Post and Beam Layout Calculator
Post spacing, post count, beam quantities, and post load capacity for rectangular post-and-beam structures.
About this calculator
This calculator lays out a rectangular post-and-beam grid and separately checks whether the chosen post size can carry the resulting load. Layout comes first: Building Length and Building Width are each divided into bays no wider than Target Bay Spacing (rounded up to a whole number of bays), and Total Posts is simply (bays along length + 1) times (bays along width + 1) -- since bay counts use a ceiling function, small spacing changes can leave the post count unchanged until they actually push the bay count up a step. Max Post Load (Interior) is the actual bay spacing (length x width, adjusted to divide the building evenly) times Total Floor/Roof Load -- it has nothing to do with the post size you picked. Post Axial Capacity, by contrast, comes purely from the post's own cross-section (Post Size squared) times the species' compression strength (Fc), so Post Size and Wood Species drive capacity while being completely inert to how much load a post actually carries.
The Fc values behind that capacity (1000, 1100, 550, and 800 psi for Douglas Fir-Larch, Southern Pine, Eastern White Pine, and Red Oak) are the American Wood Council's own published reference design values for posts and timbers 5"x5" and larger, not simplified round numbers. Post Utilization Ratio (load / capacity) is where the two halves meet: a bigger post lowers utilization without changing the load at all, while a heavier Total Floor/Roof Load raises utilization without changing capacity. Min Post Size Needed reports the smallest square post that would clear the interior load exactly, for comparison against your entered Post Size.
Inputs
Results
Total posts
15
Max post load (interior)
6,000 lbs
≈ 6 grand pianos
Figures current as of 2018. Source: American Wood Council, National Design Specification (NDS) Supplement: Design Values for Wood Construction, 2018 Edition, Table 4D (Reference Design Values for Visually Graded Timbers, 5" x 5" and larger)
How to Use This Calculator
- Enter Building length, Building width, and Target bay spacing.
- Set Total floor/roof load, Post size (square), and Wood species.
- Review Total posts and Max post load (interior) (lbs).
- Use Post axial capacity (lbs) and Post utilization ratio to inform your decision.
How the result changes with Building width
| Building width | Total posts | Max post load (interior) |
|---|---|---|
| 12 | 10 | 6,000 lbs |
| 18 | 15 | 4,500 lbs |
| 36 | 20 | 6,000 lbs |
| 60 | 30 | 6,000 lbs |
What each input means
- Building length
- Overall building length along the long axis.
- Building width
- Overall building width along the short axis.
- Target bay spacing
- Desired distance between posts. Actual spacing may be adjusted to divide evenly.
- Total floor/roof load
- Combined dead + live load carried by the frame.
- Post size (square)
- Nominal square post dimension (e.g., 8 for an 8×8 post).
- Wood species
- 0 = Douglas Fir-Larch, 1 = Southern Pine, 2 = Eastern White Pine, 3 = Red Oak.
What each result means
- Total posts
- Number of posts in the rectangular grid.
- Max post load (interior)
- Axial load on a fully-loaded interior post.
- Post axial capacity
- Compression capacity: A × Fc (short column).
- Post utilization ratio
- Load / capacity. Must be ≤ 1.0.
- Min post size needed
- Minimum square post side to carry the interior load.
- Bays along length
- Number of structural bays along the building length.
- Bays along width
- Number of structural bays along the building width.
- Actual bay spacing (length)
- Adjusted bay spacing to divide length evenly.
- Actual bay spacing (width)
- Adjusted bay spacing to divide width evenly.
- Total beam segments
- Number of individual beam segments in the grid.
- Total beam length
- Total linear feet of beams needed.
- Building footprint
- Length × width floor area.
- Beam lines along length
- Number of beam grid lines running along the length axis.
- Beam lines along width
- Number of beam grid lines running along the width axis.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersBuilding length = 40, Building width = 24, Target bay spacing = 12, Total floor/roof load = 50 = 6 input(s) provided
- Calculate Total postsTotal posts = postsAlongLength * postsAlongWidth15 = 15
- Calculate Max post loadMax post load = maxTributaryArea * totalLoadPsf6000 = 6000
- Calculate Post axial capacityPost axial capacity = postArea * Fc64000 = 64000
- Calculate Post utilization ratioPost utilization ratio = interiorPostLoad / postCapacity0.094 = 0.094
Figures and sources
- Compression parallel to grain (Fc) reference design values for Douglas Fir-Larch, Southern Pine, Eastern White Pine, and Red Oak posts and timbers (2018) — American Wood Council, National Design Specification (NDS) Supplement: Design Values for Wood Construction, 2018 Edition, Table 4D (Reference Design Values for Visually Graded Timbers, 5" x 5" and larger)
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does choosing a bigger post size change the load it needs to carry?
No. Max Post Load (Interior) comes entirely from the tributary floor/roof area (actual bay spacing in both directions) times Total Floor/Roof Load -- Post Size only affects Post Axial Capacity (post area x the species' compression strength) and therefore Post Utilization Ratio. A bigger post lowers utilization by raising capacity, not by lowering the load it has to resist.
Why didn't Total Posts change when I adjusted Target Bay Spacing slightly?
Bays Along Length and Bays Along Width are computed with a ceiling function (building dimension divided by target spacing, rounded up), so Total Posts only changes when a spacing adjustment is large enough to cross a whole-bay boundary. A building that needs 4 bays at both 10 ft and 12 ft target spacing will report the same post count for either value, with only the reported Actual Bay Spacing shifting between them.
Does wood species affect how many posts the layout needs?
No. Total Posts, Bays Along Length, Bays Along Width, and Actual Bay Spacing are purely geometric results driven by Building Length, Building Width, and Target Bay Spacing. Wood Species only enters through the species' compression strength (Fc), which affects Post Axial Capacity, Post Utilization Ratio, and Min Post Size Needed -- the grid layout itself stays identical regardless of which species you select. Those Fc values (1000 psi for Douglas Fir-Larch, 1100 for Southern Pine, 550 for Eastern White Pine, 800 for Red Oak) come from the American Wood Council's NDS Supplement reference design values for posts and timbers.
What raises Post Utilization Ratio the fastest?
Total Floor/Roof Load moves utilization directly and linearly -- doubling the load doubles the interior post's demand while its capacity stays fixed. Increasing Building Length or Building Width can also raise utilization by widening the actual tributary area each interior post carries, though that effect is dampened by the calculator's own bay re-spacing (it never lets actual spacing exceed your Target Bay Spacing).
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