A post and beam span calculator estimates the maximum safe span for a beam based on its lumber size, species, load type, and tributary width. For engineered or load-bearing applications, always consult a structural engineer.
Beam Specifications
Total floor/roof area width the beam supports (both sides combined)
Span Results
Enter beam specifications to calculate span.
These are estimates for preliminary planning only. All load-bearing structural beams must be verified by a licensed structural engineer before construction.
How to Use the Post and Beam Span Calculator
Beam sizing is one of the most critical structural decisions in any construction project. Under-sized beams deflect excessively, crack finishes, and can fail catastrophically under load.
Understanding Tributary Width
Tributary width is the total width of floor or roof area that a beam supports. If joists frame into a beam from both sides, add the half-span from each side. A beam with 4-foot joists on the left and 6-foot joists on the right has a tributary width of 10 feet total. More tributary width means more load, which means you need a stronger beam or shorter span.
Species Matters Significantly
Douglas Fir-Larch has about 25% higher bending strength than Hem-Fir and 15% more than Spruce-Pine-Fir. For the same beam size and span, DF-L carries significantly more load. If your lumber yard only carries SPF, you may need to upsize the beam to achieve the same span.
Built-Up Beams
Three 2×10s nailed together create a beam approximately equivalent to a solid 6×10 but with better moisture resistance and easier installation. Built-up beams must be fastened with at least two rows of 16d nails at 12-inch spacing, or per manufacturer spec for engineered fasteners.
Frequently Asked Questions
Is this beam span calculator free?
Yes, completely free with no signup required. Values are based on standard span tables for common residential lumber species and grades.
How do I know if my beam is strong enough?
A beam fails in two ways: excessive deflection (sag) and stress failure. Codes limit deflection to L/360 for floors and L/240 for roofs, where L is the span. A 12-foot beam can deflect at most 0.4 inches for a floor (12×12/360 = 0.4). Always check with a structural engineer for any load-bearing applications.
What is the difference between a built-up beam and a solid beam?
A built-up beam consists of multiple 2x lumber nailed or bolted together — like three 2×10s nailed to form a 6×10 equivalent. Solid beams are cut from a single timber. Built-up beams are often stronger than solid beams of the same dimension because they're made from multiple pieces and are more dimensionally stable. They're also easier to obtain and handle.
Does beam span depend on what's above it?
Yes, significantly. Tributary width (the area of floor or roof the beam must support) is the key load variable. A beam supporting a 6-foot tributary width on each side carries twice the load of one with a 3-foot tributary on each side. The calculator accounts for this with the tributary width input.
Which wood species makes the strongest beams?
Douglas Fir-Larch and Southern Yellow Pine (SYP) are the strongest and stiffest common framing species — ideal for beams. Hem-Fir and Spruce-Pine-Fir (SPF) are weaker. LVL (Laminated Veneer Lumber) and glulam beams are engineered products with higher and more consistent strength ratings than solid-sawn lumber.