Framing Calculators
Free span calculators for framing a house, an addition or a deck: what size rafters, headers, floor joists, beams and deck framing you need, checked to the 2018 National Design Specification (NDS) for wood. No account needed.
A house, from the roof down
Each piece holds up the one above it, so work in that order. Every result has a button to the next calculator, with the spans and loads it already knows filled in.
Footings
How big a footing does each post need?
The footing size chart gives the size for a load and soil. The footing designer checks one in full, and the wood beam calculator opens it with the post's load.
When 2x lumber isn't enough
Try a flitch beam, a glulam or LVL beam in the beam designer, or a steel beam.
A deck
Size the whole deck at once, or just the joists.
Which calculator do I need?
| You want to know | Use |
|---|---|
| How far roof rafters can span, or what size they should be | Rafter span calculator |
| What size header goes over a door, window or garage door | Header span calculator |
| How far floor joists can span | Floor joist span calculator |
| What size beam carries the joists, or replaces a wall you're taking out | Wood beam span calculator |
| How big the footing under a post should be | Footing size chart |
| Everything for a deck: joists, beams, posts and footings | Deck framing calculator |
| Just the deck joists | Deck joist span calculator |
How the calculators work
- Each member is checked for bending, shear and sag with the NDS allowable stress method, using the wood's design values for its species, grade and size.
- Loads follow the American Society of Civil Engineers standard ASCE 7-16: dead load, floor live load, and roof live load or snow, combined the way the standard requires.
- Outdoor deck lumber is checked wet; indoor framing is checked dry.
- Each calculator shows its work: every check, how much of the member's capacity it uses, and a drawing of the member under its load.
Questions
What size lumber do I need for a 12 ft span?
It depends on what it holds up. Floor joists 16 in apart under a 40 psf floor need 2x8s for 12 ft and 2x12s for 16 ft (Douglas Fir-Larch No.2). Rafters and beams carry different loads; pick the calculator for the member.
Which calculator do I start with?
Start at the roof and work down, the way the load travels: rafters, then the headers in the walls under them, then the floor joists and the beams under those, then the footings. Each result has a button that opens the next calculator with what it already knows filled in.
Do these match the span tables in the building code?
They follow the same method as the International Residential Code (IRC) span tables: the 2018 National Design Specification (NDS) for wood with typical house loads. Spans come out close to the code tables but not identical, so check with your building department, which has the final say.
What wood species and grades are included?
Douglas Fir-Larch, Southern Pine, Hem-Fir and Spruce-Pine-Fir, in Select Structural, No.1 and No.2.
What if no 2x size works?
Move the members closer together, add a beam or wall to shorten the span, or use a stronger beam: a flitch beam (wood with a steel plate), a glulam or laminated veneer lumber (LVL) beam in the beam designer, or a steel beam.
Are the calculators free?
Yes. They need no account, and every result has a link you can copy to save or send it.