Class 3 and Class 4 are strength classes for the chair’s gas cylinder, defined in the European standard EN 16955 (which replaced the older DIN 4550). Supplier spec sheets certify Class 3 cylinders for users up to 120 kg (about 265 lb) and Class 4 for users up to 150 kg (about 330 lb). So for a heavier user, Class 4 is the better cylinder. It is still only one part of the chair: the number that tells you whether a chair suits you is the chair maker’s published weight rating.

What the classes mean

EN 16955 covers the tapered pressure tube of the self-supporting gas springs used to adjust seat height. It defines strength classes 2, 3 and 4. Each class is tied to an alternating bending load the tube must survive in fatigue testing and to a maximum seat-offset dimension, and Annex B of the standard is a guide to choosing a class from user weight and that offset. Class 4 covers users up to 150 kg. The standard also requires each tube to be marked with the maker, type, a reference to EN 16955 and the strength class, which is why you see “CLASS 4” stamped on many cylinders.

ACP International, a European chair components supplier, states the numbers plainly on its cylinder spec pages: its standard cylinders meet at least Class 3 (certified for up to 120 kg user weight) or Class 4 (certified for up to 150 kg user weight).

You will also see the class described as a wall-thickness rating. Chairpartsonline’s stamping guide says the class relates to piston wall thickness, with Class 4 the thickest. Published thickness figures vary from supplier to supplier, so use the class marking and the certified user weight rather than comparing millimeters.

BIFMA tests the whole chair, not the cylinder class

The U.S. office chair standards from BIFMA do not use cylinder classes. They test the complete chair. Micom, a furniture test lab, summarizes the difference: ANSI/BIFMA X5.1, the general-purpose office chair standard, is based on a 253 lb user, while ANSI/BIFMA X5.11, the large-occupant standard, is based on a 400 lb user and uses heavier loads in tests such as swivel cycling (400 lb on the seat vs 250 lb in X5.1).

That is the practical split for heavy users:

Question Where the answer comes from
Is the cylinder strong enough? Its EN 16955 class: Class 3 to 120 kg, Class 4 to 150 kg on supplier spec sheets
Is the chair rated for me? The chair maker’s published weight rating
Was the chair tested for larger users? A stated BIFMA X5.11 (large occupant) claim, or the maker’s own test statement
I weigh more than 150 kg (about 330 lb) Outside the classes; EN 16955 refers these cases to the gas spring maker. Choose a chair rated for your weight

Start with the chair’s weight rating

A Class 4 cylinder in a chair rated for 250 lb does not make it a 330 lb chair. The base, seat plate, tilt mechanism and casters all share the load, and the maker’s rating applies to the assembled chair. Read the rating for the exact model and configuration, and check what it covers. For more on reading those labels, see what 250- and 300-lb capacity labels tell you and StackAudit’s 300 lb vs 350 lb weight capacity comparison .

If a listing names a Class 4 cylinder but gives no chair rating, treat the chair rating as unknown, not as 150 kg.

Replacing a cylinder with a Class 4

When a cylinder fails, a Class 4 replacement is a reasonable choice for any user, and parts suppliers lean that way: chairpartsonline says all its cylinders are Class 4, and its sizing page points users over 250 lb to its heavy-duty versions. Class does not change the fit. These do:

  • Taper. Current standard cylinders carry the “TYP 17-4” top taper. Chairpartsonline lists 17-2 and 17-26 as obsolete tapers that require replacing the mechanism along with the cylinder.
  • Diameter. A 2 inch (50 mm) outer column means a standard cylinder; the piston rod is about 1.1 inches (28 mm).
  • Travel. Measure seat height fully down and fully up; the difference is the travel (stroke). Match it unless you want a different range.
  • Length. Compare the column length and extended length with the old cylinder, or measure from the underside of the mechanism to the underside of the base at both extremes.

Some Steelcase and Herman Miller chairs need brand-specific cylinders. For the removal and installation steps, see how to diagnose and fix desk chair gas lift problems .

The short answer

Choose Class 4 when you have the option, especially for a heavier user or a main work chair, because it is certified for more user weight. Then check the chair’s published weight rating, because that, not the cylinder stamp, is the limit. Above about 330 lb, look for a chair rated and tested for your weight.

FAQ

Is Class 4 always the better pick for heavy users?

For the cylinder itself, yes: suppliers certify Class 4 cylinders for heavier users (150 kg, about 330 lb) than Class 3 (120 kg, about 265 lb). But the chair’s published weight rating is the limit that applies.

Does a Class 4 gas lift make the whole chair heavy-duty?

No. The base, mechanism, seat and casters carry the load too. Swapping in a Class 4 cylinder does not raise the chair’s documented weight rating.

Where do I find the class on my cylinder?

It is stamped on the cylinder tube. EN 16955 requires each tube to be marked with the maker, type, a reference to the standard and the strength class.

What if I weigh more than 330 pounds?

That is beyond the Class 4 certification suppliers publish, and EN 16955 points heavier applications to the gas spring maker. Choose a chair whose maker rates it for your weight, such as one tested to BIFMA X5.11.

Sources

Sources checked October 3, 2026.