Office chairs

Which Armrest Adjustments Matter on an Office Chair (2D, 3D, 4D, Flip-Up), and Can You Upgrade Them?

Published · By Ergocrew · Specs and guidance as of that date, from the pages under Sources

Summary

Start with height and width: OSHA asks for armrests low enough for "relaxed shoulders" and close enough to support the lower arms. Forward-back travel and swivel can then help place the pads where you use them. Flip-up arms are useful when the desk edge blocks the chair. Compare the named movements and their ranges rather than the D count, and buy a replacement set only after confirming your exact chair's compatibility.

Where guidance puts the arms and elbows

OSHA's chair guidance asks for armrests that are adjustable in position so they support the lower arm. In OSHA's terms, they should be wide enough for getting in and out of the chair, close enough to support the lower arms, and low enough for relaxed shoulders. Its page on working postures gives an elbow angle between 90 and 120 degrees. OSHA also says "Armrests should be made of a soft material and have rounded edges."

CCOHS, Canada's occupational health agency, puts the elbows at about 90 degrees in its chair-adjustment guidance. It also notes that armrests extending 10 inches (25 cm) or less from the back of the seat may be appropriate.

An armrest's minimum height matters as much as its upward travel. If it remains too high, a pivoting pad does not lower it to your forearm. Our guide to adjusting a chair covers where armrest height fits in the overall setup.

What 2D, 3D and 4D mean on a listing

The number in front of "D" is the maker's own count of directions. One maker's 4D is not necessarily another's. These are the four movements that usually lie behind the labels:

The listings show how labels vary. Sihoo describes the M18's arms as height-adjustable and the M59AS's as 3D flip-up, with up-down, left-right and forward-back movement. ProtoArc lists the EC200 Pro's 4D ranges as 2.76" vertically, 1.97" forward-back, 0.8" laterally and up to 30° swivel. Sihoo calls the C300 Pro V2's arms 8D and the B300 Pro's 6D. Compare those makers' named movements rather than assuming that each D count measures the same thing.

Flip-up armrests and desk clearance

A fixed armrest can hit the desk edge before the seat is close enough to type. Flip-up arms rotate out of the way, so the chair can move in further. For the M59AS, Sihoo says the "flip-up armrests tuck under the desk, saving space." Sihoo lists the B100 with "2D flip-up armrests" and the B100 Pro with "4D flip-up armrests" that "flip up out of the way."

Specs below are as each maker lists them (read 2026-10-23). Prices are at each maker's US store, checked 2026-10-13.

ModelArmrest type as the maker states itFlip-upPrice at the maker's US store, checked 2026-10-13
Sihoo M18Height-adjustableConfirm with Sihoofrom $169.99
Sihoo B1002D flip-upYesfrom $199.99
Sihoo M59AS3D flip-up (up-down, left-right, forward-back)Yes$239.99
Sihoo B100 Pro4D flip-upYesfrom $259.99
Sihoo B300 Pro6D, including flip upYesfrom $259.99

On these entry prices, moving from the B100 to the B100 Pro costs $259.99 - $199.99 = $60. That gap covers more than the arms, because Sihoo lists a 2D headrest on the B100 and a 3D-adjustable headrest on the B100 Pro. The M59AS's 3D flip-up arms come at $239.99 - $199.99 = $40 more than the B100's starting option. ProtoArc's EC200 Pro, with the 4D arms described above, was $249.99 at ProtoArc's US store, checked 2026-10-13.

If desk clearance is the problem, compare the Sihoo B100's flip-up arms at a starting price of $199.99 with the M59AS's forward-back and side-to-side adjustments at $239.99, both at Sihoo's US store, checked 2026-10-13. Measure the space beneath your desk, then confirm the armrest positions and selected option with Sihoo.

Replacing or upgrading armrests

A replacement armrest needs to match the chair's mounting points, not just its D label. ProtoArc's 4D Armrest Set was $49.99 at its US store, checked 2026-11-01, and lists height, depth, width and angle adjustment. The page title and main compatibility notice name the EC200 and EC300, while the dimensions and lower compatibility notice name only the EC200. If you own an EC300, ask ProtoArc to confirm the exact version and kit before ordering; do not assume the separately named EC200 Pro is included either. Also confirm the kit's return and warranty terms: its product badges and the general policy for replacement parts differ.

Before budgeting for an upgrade, give the maker your chair's exact model and ask for the compatible part number. A set made for another chair may not match its bolt spacing or arm length.

What to check before buying

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Related guides

Sources

  1. ProtoArc, Warranty Policy
  2. OSHA Computer Workstations eTool, Chairs
  3. OSHA Computer Workstations eTool, Neutral Working Postures
  4. CCOHS, Office Ergonomics - Adjusting the Chair
  5. Sihoo, M18 Ergonomic Office Chair
  6. Sihoo, B100 Ergonomic Office Chair
  7. Sihoo, B100 Pro Ergonomic Office Chair
  8. Sihoo, M59AS Ergonomic Office Chair
  9. Sihoo, B300 Pro Ergonomic Office Chair
  10. Sihoo, Doro C300 Pro V2
  11. ProtoArc, EC200 Pro Ergonomic Office Chair
  12. ProtoArc, 4D Armrest Set for EC200 Ergonomic Chair

We have not tested these products. Specs and guidance are as published on these pages on the date above and can change; the manufacturer's page for your model has the current details.

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