Timber Post CalculatorTimber Post & Column Size Calculator
Size a timber post for a vertical load. Checks compression and buckling across standard post sizes in a clear pass/fail table, accounting for height, end fixity and timber grade.
Why this calculation matters
Column capacity decides section size and whether a mid-height restraint is needed. Slender columns buckle long before the timber crushes.
Timber Post Calculator
Timber Post & Column Size Calculator
Vertical compression on post. 1 kN ~ 102 kg.
| Post Size | Capacity | Slenderness | Result |
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Where these numbers come from. Stiffness (E) is the mean modulus of elasticity parallel to grain, E0,mean, from EN 338:2016 Table 1. The permissible grade stresses are BS 5268-2 values. Those two are compatible for this purpose: stiffness does not carry the permissible-stress reduction that strength does, and BS 5268 and EN 338 agree on E to within about 2%. This sizes a member; it is not a design check. For anything you are building, have the section verified against EN 1995 (Eurocode 5) by someone who will sign it off.
About Timber Post Calculator
A timber post under vertical load can fail two ways: by crushing (compression) or by buckling sideways if it is too slender. This calculator checks a range of standard square post sizes against your load, height and end fixity, showing in a pass/fail table which sizes are safe and highlighting the smallest economical post. It combines compression capacity with Euler buckling theory. It answers the practical question of wood column load capacity, using an Euler buckling calculator approach with the slenderness check built in.
Where Is This Used?
Formulas Used
Effective length Le = K x Height (K by end condition)Slenderness = Le / radius of gyration (r = sqrt(I/A))Compression (squash) capacity = fc x AreaEuler buckling load Pcr = pi squared x E x I / Le squaredSafe if capacity >= load AND slenderness <= 180How the calculation works
A short post fails by crushing; a long one fails by bowing sideways long before the timber is crushed. The calculation works out both and takes whichever is smaller. Because buckling depends on the square of the unrestrained length, adding a restraint at mid-height usually does more than moving up a section size.
Show the formula
Area A = b^2. Second moment I = b^4 / 12. Radius of gyration r = sqrt(I/A). Effective length Le = height x K, where K is 1.0 pinned, 0.85 fixed-pinned, 0.7 fixed-fixed, 2.0 cantilever. Slenderness = Le / r. Euler critical load Pcr = pi^2 E I / Le^2. Capacity = lesser of (allowable compression x A x reduction) and 0.6 Pcr, where the reduction is 2500/slenderness^2 above a slenderness of 50.E values are EN 338 mean moduli. The 0.6 factor on the Euler load and the 2500/slenderness^2 reduction are conservative simplifications, not the kc buckling procedure of EN 1995-1-1. Square sections, axial load only, slenderness capped at 180.
References and what each one provides
- EN 338CEN (European Committee for Standardization)Structural timber strength classes — characteristic bending strength, mean modulus of elasticity and density for C and D classes.
- EN 1995-1-1 (Eurocode 5)CENDesign of timber structures: fastener spacing and edge distances, connection capacity, and deflection limits.
- IS 883Bureau of Indian StandardsIndian code of practice for design of structural timber, including permissible stresses, joint design and deflection limits.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Preliminary buckling check for an axially loaded column. It is not a structural design. It does not account for eccentric loading, connection design, foundation capacity, fire rating, or lateral restraint conditions other than those entered.
Do not use this for Structural design approval, temporary works design, or scaffold and propping certification.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
- EN 338 strength classes
- Euler buckling with effective-length factors
Formula checked against the sources above by an automated regression test (tests/test-calculators.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
- Enter the job details below, then press Calculate.
- Pick your end condition and strength grade from the dropdown.
- Fill in axial load, post height.
- Press Calculate.
Worked example
Common mistakes to avoid
- Ignoring buckling on tall slim posts. A post can crush-resist the load yet still buckle sideways.
- Using unbraced length equal to full height when mid-height bracing exists — or worse, the opposite.
- Assuming end fixity you do not have. A post loosely sitting in a shoe is not a fixed end.
Frequently Asked Questions
Wood species data
Density, hardness and movement for 60 timbers
Full listing in the strength properties reference.