A cascade bottle does not give you every cubic foot it holds — the last stretch of pressure is too low to top off a player's tank. What matters is usable air divided by what one fill takes, and that division always rounds down.
An 80 cu ft bottle with a 15% reserve holds 68 usable feet — 19 fills at 3.5 feet each, not the 22 the label suggests.
The example
Three bottles on the fill station, each with a reserve you cannot push into a player tank and a typical fill size.
| A | B | C | D | E | |
|---|---|---|---|---|---|
| 1 | Bottle | Cu ft | Reserve | Cu ft per fill | Fills |
| 2 | 80 cu ft | 80 | 15% | 3.5 | 19 |
| 3 | 100 cu ft | 100 | 15% | 3.5 | 24 |
| 4 | 80, big tanks | 80 | 15% | 5 | 13 |
The formula
Usable air first, then fills:
How it works
Left to right:
B2*(1-C2)is usable air. The reserve is the tail of the bottle whose pressure is below what a player tank needs — typically 10–20% depending on your fill pressure./D2is one fill. A 48 ci tank at 3000 psi is roughly 3.5 cu ft; a 68 ci at 4500 psi is closer to 5.ROUNDDOWN(...,0)is the whole point. A partial fill is a player walking back to the station mid-game, so it does not count.- Multiply the fills by your air price and the bottle stops being a cost and becomes a margin line.
Log the actual fills you get off one bottle for a weekend and back-solve the reserve. Your compressor, your fill whip, and your player mix all move that number.
Try it: interactive demo
Enter the bottle size, the reserve share you cannot use, and the cubic feet one player fill takes.
Variations
Air revenue per bottle
Multiply the fills by what you charge for air to see the bottle's ceiling.
Bottles to bring for a booked group
Flip it: total fills needed divided by fills per bottle, rounded up.
Pitfalls & errors
Cubic feet and cubic inches are different units. A “68/4500” player tank is 68 cubic inches of volume at 4500 psi, which works out to about 5 cubic feet of free air. Mixing them silently multiplies your fill count by a factor of twelve.
Cascade systems drain bottles unevenly. Rate the bank as a whole rather than one bottle if you fill from a manifold.
Never ROUNDUP here. Rounding a 19.4 into 20 puts the last player of the day on a half-pressure tank.
Practice workbook
Frequently asked questions
How do I estimate cubic feet per fill?
Does the reserve change with the tank I am filling?
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