How many kegs does a 5 lb CO₂ tank do?
It depends on what you're asking the gas to do, and that is the whole story of this page. A cylinder has three jobs: carbonating the beer, dispensing it (every liter poured is replaced by a liter of gas at the keg's pressure), and purging the empty keg of air before the next batch. For a 5-gallon corny at 12 psi and 38 °F those are 61 g, 67 g and — for the usual three blasts to 30 psi — 232 g. Purging, the one nobody counts, is the biggest by a factor of three. So:
| Per 5-gallon corny | Gas per keg | 2.5 lb | 5 lb | 10 lb | 20 lb |
|---|---|---|---|---|---|
| Dispense only (commercial or primed keg, no purge) | 67 g · 2.4 oz | 16 kegs | 33 kegs | 67 kegs | 135 kegs |
| Carbonate from fermenter-fresh + dispense | 128 g · 4.5 oz | 8 kegs | 17 kegs | 35 kegs | 71 kegs |
| + liquid purge (fill with sanitizer, push out at 5 psi) | 178 g · 6.3 oz | 6 kegs | 12 kegs | 25 kegs | 50 kegs |
| + three 30 psi pressurize-and-vent purges | 359 g · 12.7 oz | 3 kegs | 6 kegs | 12 kegs | 25 kegs |
19 L of beer at 12 psi and 38 °F; carbonating to 2.5 volumes from the 0.86 a fermenter-fresh keg holds. Whole kegs. Nothing allowed for leaks. Computed by this page's engine.
A 5 lb cylinder dispenses 33 cornies, or carbonates and dispenses 17, or does the whole job with a three-blast purge for 6. Every one of those numbers is on the internet somewhere as "the" answer, and none of the pages say which job they counted.
Why the internet says anything from 2 to 22
For a 5 lb cylinder the ranking pages print "6–8 half barrels" (KegWorks), "5–7" (KegWorks' own chart, two paragraphs later), "4 or 5" (Kegerators.com), "2 to 4 full-sized kegs" (a search snippet), "15–22" 5-gallon kegs (BeverageCraft), and both "7–11" and "15–22" 5-gallon kegs on the same BeverageFactory FAQ. They aren't all wrong. They're answers to different questions. The industry figure — "half a pound of CO₂ per half barrel," which BeverageFactory, BeverageCraft and the Wholesale Beer Parts estimator all use — is dispensing only, because a commercial keg arrives carbonated and nobody purges it. Run the gas law on a 15.5-gallon keg at 12 psi and 38 °F and you get 0.46 lb: the half pound is physics, and a 5 lb cylinder dispenses 11 half barrels of it. The "6–8" the same sources quote is that number with a real-world allowance for purging the lines, the gas left in a "finished" cylinder, and small leaks.
A homebrewer's corny is a different question. It arrives flat-ish and needs carbonating; it was empty and open before that and needs purging. Count all three and the same cylinder does a third as many kegs as it "should." That's not a leak — though the next section is.
A CO₂ cylinder holds liquid, with vapor above it, and the high-pressure gauge reads the vapor: the pressure at which CO₂ boils at the cylinder's temperature — about 816 psi at 20 °C, 538 psi in a 38 °F fridge. It reads that whether the cylinder is full or nearly empty, then collapses in the last few pours once the liquid is gone. (A cylinder moved from the fridge to a warm room "gains" 300 psi without gaining a gram.) The only honest fullness gauge is a bathroom scale: the tare weight is stamped on the collar, and 5 lb of gas is 5 lb.
How much CO₂ does it take to dispense a keg?
Exactly the keg's volume of gas at the keg's absolute pressure — gauge plus atmosphere — at the beer's temperature. Ideal gas, no rule of thumb needed:
grams = Pabs × V × 44.01 ÷ (R × T) · 18.9 L at 12 psi gauge, 38 °F → 67 g
| Keg | 10 psi | 12 psi | 15 psi | 20 psi | 30 psi |
|---|---|---|---|---|---|
| 5 gal corny (19 L) | 62 g 2.2 oz | 67 g 2.4 oz | 74 g 2.6 oz | 87 g 3.1 oz | 112 g 3.9 oz |
| 1/6 barrel (5.16 gal · 19.5 L) | 64 g 2.2 oz | 69 g 2.4 oz | 77 g 2.7 oz | 90 g 3.2 oz | 115 g 4.1 oz |
| 1/4 barrel (7.75 gal · 29 L) | 96 g 3.4 oz | 103 g 3.6 oz | 115 g 4.1 oz | 134 g 4.7 oz | 173 g 6.1 oz |
| 1/2 barrel (15.5 gal · 58.7 L) | 191 g 6.7 oz | 207 g 7.3 oz | 230 g 8.1 oz | 269 g 9.5 oz | 346 g 12.2 oz |
| 50 L (EU) | 163 g 5.7 oz | 176 g 6.2 oz | 196 g 6.9 oz | 229 g 8.1 oz | 295 g 10.4 oz |
Beer at 38 °F, sea level. Higher serving pressure costs proportionally more gas per pour — a 30 psi wheat beer is two and a half times the 12 psi lager. Computed by this page's engine.
This is also why a 16 g cartridge — the picnic-tap kind — dispenses about a quarter of a corny (24%), not the "one third" the cartridge sellers quote, and why a 68 g cartridge is one keg, not "1 to 4."
How much CO₂ to force-carbonate a keg?
The gas that ends up in the beer: the volumes you want, minus the 0.86 a keg straight from a 68 °F fermenter already holds, times 1.96 g per liter per volume. Count from zero — as most people do — and you'll budget 93 g where 61 g is the truth. The residual is the same number the priming-sugar calculator uses, so the two pages can't disagree.
| Keg | to 2.0 vol | to 2.5 vol | to 3.0 vol | to 4.0 vol |
|---|---|---|---|---|
| 5 gal corny (19 L) | 42 g · 1.5 oz | 61 g · 2.2 oz | 80 g · 2.8 oz | 117 g · 4.1 oz |
| 1/2 barrel (15.5 gal · 58.7 L) | 131 g · 4.6 oz | 189 g · 6.7 oz | 247 g · 8.7 oz | 362 g · 12.8 oz |
From 0.86 residual volumes. A primed or naturally carbonated keg needs none. Computed by this page's engine.
Purging a keg: the gas nobody counts
An empty 19 L keg is 19 L of air, and air is a fifth oxygen. The common fix is to pressurize it to 30 psi and vent, three times. Each cycle throws away a keg-volume of gas at the purge pressure, and leaves behind (atmosphere ÷ absolute pressure) of the oxygen that was there — at 30 psi, a third of it. Three cycles: 3.6% of the oxygen left, for 232 g of CO₂. That's more than three times what dispensing the whole keg costs.
| Purge, empty 19 L keg | Gas | Oxygen left |
|---|---|---|
| 1 × 30 psi | 77 g · 2.7 oz | 32.9% |
| 3 × 30 psi | 232 g · 8.2 oz | 3.6% |
| 5 × 30 psi | 386 g · 13.6 oz | 0.4% |
| 3 × 10 psi | 77 g · 2.7 oz | 21.1% |
| 5 × 10 psi | 129 g · 4.5 oz | 7.5% |
| 8 × 10 psi | 206 g · 7.3 oz | 1.6% |
| Fill with sanitizer, push out at 5 psi | 51 g · 1.8 oz | ~0% |
Oxygen left as a fraction of the air originally in the keg; ~21 % of that is the actual O₂. 38 °F, sea level. Computed by this page's engine.
Fill the keg to the top with sanitizer, seal it, and push the liquid out through the beer post with CO₂ at a few psi. The gas that replaces the liquid is one keg-volume at 5 psi — 51 g — and there is no air left to dilute, because there was never any headspace. It costs a fifth of the three-blast method, leaves less oxygen than 5 blasts at 30 psi, and turns a 5 lb cylinder from 6 kegs into 12. Lower pressure and more cycles helps too: 5 × 10 psi costs 129 g for 7% left, versus 232 g for 3.6% at 3 × 30.
My tank ran out early — leak, or normal?
Check it against the table first: a homebrewer who purges hard and force-carbonates every keg gets 6 cornies from 5 lb, and that is normal, not a leak. Below that, it's a leak, and here is the number that makes leaks easy to believe: emptying a 5 lb cylinder overnight — ten hours — is a flow of 227 g an hour, the gas it takes to dispense 1.1 half barrels, every hour, through a fitting you can't hear. A regulator nut, a hose clamp, a keg lid seated a hair off — any of them will do it. The high-pressure gauge won't warn you, for the reason in the box above; it reads 538 psi right up to the last pour.
The test that works: pressurize the whole system, close the cylinder valve, and leave it. If either gauge moves in an hour, spray every joint with soapy water and watch for bubbles — start at the regulator, then the disconnects, then the keg lid. Then weigh the cylinder and write the number on it. A 5 lb cylinder that's lost 2 lb has done 2 kegs' worth of work at the full budget or 13 at dispensing alone; if it's done fewer, the difference went into the room.
- 5 lb of CO₂ is 1,200 liters of gas. Released into a sealed 10 m³ closet — a small pantry with a keezer in it — that's 12% CO₂, well past the concentration that's immediately dangerous to life. Cylinders live in ventilated space, and a leak you can hear in a small room is a room you leave.
- Upright and secured. A knocked-over cylinder with the valve open is a projectile; a chain or strap costs nothing. Never lay one on its side on gas, and never heat one.
- Every fitting gets the soapy-water test the first time it's under pressure and any time it's been touched. Tighten with the cylinder valve closed.
- Refill, don't top up. Only a fill station with a scale fills a cylinder; the weight on the collar is the limit, because liquid CO₂ expands hard when it warms.
The honest summary: the gas laws are exact and the world around them leaks. Budget three line items, buy the cylinder one size up from what the table says, weigh it now and then, and treat a tank that's short of the number as a fitting to find rather than a mystery.