OSHA does not set a single minimum manway dimension that applies to every confined space. The federal rules on manway size requirements are performance-based: the opening must be large enough for a worker to bodily enter and perform the assigned work, and large enough to allow safe rescue. The most concrete number OSHA provides sits in Appendix F to 29 CFR 1910.146, which treats any opening 24 inches or less in its smallest dimension as a “restricted” portal — too small for a rescuer in a self-contained breathing apparatus and too small for normal spinal immobilization of an injured worker.1Occupational Safety and Health Administration. Appendix F to 1910.146 – Rescue Team or Rescue Service Evaluation Criteria That 24-inch figure is the number most safety professionals treat as the practical minimum.
The 24-Inch Threshold and Where It Comes From
Appendix F is written to help employers evaluate rescue services, and it splits portals into two categories. A restricted portal is 24 inches or less in its least dimension. An unrestricted portal is greater than 24 inches in its least dimension and allows relatively free movement in and out.1Occupational Safety and Health Administration. Appendix F to 1910.146 – Rescue Team or Rescue Service Evaluation Criteria
Appendix F is labeled non-mandatory guidance. OSHA will not issue a citation solely because a portal measures less than 24 inches. But the classification still carries real weight. When OSHA evaluates whether a rescue plan is adequate, its own framework asks whether the rescue service can properly package and retrieve victims from a permit space with a limited-size opening of less than 24 inches in diameter.1Occupational Safety and Health Administration. Appendix F to 1910.146 – Rescue Team or Rescue Service Evaluation Criteria If the answer is no, the entry program has a gap OSHA can cite.
This is where many programs run into trouble. An employer might have a perfectly legal 20-inch manway on an existing tank. If the written rescue plan assumes the rescue team can enter in air packs and extract a worker on a backboard, the plan does not match the physical reality of the opening.
What the Regulation Actually Says
The general industry confined space rule at 29 CFR 1910.146 requires that a confined space be “large enough and so configured that an employee can bodily enter and perform assigned work.”2eCFR. 29 CFR 1910.146 – Permit-Required Confined Spaces The regulation stops there on dimensions. It leaves exact measurements to the employer, who must evaluate each space against the work being performed, the equipment workers will carry, and the rescue plan in place.
OSHA training materials use 18 inches as the point at which a worker can physically enter a space, and the construction confined space workbook uses the same figure as the threshold below which an opening is too small for bodily entry. Fitting through and being safely retrieved through are two different problems, though, which is why practical minimums run larger.
A space is “confined” under OSHA rules when it meets three criteria: it is large enough for a worker to enter and do work, it has limited or restricted means of entry and exit, and it was not designed for continuous occupancy. Tanks, silos, vaults, pits, and manholes are common examples. A confined space becomes permit-required when it contains or could contain a hazardous atmosphere, an engulfment material, an internal shape that could trap a worker, or any other recognized serious hazard.2eCFR. 29 CFR 1910.146 – Permit-Required Confined Spaces The permit-required designation is what drives most sizing decisions, because those spaces must accommodate rescue and retrieval equipment, not just routine entry.
What the Opening Has to Fit
Manway sizing is really a question of what has to pass through it. For a permit-required space, that list usually includes the worker, a chest or full-body harness, a retrieval line, and the equipment the worker will use inside. For rescue, add either a rescuer in protective equipment or a stretcher.
A worker wearing a self-contained breathing apparatus adds roughly 10 to 12 inches of bulk to their profile. That is why Appendix F flags any portal of 24 inches or less as too small for entry rescue by an SCBA-equipped rescuer.1Occupational Safety and Health Administration. Appendix F to 1910.146 – Rescue Team or Rescue Service Evaluation Criteria For a space with a hazardous atmosphere where SCBA is required for both entrants and rescuers, an opening below 24 inches effectively rules out entry rescue and forces a non-entry retrieval approach.
Confined space rescue stretchers built for tight-opening extraction typically run 18 to 19 inches wide. A 24-inch circular opening fits the stretcher, but only just, and only when oriented correctly. Openings much below 24 inches may force extraction on a harness alone, with no spinal immobilization.
For very narrow exits, NIOSH guidance recommends a wrist-type harness in place of a chest or full-body harness when the opening is less than 18 inches in diameter. OSHA’s own rules permit wristlets or anklets in place of a standard harness when the employer can show the harness is infeasible or creates a greater hazard.3eCFR. 29 CFR Part 1926 Subpart AA – Confined Spaces in Construction
Vertical entries deeper than five feet require a mechanical retrieval device, typically a winch mounted on a tripod over the opening.2eCFR. 29 CFR 1910.146 – Permit-Required Confined Spaces The retrieval line must run through the opening alongside the worker, which reduces the effective clear space.
The Consensus Number: ANSI/ASSP Z117.1
Because OSHA leaves the dimensions open, many employers and equipment designers work to ANSI/ASSP Z117.1. The current edition recommends that vertical confined space openings be at least 24 inches in diameter, or 24 by 24 inches for non-circular openings. It also requires that the opening accommodate the largest worker expected to enter, including any tools or equipment they will carry.
ANSI/ASSP Z117.1 is a voluntary consensus standard, not a regulation with the force of law. OSHA can, however, cite it as evidence of recognized industry practice when evaluating a confined space program under the general duty clause. Employers frequently adopt Z117.1 as the baseline for new equipment specifications and facility design because it supplies the concrete numbers the regulation deliberately omits.
Construction and Shipyard Work
The construction confined space standard at 29 CFR Part 1926 Subpart AA, effective in 2015, uses the same performance-based definition as general industry: bodily entry, limited means of entry or exit, not designed for continuous occupancy. It also does not prescribe a minimum opening dimension in inches. Its rescue and retrieval provisions mirror general industry: a chest or full-body harness with a retrieval line attached near shoulder level or above the head, and a mechanical device for vertical permit spaces deeper than five feet.3eCFR. 29 CFR Part 1926 Subpart AA – Confined Spaces in Construction
One requirement in the construction rule matters directly for opening size: rescue training must involve representative permit spaces that simulate the opening size, configuration, and accessibility of the actual spaces where rescue might be needed.3eCFR. 29 CFR Part 1926 Subpart AA – Confined Spaces in Construction If a site has 20-inch manways, the rescue team has to practice through 20-inch manways.
Shipyard employment falls under 29 CFR Part 1915, which covers ship repairing, shipbuilding, and shipbreaking.4eCFR. 29 CFR Part 1915 – Occupational Safety and Health Standards for Shipyard Employment The shipyard rule at 29 CFR 1915.76 requires more than one means of access to a confined space when work may generate a hazardous atmosphere, unless the vessel’s structure makes a second opening impractical. Where ventilation ducts pass through access openings, the ducts must be arranged so workers can still move freely through at least two of those openings.5eCFR. 29 CFR 1915.76 – Access to Cargo Spaces and Confined Spaces The shipyard standard itself does not specify minimum opening dimensions. Vessel manhole sizes are largely dictated by design codes such as ASME, which allows manholes as small as 15 inches in diameter or elliptical openings of roughly 11 by 15 inches for certain vessel types — dimensions set for inspection access rather than worker rescue.
What Happens If the Opening Is Too Small
Confined space violations consistently rank among OSHA’s most frequently cited standards. The manway itself is rarely cited in isolation; the citation usually comes through the rescue plan, the entry procedure, or the failure to protect entrants. As of the January 2025 adjustment, OSHA’s maximum penalties are up to $16,550 per serious or other-than-serious violation, up to $165,514 per willful or repeated violation, and up to $16,550 per day for failure to abate past the deadline.6Occupational Safety and Health Administration. OSHA Penalties These amounts adjust annually for inflation.
An entry through an opening that prevents effective retrieval can be cited as serious. If the employer knew the opening was inadequate and allowed entry anyway, a willful classification raises the ceiling roughly tenfold. Each exposed worker can be treated as a separate violation, so a single bad entry with a four-person crew multiplies quickly.6Occupational Safety and Health Administration. OSHA Penalties
The practical takeaway on sizing: measure the smallest dimension of every opening a worker will pass through, compare it to 24 inches, and match the rescue plan to what the opening actually allows. Where the opening is smaller than 24 inches, the plan has to account for it — with wrist retrieval, non-entry rescue, or engineering changes to the opening itself — rather than assume rescue equipment that will not fit.