Window Header Size Calculator Formula

Understand the math behind the window header size calculator. Each variable explained with a worked example.

Use the Window Header Size Calculator

Formulas Used

Minimum Header Depth

header_depth = load_bearing == 0 ? 4 : (span_width <= 4 ? 6 : (span_width <= 6 ? 8 : (span_width <= 8 ? 10 : 12))) + (floors_supported - 1) * 2

Recommended Header Size

header_desc = load_bearing == 0 ? 4 : (span_width <= 4 ? 6 : (span_width <= 6 ? 8 : (span_width <= 8 ? 10 : 12))) + (floors_supported - 1) * 2

Jack Studs per Side

jack_studs = span_width <= 4 ? 1 : (span_width <= 8 ? 2 : 3)

Variables

VariableDescriptionDefault
span_widthOpening Span(feet)4
floors_supportedFloors Supported Above1
load_bearingLoad-Bearing Wall?1
base_depthDerived value= load_bearing == 0 ? 4 : (span_width <= 4 ? 6 : (span_width <= 6 ? 8 : (span_width <= 8 ? 10 : 12)))calculated

How It Works

How to Size a Window or Door Header

General Rules of Thumb

For load-bearing walls, header depth increases with span:

  • Up to 4 ft span: 2x6 header (or doubled)
  • 4-6 ft span: 2x8 header
  • 6-8 ft span: 2x10 header
  • 8+ ft span: 2x12 header or engineered lumber (LVL)

    • Add 2 inches of depth for each additional floor supported above the opening. Non-bearing walls typically need only a flat 2x4 header for plate support.

    Important Note

    This calculator provides general guidelines. Always consult local building codes and an engineer for final header sizing, especially for spans over 8 feet or multi-story loads.

    Worked Example

    A 4-foot window opening in a load-bearing wall supporting one floor above.

    span_width = 4floors_supported = 1load_bearing = 1
    1. 01Span = 4 ft, load-bearing wall, 1 floor above
    2. 02Base header depth for 4 ft span = 6 inches
    3. 03Additional depth = (1 - 1) x 2 = 0 inches
    4. 04Header size = 2x6 (doubled) or equivalent LVL
    5. 05Jack studs needed = 1 per side

    Ready to run the numbers?

    Open Window Header Size Calculator