A header (lintel) carries the load above a door or window opening over to the jack studs. For non-structural interior walls and light loads, framers use a quick rule: the span in inches can be up to twelve times the header’s nominal depth. Load-bearing walls must follow the header span tables in the building code (IRC R602.7) or an engineer’s design, which depend on building width, snow load and the number of floors supported.
How to use the Door Header Size Calculator
- Header size (nominal)
- Rough opening width
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose a header size to see its maximum span, and enter a rough opening to get the recommended header. The rough opening is the full width between the jack studs.
Rule-of-thumb spans
| Header (nominal) | Max rough opening |
|---|---|
| 4 × 4 | 48 in (121.9 cm) |
| 4 × 6 | 72 in (182.9 cm) |
| 4 × 8 | 96 in (243.8 cm) |
| 4 × 10 | 120 in (304.8 cm) |
| 4 × 12 | 144 in (365.8 cm) |
Important
This rule of thumb is for light, non-structural situations. For load-bearing walls use the IRC Table R602.7(1) header spans or an engineered beam.
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Header size (nominal) | Choose one: 4 × 4 (or doubled 2×4), 4 × 6 (or doubled 2×6), 4 × 8 (or doubled 2×8), 4 × 10 (or doubled 2×10) … |
| Rough opening width | Units: in, cm, mm, ft, m |
Formula
Maximum span (in) = nominal header depth (in) × 12Recommended depth = rough opening (in) ÷ 12, rounded up to a standard sizeWorked example
Inputs — Header size (nominal): 4 × 6 (or doubled 2×6); Rough opening width: 38 in.
Results — Maximum span for this header = 72 in; Recommended header depth (nominal) = 4 in; Selected header covers the opening (1 = yes) = 1.
Frequently asked questions
What size header for a 36 in door?
The rough opening is about 38 in, so a 4×4 (48 in) is enough for a non-bearing wall; bearing walls usually use doubled 2×6 to 2×10 per code tables.
What is a rough opening?
The framed opening for the door including space for the jamb and shims – typically the door width plus 2 in.
Does a non-load-bearing wall need a header?
Codes allow a flat 2× on edge or a single plate in non-bearing walls, but a header makes hanging the door easier.
What formula does the Door Header Size Calculator use?
Maximum span (in) = nominal header depth (in) × 12.