How to use this calculator
- Choose the member size. Pick a common dressed 2x size or enter actual custom width and depth.
- Enter spacing and loads. Use on-center spacing plus live and dead area loads in kPa or psf.
- Enter design values. Replace the example E, Fb and Fv values with adjusted values for the actual species, grade, size and design basis.
- Set deflection limits. Use limits such as L/360 for live-load deflection and L/240 for total-load deflection when appropriate.
- Read the governing span. Compare bending, shear, live-deflection and total-deflection spans, then review utilization at the entered check span.
How it works
This wood beam span calculator screens a single sawn rectangular member as a simply supported beam with uniform load. It starts by converting the live and dead area loads into line loads from the member spacing:
w_L = q_L x spacing
w = (q_L + q_D) x spacing
For the selected actual width b and depth d, the
rectangular section properties are:
I = b x d^3 / 12
S = b x d^2 / 6
The simple-span uniform-load checks use:
M = w x L^2 / 8
V = w x L / 2
fb = M / S
fv = 1.5 x V / (b x d)
delta = 5 x w x L^4 / (384 x E x I)
The calculator back-solves a span for bending, shear, live-load deflection and total-load deflection, then reports the shortest one as the allowable span for the entered assumptions.
Worked example
Verified against the live calculator
A 2x10 member at 16 in on-center carries
40 psf live load plus 10 psf dead load. With example
design values E = 1.2 Mpsi, Fb = 1000 psi and
Fv = 135 psi, the total line load is 66.7 lb/ft.
The calculator compares the bending, shear, live-deflection and total-deflection
span limits and reports the shortest governing value. At a 12 ft
check span it also reports bending, shear and deflection utilization so you can
see which limit is closest.
Frequently asked questions
How do you calculate wood beam span?
For this screening method, area load is multiplied by member spacing to get uniform line load. The calculator back-solves span from bending stress, shear stress, live-load deflection and total-load deflection, then reports the shortest governing span.
Is this the same as an AWC or code span table?
No. Span tables include code assumptions, species/grade design values, adjustment factors and prescriptive limits. This calculator uses the E, Fb and Fv values you enter, so it is a transparent engineering screen rather than an official span-table selector.
What values should I enter for E, Fb and Fv?
Use adjusted design values from the same code/design edition for the actual species, grade, size, service condition, duration, temperature and other project factors. Do not mix design values and provisions from different editions.
Does spacing affect allowable span?
Yes. The member carries a tributary width equal to its spacing, so 24 in on-center spacing produces 50% more line load than 16 in on-center for the same psf load.
Does this work for rafters?
It can screen a simple uniformly loaded sloped member using the projected area load basis you enter, but it does not handle roof-specific code load combinations, uplift, ties, bracing, birdsmouth limits or snow drift. Use the rafter calculator for geometry only.
Does it check point loads or built-up beams?
No. This page is for a single rectangular sawn member under uniform load. Use the LVL beam calculator or a project-specific beam design for built-up beams, point loads, multiple spans and connection schedules.
Method & assumptions
- Simple-span, single rectangular sawn member with uniform load only.
- Common 2x sizes use dressed North American actual dimensions; custom mode uses the actual dimensions you enter.
- Area loads are multiplied by member spacing to create uniform line load. Verify the tributary width for beams, headers and nonparallel load paths.
- Defaults for E, Fb and Fv are examples only. Use adjusted values from your adopted design standard, species, grade and project conditions.
- Does not include NDS adjustment factors, repetitive-member effects, bearing compression, notches, holes, lateral restraint, vibration, fire, treatment, wet service, connections, snow drift, wind uplift, seismic load combinations or local code review.
- For roof geometry, use the rafter calculator. For engineered lumber, use the LVL beam calculator. For lumber quantity, use the board feet calculator.