Welding Reference Library · Measure & Identify

Identifying Cylinder Sizes

How welding gas cylinders are sized and named, why size letters and nicknames differ between gas suppliers, and which markings to use to confirm a cylinder's size.

Ref WG-030649 CFR 178

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

High-pressure gas cylinders are built to DOT specifications in 49 CFR 178. The specification sets construction and service pressure, not a size name. Size letters (such as K, T, Q, S) and nicknames (such as "80", "125" or "300") are supplier conventions. The same letter can mean different cylinders at different suppliers, so a customer asking for "a K" or "a 125" may get a cylinder that does not fit their cart or exchange program.

How to identify

Read the DOT stamp on the shoulder: specification and service pressure (see the DOT markings sheet).
Look for a stamped or labeled water capacity or tare weight, if present.
Measure the overall height (without cap) and the outside diameter.
Read the supplier's label or neck ring for their size code.
Compare to that supplier's own size chart.

Ways cylinders are described

Description	Example	What it really tells you	Reliable across suppliers?
Size letter	K, T, Q, R, S	Supplier's code for a cylinder model	No
Nominal gas content	"80 cu ft", "125", "300"	Approximate gas volume when full, which varies by gas and fill pressure	Roughly, for the same gas
Height and diameter	55 in by 9 in	Physical size, fits carts and racks	Yes
Water capacity	Stamped or listed in liters or pounds of water	Internal volume	Yes
DOT spec and service pressure	DOT-3AA 2265	Construction and rated pressure	Yes, but not size
Acetylene size	MC (about 10 cu ft), B (about 40 cu ft), larger by cubic feet	Common fuel cylinder names	Mostly; MC and B are widely used

Key points

Gas content depends on the gas. The same cylinder holds a different volume of argon, CO2 or helium.
CO2 is sold by weight because it is a liquid in the cylinder.
Acetylene cylinders are different construction (porous filler with solvent). They are not interchangeable with high-pressure cylinders.
Owned cylinders need current requalification dates to be refilled. Exchange programs may not accept cylinders from another supplier.
Carts, racks and chains are sized for diameter. Confirm before quoting a cart.

Common mistakes

Quoting by size letter across suppliers.
Comparing prices for "a 125" without checking fill pressure.
Assuming a customer-owned cylinder can be swapped at any supplier.
Selling a cart before measuring the cylinder diameter.

Governing standards: 49 CFR 178; 49 CFR 180 (requalification).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

More in Measure & Identify

← All Welding sheets