.28 | 55 | 1 |
The formula
Total gas first, then tanks:
How it works
Three inputs and one guardrail:
B2*C2is total helium in cubic feet. Keep a cubic-feet-each figure per balloon size — 9 inch is about 0.28, 11 inch about 0.5, 18 inch mylar about 1.0, 36 inch about 10.5./D2converts to tanks. Rental cylinders are commonly 55, 125, or 291 cubic feet; the label tells you which.ROUNDUP(...,0)because a tank is a rental unit, not a continuous quantity.- Give each balloon size its own row and sum the cubic feet before dividing if one install mixes sizes.
Tanks are rented by the day, so rounding up costs one day's rental — far cheaper than a second trip in the middle of an install.
Try it: interactive demo
Enter the balloon count, cubic feet per balloon, and tank capacity.
Variations
Mixed sizes on one install
Sum the cubic feet across the size rows first, then convert once.
Cost of the gas
Multiply tanks by the daily rental plus fill charge.
Pitfalls & errors
Air-filled work needs no helium at all. Garlands and organic columns are usually air on a frame — only the floating pieces belong in this calculation, and mixing them in inflates the tank count badly.
Float time depends on treatment, not tank size. A treated 11-inch latex floats a day or more; untreated it can sag in six hours, which affects when you inflate, not how much gas you buy.
Practice workbook
Frequently asked questions
How many 11-inch balloons does a 55 cu ft tank fill?
Should I plan for waste?
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