Welding Gas Cylinder Size Chart: T, K, Q, MC & More

How Cylinder Sizes Are Named and Measured

Welding gas cylinders are steel or aluminum high-pressure vessels built and stamped to US DOT specifications, most commonly DOT-3AA (seamless steel) and DOT-3AL (aluminum). The stamp on the shoulder tells you the specification, the service pressure (typically 2015 to 2265 PSIG for common inert-gas tanks), the manufacture date, and the retest date. Under CGA and DOT rules these cylinders must be hydrostatically retested, usually every 5 or 10 years depending on the marking, before a distributor will refill them.

The confusing part is that there is no single national naming convention. The same physical bottle can be called by a letter code, a number, or its nominal cubic-foot capacity depending on the region and the gas company. A '#5' at one supplier is a '300' at another and a 'K' at a third, and they are all roughly the same tank. What actually matters is the water volume of the cylinder and its fill pressure, because those two numbers determine the cubic feet of gas you get.

Capacity is expressed in cubic feet (cf) of gas at standard temperature and pressure once the compressed contents are expanded to atmosphere. A tank marked 80 cf holds roughly 80 cubic feet of usable argon or CO2 mix when full. For pure CO2, capacity is rated by weight in pounds of liquid rather than cubic feet, because CO2 is stored as a liquid under its own vapor pressure of about 830 PSI at 70 F, so a '20 lb CO2' tank is a common MIG size.

• DOT-3AA: Seamless high-strength steel spec used for most argon, argon-blend, oxygen, and inert-gas cylinders at 2015 to 2400 PSIG service pressure.
• DOT-3AL: Seamless aluminum spec, lighter for the same volume; common in smaller portable and medical-style bottles.
• Hydrostatic retest: Required every 5 or 10 years per DOT/CGA before refill; check the most recent date stamped on the cylinder shoulder.
• CGA valve outlet: The connection standard matters: CGA-580 for argon/inert mixes and nitrogen, CGA-320 for CO2, CGA-540 for oxygen, CGA-350 for shielding-grade hydrogen blends.

The Cylinder Size Chart: Capacity, Dimensions, and Weight

Below is the practical size chart distributors across the US use for high-pressure inert and shielding-gas cylinders (argon, argon/CO2 blends like C25, tri-mix, and 100% CO2 where noted). Dimensions and weights are nominal and vary a little between manufacturers and between steel and aluminum, but they are close enough to plan a purchase around. The letter codes, number codes, and cubic-foot names are cross-referenced because you will hear all three.

Note that CO2 cylinders in the smaller range are rated by liquid weight rather than cubic feet: a 20 lb CO2 tank yields roughly 175 cf of gas, and a 50 lb CO2 tank roughly 435 cf, so a CO2 bottle punches well above an argon bottle of the same physical size.

The full-height T (also sold as K or #5/300) is the workhorse of professional shops. It is the tallest tank most people will handle without a forklift, and a loaded one weighs around 130 to 150 lbs, which is why it must be chained upright and moved on a cart or bottle dolly per OSHA 1910.253 handling rules.

How Long a Cylinder Lasts at Your Flow Rate

Runtime is simple arithmetic once you know two numbers: the cylinder's capacity in cubic feet and your gas flow in cubic feet per hour (CFH). Divide capacity by CFH to get hours of actual arc-on time. The catch is that arc-on time is not shift time. On typical MIG work you are welding maybe 20 to 40 percent of the clock, so a tank that gives 15 arc-hours can easily stretch across a couple of weeks of normal shop use.

Most short-circuit MIG runs 20 to 35 CFH of C25 (75% argon / 25% CO2). TIG on steel or stainless with 100% argon usually runs 15 to 20 CFH, while aluminum TIG and larger cups climb to 20 to 30 CFH. Flux-cored self-shielded wire (E71T-11, for example) uses no bottle at all. Set flow with a flowmeter at the torch under actual welding conditions, not just the regulator gauge, because drafts and nozzle size change what you truly need; more gas past roughly 35 to 40 CFH usually just creates turbulence and porosity, not better coverage.

The table below shows approximate arc-on hours for common tank sizes at three realistic flow rates. Remember to plan a refill before you hit zero: performance falls off and you risk drawing contaminants once a cylinder drops below about 100 to 150 PSI, so treat the last bit of the gauge as reserve, not usable gas.

Matching Size to Your Actual Usage and Portability

The single most common mistake buyers make is going too small. A cylinder that seems generous on paper disappears fast once you factor in purge starts, tacking, and the fact that you cannot use the last 100 PSI. As a rule of thumb, buy the largest cylinder you can reasonably store, secure, and transport, because the gas itself is a small fraction of the cost and larger fills are dramatically cheaper per cubic foot.

Portability is the real constraint. An MC or R bottle rides in a truck door or a roll-cage kit and is ideal for mobile repair, fence work, and site tacking, but you will refill it constantly. An 80 cf (#3) is the sweet spot for a hobbyist who wants a tank they can still carry up basement stairs and lift into a vehicle. A 125 cf (#4) suits a serious home shop or part-time fabricator. Once you are welding most days, the 330 cf T/K pays for itself and cuts refill trips to a handful per year, but it needs a cart and a chain and realistically stays put.

Think about the regulator and cart too. A high-flow inert-gas flowmeter regulator with a CGA-580 nut runs about 60 to 150 dollars; a CO2 regulator with CGA-320 is similar. Budget for a bottle cart (40 to 120 dollars) or a welder-mounted rack, and always secure cylinders upright with a chain or strap per OSHA 1910.253(b)(2), whether stored, transported, or in use. Never lay a full high-pressure cylinder on its side to weld from it, and keep the valve cap on any cylinder that is being moved or stored.

• Mobile / field repair: MC (10 cf) or R (20 cf) aluminum for weight; accept frequent refills as the price of true portability.
• Hobbyist / occasional: 80 cf (#3) balances runtime and one-person handling; upgrade to 125 cf (#4) if you weld weekly.
• Part-time fabricator: 125 cf (#4) or a 330 cf T if you have a cart; far cheaper per cf than swapping small bottles.
• Full-time production shop: 330 cf T/K, or step up to bulk liquid/manifold systems once you run multiple booths daily.

Own vs Lease vs Exchange: The Economics

There are three ways to get gas from suppliers across the US, and the right one depends entirely on how much you weld. In a cylinder exchange (the model at most welding-supply retailers and big-box stores), you swap your empty for a full one and pay only for the gas fill. You never own a specific bottle, so you never pay for hydro testing, and you are not stuck with a cylinder a distributor refuses to fill. This is the best model for hobbyists and light users.

Leasing (or renting) means the distributor owns the cylinder and charges an annual fee, typically 60 to 150 dollars per bottle per year, plus the gas each time you refill. Leasing makes sense for larger T/K tanks and specialty gas mixes because you always have a certified, in-test cylinder and the gas company handles maintenance, but those annual fees add up fast if the bottle sits idle. Read the lease: some contracts have demurrage or minimum-usage clauses.

Owning your cylinder means you buy the physical tank up front (roughly 150 to 250 dollars for an 80 cf, 250 to 400 for a 125 cf, and 300 to 500-plus for a full 330 cf, steel), then pay only for refills for life. Owning wins for steady long-term users, but confirm before you buy that local distributors will refill customer-owned bottles; some only exchange, and a customer-owned tank must still pass hydro to be filled. The break-even against leasing on a T-size tank is often two to three years, so if you will weld for years, ownership is usually the cheapest path per cubic foot.

On refill pricing itself, expect rough ranges of 15 to 30 dollars for an 80 cf argon/C25 fill, 30 to 55 for a 125 cf, and 45 to 90 for a full 330 cf, with 100% argon and tri-mix costing more than C25, and pure CO2 the cheapest per cubic foot. Prices vary by region and by whether you exchange or wait for a fill of your own bottle. Always compare price per cubic foot, not price per fill, and buy the biggest cylinder your situation allows to drive that number down.

• Exchange: Pay only for gas, no test costs, no ownership; ideal for light and intermittent users. Downside: you get whatever bottle is on the shelf.
• Lease / rent: Annual fee (~60-150 dollars/bottle) plus gas; distributor maintains and certifies. Good for specialty mixes and large tanks with steady use.
• Own: Buy the tank (150-500-plus dollars), then refills for life. Cheapest long-term, but confirm refill acceptance and hydro status first.

Safety, Standards, and Handling You Cannot Skip

Compressed-gas cylinders store enormous energy and are governed by real, enforceable rules. OSHA 29 CFR 1910.101 and 1910.253 cover storage, handling, and use; NFPA 55 covers compressed-gas storage in facilities; and DOT regulations govern transport on public roads. The practical takeaways are consistent: keep cylinders upright, secure them with chain or strap so they cannot fall, keep the protective valve cap on when the regulator is off, and store oxygen at least 20 feet from fuel gases like acetylene or separate them with a 5-foot, half-hour fire-rated barrier.

Acetylene deserves special mention because it is not just another cylinder. Acetylene is dissolved in acetone inside a porous mass and must always be used upright, drawn at no more than 1/7 of the cylinder capacity per hour, and kept below 15 PSIG working pressure per its own safe-handling limits. Standing a laid-down acetylene cylinder upright for at least 30 minutes before use is not optional; using it too soon can pull liquid acetone into the line.

For shielding-gas work, match the CGA valve outlet to the gas, never adapt or force fittings, open cylinder valves slowly to avoid regulator shock, and 'crack' the valve briefly before attaching a regulator to blow out debris (except on oxygen near oil). Leak-check connections with approved leak-detector solution, never a flame. When a cylinder reaches its retest date, a reputable distributor will pull it from service; do not attempt to keep an out-of-test or visibly damaged cylinder in use.

Frequently Asked Questions