Welding gas cylinders come in standardized sizes, but the naming conventions are confusing. Different suppliers use different letter designations, and the same physical cylinder gets called by different names depending on the gas inside. This chart sorts it out with actual dimensions, capacities, and weights.
Inert Gas Cylinders (Argon, Helium, Ar/CO2 Mixes)
These are high-pressure cylinders filled to approximately 2,015-2,640 PSI. The gas is stored as a compressed gas (not liquid) at room temperature. Capacity is measured in cubic feet (CF) at standard atmospheric pressure.
| Common Name | Capacity (CF) | Height (approx.) | Diameter | Full Weight | Pressure (full) |
|---|---|---|---|---|---|
| R-size | 20-22 | 18" | 5" | 15-20 lbs | 2,015 PSI |
| R-size (tall) | 40 | 24" | 7" | 25-30 lbs | 2,015 PSI |
| Q-size | 80 | 33" | 7" | 55-65 lbs | 2,265 PSI |
| S-size | 125 | 40" | 7" | 80-95 lbs | 2,265 PSI |
| K-size | 250 | 51" | 9" | 140-165 lbs | 2,400 PSI |
| T-size (300) | 330 | 55" | 9" | 170-195 lbs | 2,640 PSI |
Heights include the valve but not the protective cap. Weights are approximate and vary by manufacturer. Cylinder tare weight is stamped on the shoulder of each cylinder.
R-size (20-40 CF): Portable enough for fieldwork and small enough to fit under a workbench. Empties fast at standard flow rates. A 20 CF cylinder at 25 CFH lasts under an hour. These are backup tanks or travel tanks, not primary shop cylinders.
Q-size (80 CF): The standard home-shop cylinder. At 25 CFH, it provides roughly 3 hours of arc time. Manageable weight for one person to load into a truck or strap to a cart. This is where most hobbyists start.
S-size (125 CF): The step up for anyone who welds regularly. About 5 hours of arc time at 25 CFH. Still manageable by one person with a cylinder cart. The price-per-fill jump from 80 CF is modest, making this the better value.
K-size (250 CF): The standard production-shop cylinder. Heavy enough to require a cart and a second person for loading. About 10 hours of arc time. Fills cost more in absolute terms but less per cubic foot.
T-size (330 CF): The largest standard high-pressure cylinder. Production shops and welding schools use these. At 170+ lbs full, they stay on a cart permanently.
CO2 Cylinders
CO2 is stored as a liquid under pressure, not as a compressed gas. This means CO2 cylinders are measured by weight (pounds of liquid CO2), not cubic feet. As the liquid evaporates, it produces gas at a predictable ratio.
1 lb of liquid CO2 produces approximately 8.7 cubic feet of gas at standard temperature and pressure.
| Cylinder Size | CO2 Weight | Gas Volume (CF) | Height (approx.) | Full Weight |
|---|---|---|---|---|
| 5 lb | 5 lbs | ~44 CF | 18" | 15-18 lbs |
| 10 lb | 10 lbs | ~87 CF | 24" | 25-30 lbs |
| 20 lb | 20 lbs | ~174 CF | 28" | 45-55 lbs |
| 35 lb | 35 lbs | ~305 CF | 35" | 75-85 lbs |
| 50 lb | 50 lbs | ~435 CF | 48" | 120-140 lbs |
| 75 lb | 75 lbs | ~653 CF | 52" | 165-185 lbs |
CO2 cylinders must stand upright during use. Tilting a CO2 cylinder lets liquid CO2 reach the valve, which can send liquid into your regulator, freeze the diaphragm, and produce wildly inaccurate flow readings. Some cylinders have internal siphon tubes for liquid withdrawal, but standard welding CO2 cylinders don’t.
5 lb and 10 lb: Common in homebrew beer dispensing and occasionally repurposed for welding. Too small for regular MIG use but fine as an emergency backup.
20 lb: The most common CO2 cylinder size for MIG welding. Produces roughly 174 CF of gas, comparable to a 125 CF inert gas cylinder. Available at welding suppliers and some home improvement stores through exchange programs.
50 lb and 75 lb: Production welding with straight CO2. These stay on a cart permanently.
Oxygen and Acetylene Cylinders (Oxy-Fuel)
These aren’t shielding gas, but they’re common enough in welding shops to include here.
| Gas | Common Size | Capacity | Height | Full Weight | Pressure |
|---|---|---|---|---|---|
| Oxygen | R-size | 20 CF | 18" | 15 lbs | 2,200 PSI |
| Oxygen | Q-size | 80 CF | 33" | 55 lbs | 2,200 PSI |
| Oxygen | K-size | 244 CF | 51" | 135 lbs | 2,200 PSI |
| Acetylene | MC (mini) | 10 CF | 13" | 8 lbs | 250 PSI |
| Acetylene | B-size | 40 CF | 24" | 25 lbs | 250 PSI |
| Acetylene | WC-size | 75 CF | 33" | 65 lbs | 250 PSI |
| Acetylene | WS-size | 120 CF | 40" | 100 lbs | 250 PSI |
| Acetylene | K-size | 300+ CF | 47" | 220 lbs | 250 PSI |
Acetylene cylinders are packed with a porous material (calcium silicate) saturated with acetone, which absorbs the acetylene gas. This makes acetylene safe to store at moderate pressure. Never exceed 15 PSI at the regulator with acetylene, and never lay an acetylene cylinder on its side. Liquid acetone can reach the valve, contaminate the gas, and create a safety hazard.
Valve Connections
Different gases use different valve outlet connections to prevent accidentally connecting the wrong regulator to the wrong gas. The Compressed Gas Association (CGA) standardizes these fittings.
| Gas | CGA Connection | Thread Direction |
|---|---|---|
| Argon, Ar/CO2 mixes, Helium | CGA 580 | Right-hand (standard) |
| CO2 | CGA 320 | Right-hand (standard) |
| Oxygen | CGA 540 | Right-hand (standard) |
| Acetylene | CGA 510 | Left-hand (reverse) |
| Nitrogen | CGA 580 | Right-hand (standard) |
Fuel gases (acetylene, propane, hydrogen) use left-hand threads. All fittings on fuel gas systems have a notched nut to indicate reverse threading. This prevents cross-connection with oxidizing gases. Never force a fitting. If it doesn’t thread on smoothly, you have the wrong connection.
Reading Cylinder Markings
Every DOT-certified cylinder has markings stamped into the shoulder. Here’s what they mean:
- DOT-3AA or DOT-3AL: Specification type (3AA = steel, 3AL = aluminum alloy)
- 2015 or 2265: Service pressure rating in PSI
- Serial number: Unique identifier for tracking
- Manufacturer symbol: Who built the cylinder
- Test date: Month/year of last hydrostatic test, plus inspector’s mark
- Tare weight: Empty cylinder weight (important for CO2 to calculate fill level)
Cylinders must pass a hydrostatic test every 5 or 10 years depending on type. Expired cylinders won’t be filled by reputable suppliers until retested. Check the test date before transporting a cylinder to a supplier for refill.
Choosing the Right Size
Factor in three things: how often you weld, your flow rate, and your physical ability to move the cylinder.
Hobby welder (a few hours per month): An 80 CF argon or 75/25 cylinder lasts months between fills. It’s light enough to carry to a truck and fits on a small welding cart. A 20 lb CO2 cylinder is the equivalent if you’re running straight CO2.
Small shop (several hours per week): A 125 CF cylinder provides a reasonable interval between fills. If you’re welding daily, jump to 250 CF and keep a backup cylinder on hand so you don’t run out mid-job.
Production (multiple hours per day): 250 CF or 330 CF cylinders minimize the downtime of frequent changes. Some shops run manifolded cylinder banks or bulk liquid systems for uninterrupted supply.
Cylinder Safety
Compressed gas cylinders store enormous energy. A standard 80 CF argon cylinder at 2,265 PSI contains enough pressure to turn an unsecured cylinder into a missile if the valve shears off.
Mandatory safety practices:
- Chain or strap every cylinder upright to a wall, cart, or rack. No exceptions.
- Keep the valve protection cap on during transport and storage.
- Never roll, drag, or drop cylinders.
- Close the cylinder valve when not in use, even for short breaks.
- Transport in an upright position secured to the vehicle. Never transport in a closed trunk or passenger compartment.
- Keep cylinders away from electrical circuits and welding ground clamps. Stray arc strikes on a cylinder can weaken the wall.
- Store full and empty cylinders separately to avoid confusion.
- Keep cylinders out of direct sunlight and away from heat sources exceeding 125F.