Framing & structure
Wood post & column size calculator
Estimate the axial load a 4x4 to 8x8 wood post can carry with the NDS column stability factor, and the lightest size that works.
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How much load can a wood post carry?
A wood post size calculator estimates the axial load a post can carry and the lightest size that works. Enter the load, or the tributary area with dead and live loads, the unbraced height and the species and grade, and it returns the allowable load, utilisation and column stability factor for each common size from 4x4 to 8x8, or 90 to 150 mm square.
Height matters more than most people expect. A post fails by buckling long before the wood crushes, so the allowable stress drops quickly as the post gets taller and thinner. The calculator uses the NDS column stability factor CP, which combines crushing strength and buckling stiffness: a 10 ft Douglas Fir-Larch No. 2 4x4 keeps only about a quarter of its crushing strength, roughly 4,665 lb, while a 6x6 of the same height carries about 15,660 lb.
This is a preliminary sizing aid for concentric axial load on a single sawn post. It does not check bearing on the beam or footing, connections, post bases, bending from lateral or eccentric load, or code requirements such as minimum post sizes for decks. Use it to compare options, then confirm with the span tables or an engineer your building official accepts.
How to use the wood post & column size calculator
- Choose Imperial or Metric. Imperial is the default, and your choice carries across the free tools.
- Set Load to work from the Tributary area with Dead load and Live or snow load, or enter the Axial load directly. The tributary area is half the beam span on each side of the post times half the joist span it supports.
- Enter the Unbraced post height, usually from the top of the footing to the underside of the beam, and choose the Species and grade.
- Choose the Load duration factor CD for the governing load (1.0 for occupancy live load, 1.15 for snow), the Service condition, and whether the post is Incised pressure-treated lumber.
- Read the design load, the lightest listed size that works and its allowable load, utilisation, CP and slenderness, then compare every size in the table. Download the PDF or CSV for your notes.
Worked example
Imperial: A 10 ft Douglas Fir-Larch No.2 post carrying 80 ft² at 10 psf dead + 40 psf live (4,000 lbf) has le/d 34.3 and CP 0.245 as a 4x4, an allowable load of 4,665 lbf and 86% utilisation.
Metric: A 3 m C24 post carrying 7.5 m² at 0.5 kPa dead + 2.0 kPa live (18.75 kN) has le/d 33.3 and CP 0.334 as a 90 × 90 mm section, an allowable load of 25.0 kN and 75% utilisation.
Allowable load for 4x4, 4x6, 6x6 and 8x8 posts
These are the calculator's results for Douglas Fir-Larch No. 2 in dry service with CD = 1.0 and pinned ends. The 4x sections use dimension-lumber values (NDS Supplement Table 4A, Fc = 1,350 psi with a 1.15 or 1.1 size factor); 6x6 and larger use the posts and timbers values (Table 4D, Fc = 700 psi). Even with the lower timber stress, a 6x6 carries far more because it is much stiffer against buckling.
Treat these as comparisons, not ratings. Wet exposure, incising, a different species or grade, and the bearing and connection details can all reduce the load a real post can take.
| Post | 8 ft high | 10 ft high | 12 ft high |
|---|---|---|---|
| 4x4 | 6,960 | 4,665 | 3,311 |
| 4x6 | 10,866 | 7,306 | 5,191 |
| 6x6 | 17,995 | 15,659 | 12,960 |
| 6x8 | 24,538 | 21,353 | 17,673 |
| 8x8 | 36,588 | 34,610 | 31,872 |
4x4 vs 6x6 by species, and metric posts
Species changes the answer through both crushing strength (Fc) and stiffness (Emin). Spruce-Pine-Fir has no posts-and-timbers row in the values the calculator uses, so only 4x sections are listed for it. Metric grades use EN 338 or AS/NZS characteristic values converted to an allowable-stress equivalent, described on the page.
| Species and grade | Small post | Large post |
|---|---|---|
| Douglas Fir-Larch No. 2 | 4x4: 4,665 lb | 6x6: 15,659 lb |
| Southern Pine No. 2 | 4x4: 4,120 lb | 6x6: 12,725 lb |
| Hem-Fir No. 2 | 4x4: 3,824 lb | 6x6: 13,051 lb |
| Spruce-Pine-Fir No. 1/No. 2 | 4x4: 4,092 lb | Not listed |
| C24 (EN 338) | 90 × 90 mm: 25.0 kN | 140 × 140 mm: 117.5 kN |
| C16 (EN 338) | 90 × 90 mm: 18.5 kN | 140 × 140 mm: 89.5 kN |
| MGP10 (AS/NZS) | 90 × 90 mm: 24.9 kN | 140 × 140 mm: 110.1 kN |
The column stability factor CP explained
NDS 2018 section 3.7.1 reduces the compression design value by CP. First F*c = Fc × CD × CM × CF × Ci; then the buckling stress FcE = 0.822 E′min ÷ (le/d)², where le is the effective length and d the least side. With α = FcE ÷ F*c and c = 0.8 for sawn lumber, CP = (1 + α)/(2c) − √[((1 + α)/(2c))² − α/c]. The allowable load is F*c × CP × the area.
The slenderness ratio le/d must not exceed 50. Short, stocky posts keep most of their crushing strength; long, thin ones lose most of it. The table shows the effect for a Douglas Fir-Larch No. 2 4x4.
| le/d | 4x4 height | CP | F′c (psi) |
|---|---|---|---|
| 10 | 2 ft 11 in | 0.921 | 1,430 |
| 20 | 5 ft 10 in | 0.594 | 922 |
| 30 | 8 ft 9 in | 0.313 | 486 |
| 40 | 11 ft 8 in | 0.184 | 285 |
| 50 (limit) | 14 ft 7 in | 0.120 | 186 |
Wet service, incising and deck posts
Exterior posts are often wet and pressure-treated. For 4x dimension lumber, wet service multiplies Fc by 0.8 and Emin by 0.9, and incising for treatment multiplies Fc by 0.8 and Emin by 0.95 (NDS 4.3.8). Timbers 5 in and larger use 0.91 on Fc for wet service. Together these take about a fifth off an 8 ft 4x4.
Many deck guides and local codes also set minimum post sizes regardless of load, often 6x6 for posts that support a beam, and limit post heights. The calculator does not apply those rules, so check them before choosing a 4x4.
| Condition | 4x4 | 4x6 | 6x6 |
|---|---|---|---|
| Dry | 6,960 | 10,866 | 17,995 |
| Wet service | 6,152 | 9,593 | 16,696 |
| Incised | 6,433 | 10,024 | 17,995 |
| Wet and incised | 5,656 | 8,799 | 16,696 |
Worked example: a 10 ft post under a deck beam
A post carries 8 ft of beam on each side and 10 ft of joists: half of each gives a tributary area of 8 × 10 = 80 ft². At 10 psf dead and 40 psf live, the load is 80 × 50 = 4,000 lb. The post is 10 ft tall and Douglas Fir-Larch No. 2.
For a 4x4, le/d = 120 ÷ 3.5 = 34.3, so FcE = 0.822 × 580,000 ÷ 34.3² = 405.6 psi. F*c = 1,350 × 1.15 = 1,552.5 psi, α = 0.261 and CP = 0.245. The allowable stress is 380.8 psi, and over 12.25 in² that is 4,665 lb, so the 4x4 is 86% utilised. A 6x6 at 15,659 lb is 26% utilised and is the common choice for beam posts of this height.
Who it is for and when to use it
Deck and porch posts
Compare 4x4 and 6x6 posts for a deck beam before the permit drawing, then check the local deck guide's minimum post size and height limits.
Basement and crawlspace columns
Estimate whether a wood post under a girder carries the floor load, or whether a steel column or a larger timber is needed.
Pergolas, carports and shade structures
Check tall posts carrying roof and snow load, where slenderness rather than crushing usually decides the size. Use the ground snow load guide for the snow input.
Early design comparisons
See how height, species and wet service change the post size before an engineer finalises the frame.
Method, formulas & assumptions
Load = tributary area × (dead + live or snow), or an entered axial load (ASD, unfactored). For each size: F*c = Fc × CF × CD × CM (wet) × Ci (incised 4x lumber); FcE = 0.822 E′min ÷ (le/d)² with Ke = 1.0 and d the least side; α = FcE ÷ F*c; CP = (1 + α)/(2c) − √[((1 + α)/(2c))² − α/c] with c = 0.8 (NDS 2018 3.7.1); allowable P = F*c × CP × area, le/d ≤ 50. 4x sections use NDS Table 4A/4B dimension values; 6x6 and larger use Table 4D posts and timbers.
Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.
Common mistakes to avoid
- Using the post's full length when it is braced partway up. Enter the unbraced length between points that stop it moving sideways; bracing in one direction only does not shorten the other.
- Ignoring wet service and incising for exterior treated posts. Together they cut a 4x4's capacity by about a fifth.
- Counting the load once when a post supports two beams or a beam and a floor. Add every tributary area that bears on the post.
- Checking the post but not the bearing. The beam above and the footing below have their own limits, and the connection usually needs a post cap and base.
- Assuming a 4x6 is stronger in both directions. Buckling about the 3½ in side governs unless the post is braced in that direction.
Scope & limits
Concentric axial load on a single solid sawn post with pinned ends. Bearing on the beam and footing, post caps and bases, bending from lateral or eccentric load, built-up columns, fire and minimum post sizes in deck guides are not checked. Metric grades use characteristic values converted to an allowable-stress equivalent. Reference values for preliminary sizing; the adopted code and an engineer govern.
Frequently asked questions
How much weight can a 4x4 post hold?
It depends mostly on height. A dry Douglas Fir-Larch No. 2 4x4 carries about 6,960 lb at 8 ft, 4,665 lb at 10 ft and 3,311 lb at 12 ft under the NDS column method, before bearing and connection checks.
How much weight can a 6x6 post hold?
A dry Douglas Fir-Larch No. 2 6x6 carries about 18,000 lb at 8 ft and 15,659 lb at 10 ft. It is far less sensitive to height than a 4x4 because it is much stiffer.
Should deck posts be 4x4 or 6x6?
Many deck guides require 6x6 posts where a post supports a beam, even if a 4x4 would carry the load, because of bracing, notching for the beam and long-term performance. Check your local deck guide.
What is the column stability factor?
CP is the NDS factor that reduces a post's compression strength for buckling. It falls from close to 1 for short posts to about 0.12 at the slenderness limit of le/d = 50.
What is tributary area for a post?
It is the floor or roof area whose load flows to the post: half the beam span on each side times half the span of the joists the beam carries. Multiply it by the dead plus live or snow load.
Does it work in metric?
Yes. Switch to Metric for kilonewtons, square metres and metres, with EN 338 and AS/NZS MGP grades. A 3 m C24 90 × 90 mm post carries about 25 kN.
Does the calculator check the footing?
No. It checks the post as a column only. Footing size, bearing on the beam and the post base and cap need separate checks.
Do I need an account to download the result?
No. All calculations, local file processing and exports on this page are free, without an account.
Where are my inputs stored?
The tool keeps inputs in this browser tab. It does not upload them or create a cloud copy. Save or download anything you want to keep before leaving the page. Standard site analytics may operate independently of the tool.
Can I work in metric instead of imperial?
Yes. Use the Imperial / Metric switch above the inputs. Imperial (feet, inches, pounds, gallons) is the default; switching converts every dimensional field and result, and your choice is remembered across the free tools in this browser.
Further reading
- Deck, pergola and shade structure template
- Ground snow load explained
- Foundation types template
- All framing & structure tools
- All free architecture & construction tools
- Harth resource library
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