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Door Header Size Calculator

Table of contents

How to properly size door frame headersHow to use the door header size chartHow to use this door header size calculatorFAQs

This door header size calculator can help you determine the correct size of wood beam to use as a header for your door (or window – the same rules apply). In this calculator, you'll learn about:

  • How to properly size door frame headers;
  • How to use the door header size chart; and
  • How to use this door header size calculator.

Keep on reading to start learning.

💡 If you need to analyze an actual load-bearing wood beam, don't forget to try our wood beam span calculator.

How to properly size door frame headers

Choosing the correct door header size for any standard door opening is easy and straightforward. However, we need to ensure we use adequately sized door headers to support the potential beam load above it (whether from a transom window or just a transom wall).

We can always consider door frame headers as simply supported beams. That means we can calculate header sizes using the formulas we need to calculate beam sizes.

However, we already have a straightforward calculating procedure to quickly determine the maximum "rough opening" a piece of lumber can span when using it as a door header. That is by multiplying the lumber's height (in inches) by 12. In equation form, we express that as:

L=d×12L = d\times 12


  • LL – Maximum rough opening or header span in inches; and
  • dd – Nominal height or depth of the lumber in inches.

🙋 The "rough opening" of a doorway is equivalent to the total width of the door, including its door frame or jamb. It is also the width of the space between two studs that will house the entire door and door frame assembly.

In the next section of this text, we'll show you a much easier way to determine the door header size by using the door header size chart.

How to use the door header size chart

Although calculating the maximum rough opening a door header can span is very easy, here is the door header size chart we can use for convenience:

Maximum rough opening

Header size (nominal size)

48" (121.92 cm)

4" × 4"

72" (182.88 cm)

4" × 6"

96" (243.84 cm)

4" × 8"

120" (304.80 cm)

4" × 10"

144" (365.76 cm)

4" × 12"

Let's say you need to find the recommended door header size for a rough opening of 3 feet or 36 inches. According to our door header size chart, we'll have to use a piece of lumber with a nominal size of 4" × 4" for that opening.

💡 If you're still in the process of building your wall and need to determine your framing materials, you can perhaps use our framing calculator to help you with that. After that, our drywall calculator will also come in handy when determining the number of drywall materials you'll need.

If, on the other hand, you have pieces of lumber lying around your construction site and you want to know if you can use them as door frame headers, you can also use the door frame header size chart.

We can see that a 4" × 6" lumber's maximum rough opening is only 72" (or 6 feet). That means we cannot use it for rough openings greater than that. But we can use it even for spans shorter than that (including spans less than 48 inches).

The table is easy to remember. Keep in mind that a piece of 4-inch depth lumber is only good up to 4 feet, a 6-inch depth lumber to 6 feet, and so on. You get the point.

But what if you're using different units of measure, like millimeters or yards? Worry no more, as our door header size calculator covers that for you! 🙂

How to use this door header size calculator

Using our door header size calculator is like going through the door header size chart, but you can switch to any unit of measurement anytime you want.

Say you want to know the maximum rough opening or maximum header span for a particular header size. All you have to do is:

  1. Select that you want to find the maximum header span; and then
  2. Choose the header size you want to check.

You'll then see the maximum header span that you can convert to other units you want.

On the other hand, if you need help in choosing a header size:

  1. Select that you want to find the recommended header size; and then
  2. Enter the rough opening of your doorway.

Our tool will show you the recommended header size appropriate for your doorway. 🙂


How far can a 4x6 header span?

A 4" × 6" piece of lumber can span up to 72 inches if you want to use it as a door header. We get that value using the typical door header span chart or by multiplying the header's height (in this case, 6 inches) by 12, to get 6 inches × 12 =72 inches.

How do I calculate header size?

To calculate header size:

  1. Determine the measurement of your doorway's rough opening. Say 50 inches.

  2. Divide that measurement by 12:
    50 inches / 12 =4.1667 inches.

  3. Round up to the nearest even whole value to find the door header height.
    4.1667 inches →6 inches.

  4. Combine this value with the wall frame material thickness, typically 4 inches, to get a lumber size of 4" × 6".

Can I use a 2x8 as a door header?

Yes, you can use 2" × 8" lumber as a door header, but you have to build up a 4" × 8" beam by gluing together two pieces of it on their wider side with a half-inch spacer between them.

A 2-inch piece of lumber has an actual thickness of 1.5 inches. Therefore, twice its thickness plus half an inch results in 1.5" × 2 + 0.5" =3.5", the actual thickness of a typical door header.

What size of header do I need for a 4-foot door?

You need a 4" × 4" door header for a 4-foot door. For the length of your door header, It would be best to cut your 4" × 4" lumber to the size of 4 feet or 48 inches plus 3 inches to accommodate the thickness of the jack studs that will support your door header.

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