Hollow Metal Door Sizes: Standard Dimensions, Undercut & Rough Openings

Door sizes look like the simplest part of a commercial order, and they’re the part people get wrong most often. The dimensions themselves aren’t complicated. The trouble is that a hollow metal door is described by three different sizes, and mixing them up is how you end up with a door that’s a quarter inch short of the opening, or a frame that won’t fit the wall. This guide walks the three sizes, the standard dimensions Imperial stocks for next-day shipping, and the two things behind almost every sizing mistake: undercut and rough opening.

Standard hollow metal door sizes

Commercial hollow metal doors come in a defined set of standard sizes. Width runs from 2’0” to 4’0”. Height is 6’8”, 7’0”, or 8’0”. Thickness is 1-3/4”, the commercial standard, and the only door thickness worth speccing for a commercial opening. Anything outside this grid is custom, which Imperial can fabricate. The sizes below are what ship fast, because they’re stocked.

Imperial stocks these as 18-gauge, polystyrene-core, flush doors with Steelcraft hinge and lock locations. That’s the workhorse commercial door and the stock program. Custom cores, gauges, and prep are available to order.

How to read a door size code (the “3070” system)

Hollow metal doors are ordered by a numeric code rather than by inches, and the code reads in feet and inches, which trips up first-time buyers. A “3070” is a 3’0” × 7’0” door. A “2868” is 2’8” × 6’8”. The first two digits give width in feet and inches, the last two give height. Imperial’s full part number wraps that size code inside the door’s spec:

Read the size code correctly and you’ll never order the wrong leaf. It also explains why 3068 and 3070 show up on nearly every job. A 3’0” width is the accessible-width workhorse, and those are the two most common heights.

Nominal size vs. actual size: the undercut that catches everyone

This is the one that generates support calls. The size you order is the nominal size. The door that ships is smaller, because it’s undercut to operate in the frame. A 3070 is nominally 36” × 84”, and it arrives at 35-3/4” × 83-1/8”.

That’s not a defect. Per Imperial’s spec, the door leaf is undercut 1/8” at the top and 3/4” at the bottom, so 7/8” comes off the height — the bottom gap is there to clear flooring and thresholds. Width runs 1/4” under nominal. In the frame, the leaf sits with roughly a 3/32” gap on the hinge edge and 1/8” on the strike edge.

Two rules follow from this:

Undercut: 1/4” total width, 7/8” height (1/8” top plus 3/4” bottom).

Rough opening is set by the frame, not the door

The most common bad advice online is a single rough-opening number per door size. There isn’t one. Rough opening depends on the frame type and the wall, because the frame sets the opening, not the leaf hanging in it.

Imperial’s standard frames have a 2” face, so on a masonry (butt) frame the opening should match the frame’s outside dimension plus a small shim and grout allowance. In practice that works out to roughly:

Rule of thumb only. Confirm against the frame submittal. Drywall and wrap frames differ.

On a drywall (wrap or knock-down) frame, the frame slips over the stud partition, so the stud rough opening is sized to clear the frame throat and backbends. That’s a different number, and it shifts with your jamb depth (Imperial’s run 3-1/4” to 5-3/4”, codes 625 through 875). Because rough opening is frame-specific, confirm it against the frame’s submittal drawing before you form or frame the opening. Imperial’s catalog points to SDI-129 for current frame dimensions, so on rated or tight openings, work from the submittal rather than a generic chart.

Single doors vs. pairs

A pair (double door) is described by its total width. A “6070” pair is 6’0” × 7’0”, made of two 3’0” leaves in a paired frame with an astragal or mullion between them. Each leaf is a standard door at its own net size; the pair just uses two of them. On pairs, one leaf is usually the inactive leaf, fitted with a Z-astragal (Imperial stocks these as the IAZ variant, handed left or right). Ordering a pair also means making a handing decision on the inactive leaf, which is worth getting right the first time.

Stock sizes vs. custom

Everything in the size grid above ships from stock in Imperial’s standard 18-gauge flush build, which is the next-day program. Step outside the grid with an odd width, a height over 8’0”, or a special core or gauge, and it becomes a custom fabrication. Imperial builds those up to a 10’0” door and a 16’0” frame. On most jobs the smart move is to use stock sizes wherever the schedule allows and save custom for the openings that truly need it.

Bottom line

Three sizes, one rule for each. Nominal is what you order (the 3070 code). Actual is what ships, undercut about 1/4” in width and 7/8” in height. Rough opening is what you build, and the frame sets it, so confirm it on the submittal. Get those three straight and the door fits the first time.