Door Header Size Calculator

What Is a Door Header Size Calculator?

A door header size calculator helps you pick the right lumber beam above a door or window opening. Enter the rough opening width and get the recommended nominal header size, or choose a header size and see the maximum span it can support. Proper header sizing keeps wall framing safe and code-compliant.

Door Header Size Chart

The standard rule ties header depth to maximum rough opening: multiply the lumber depth in inches by 12 to get the max span in inches. A 4-inch depth supports up to 48 inches, a 6-inch depth up to 72 inches, and so on.

$$ L = d \times 12 $$

Where $L$ is the maximum rough opening in inches and $d$ is the nominal header depth in inches. For related framing work, see our Angle Cut Calculator and Drywall Calculator.

Frequently Asked Questions

What header size do I need for a 3-foot door?

A 36-inch rough opening needs a 4" × 4" header. Divide 36 by 12 to get 3, round up to the nearest even value (4), and pair it with standard 4-inch wall framing thickness.

How far can a 4x6 header span?

A 4" × 6" header spans up to 72 inches (6 feet). The formula is 6 inches × 12 = 72 inches maximum rough opening.

Can I use a 2x8 as a door header?

Yes, but you must build up a 4" × 8" beam by laminating two 2×8 boards with a half-inch spacer between them to reach the required 3.5-inch actual thickness.

What is rough opening vs door size?

Rough opening is the framed gap between studs that holds the entire door assembly including jambs. It is wider than the door slab itself, typically 2 to 3 inches wider than the door width.

How long should I cut the header board?

Cut the header to the rough opening width plus about 3 inches to sit on jack studs at each end. For a 48-inch opening, cut the header around 51 inches.

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What Is a Door Header Size Calculator?

Door Header Size Chart

Frequently Asked Questions

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