Standard Door Thickness: Interior vs. Exterior, and Why It Matters for Hardware

Door thickness is one of those measurements that gets ignored until it causes a problem. The wrong thickness means the wrong hardware — and discovering that after the door is hung and the lockset is already ordered is an expensive lesson. Here is what you need to know before you specify anything.

The two standard residential thicknesses

In North American residential construction, virtually every door falls into one of two thickness categories:

• 1-3/8" (35 mm) — the standard for interior doors in most homes. Hollow-core and solid-core interior slabs are almost universally manufactured at this thickness.
• 1-3/4" (44 mm) — the standard for exterior doors. Also used for interior doors in higher-end custom homes where solidity and acoustic performance matter.

If your door does not match these two measurements, you have a non-standard situation — usually an older home with custom-milled doors, or a European import. In either case, confirm the exact measurement before ordering any hardware.

Why thickness affects hardware selection

Most lever sets, knob sets, and deadbolts are designed with a specific door thickness range in mind. The spindle — the square shaft that connects the two halves of the lever — is cut to a length that fits within a standard thickness range. If your door is thicker than the hardware expects, the two halves will not pull tight against the door face. If it is thinner, the spindle will protrude or the rose will not seat correctly.

Most quality hardware manufacturers — including Emtek and Colombo Design — specify a "door thickness range" in their product documentation. Typical ranges are:

• 1-3/8" to 1-3/4" for standard lever and knob sets
• 1-3/4" to 2-1/4" for thick-door kits (available as accessories)
• 2" to 2-1/2" for some commercial-grade sets

If your door falls outside the standard range, ask about thick-door extension kits. They exist for most hardware lines and simply swap out the spindle for a longer one.

Hollow-core vs. solid-core at 1-3/8"

Both hollow-core and solid-core interior doors can be 1-3/8" thick — thickness and core construction are independent variables. Hollow-core doors are lighter and cheaper; solid-core doors are heavier, quieter, and feel substantially better when you open or close them. For any room where sound matters — a home office, a bedroom, a media room — solid-core is worth the upgrade. The hardware specification is identical regardless of core.

Exterior doors at 1-3/4"

The 1-3/4" standard for exterior doors is nearly universal. It reflects the structural and thermal requirements of an exterior opening: the door needs to be rigid enough to resist wind load and strong enough to support a multi-point locking system or a reinforced deadbolt. If you are specifying a deadbolt for an exterior door, confirm it is rated for 1-3/4" — most are, but always verify.

European doors and metric sizing

If you are working with European-origin doors — common in higher-end custom builds — the thickness may be metric: 40 mm, 45 mm, or 55 mm. These are close to but not identical to the North American standards. A 40 mm door is slightly thicker than 1-3/8" (34.9 mm). A 45 mm door is slightly thicker than 1-3/4" (44.5 mm). Most hardware will still fit, but confirm the spindle length before ordering.

How to measure

Use a tape measure or calipers. Open the door and measure the thickness at the edge — the side where the latch sits — at the midpoint of the door. This gives you the actual thickness of the door slab, not including any weatherstripping or edge banding. Write it down before you call your hardware supplier.

Quick reference

• Standard interior door: 1-3/8"
• Standard exterior door: 1-3/4"
• Heavy interior or premium custom: 1-3/4"
• Non-standard: measure first, ask about extension kits

Not sure what you have? Bring your measurements to our Oakville showroom or contact us — we have sized hardware for doors from 1-3/8" to over 2-1/2" thick.