Post-Frame Material Calculator
Count the posts, girts and purlins a post-frame or pole barn needs from its dimensions and spacing, with roof and wall areas — priced at your own rates.
8–10 ft is typical for post-frame.
8 bays per sidewall need 18 posts in total — one more than the bay count on each side, then doubled. Counting one post per bay is the classic way to come up two short.
What this calculator does
A post-frame takeoff is three counts and one piece of geometry. The counts are posts, girt rows and purlin rows; the geometry is the rafter run, which decides the roof area. This does all four from your dimensions and spacing, then prices them at rates you supply.
It counts material. It does not size any of it — see the limits section below, because on a building that distinction matters more than usual.
The post count, and the mistake in it
bays per sidewall = ceiling( length ÷ post spacing )
posts = (bays + 1) × 2A 60-foot sidewall at 8-foot spacing is 8 bays, which needs 9 posts — and with two sidewalls, 18 posts. Dividing 60 by 8 and taking 8 posts a side leaves you two short for the building.
This is the fencepost error, the same off-by-one covered in how many fence posts do I need. It is worth internalising once, because it recurs everywhere in construction takeoffs: studs at the end of a wall, joists, and rafters all behave the same way.
Rafter run: why roof area exceeds the footprint
The rafters do not span half the width — they span the sloped distance, which is longer:
rafter run = √( (width/2)² + (width/2 × pitch/12)² )On a 40-foot building at 4/12, half the span is 20 feet and the rafter run is about 21.1 feet. That 5.4% carries into the roof area, the purlin rows and every sheet of roofing you order.
It is the same pitch geometry the roofing calculator applies, and skipping it is the classic way to under-order roof sheeting. The metal panel calculator turns the roof area this produces into a panel count.
Girts and purlins
Both are simple division, and both round up:
- Girt rows = eave height in inches ÷ girt spacing. At 14 ft eaves and 24 in spacing, 7 rows — running the full length, on both sidewalls.
- Purlin rows = rafter run in inches ÷ purlin spacing, across both roof planes.
The spacing figures are not preferences. Girt and purlin spacing are set by the sheeting’s span rating and by wind and snow loads, so they come from the building plan. Entering a wider spacing here will happily produce a smaller material list for a building that does not stand up.
What this deliberately will not do
It will not tell you what size anything should be. Column dimensions, embedment depth, header sizing, truss selection, bracing, and uplift and shear connections are all structural engineering, determined by site-specific wind and snow loads and by your local code.
That is not a liability disclaimer bolted on at the end — it is the reason the calculator is shaped the way it is. Counting members is genuinely useful for budgeting and for checking a supplier’s takeoff. Sizing them is a licensed engineer’s job, and your building department will require the stamped drawing regardless of what any website tells you.
Using the output
The material cost line covers posts, girts and purlins at your rates. Not included: trusses, hardware and fasteners, concrete for the post holes or piers, sheeting, doors, or any interior work.
For the concrete, the concrete calculator sizes post holes and piers by volume. For the envelope, the metal panel calculator counts sheets and screws from the roof and wall areas this page reports. And to price the whole thing including slab, insulation and labour, the building cost calculator layers it up.
If the system itself is still undecided, the red iron vs cold-formed vs post-frame comparison puts all three on the same footprint.
How this is calculated
bays per sidewall = ceiling( length ÷ post spacing ) posts = (bays + 1) × 2 sidewalls girt rows = ceiling( eave height × 12 ÷ girt spacing ) purlin rows = ceiling( rafter run × 12 ÷ purlin spacing ) rafter run = √( (width/2)² + (width/2 × pitch/12)² )
Frequently asked questions
- Why is the post count one more per side than the number of bays?
- Because a run of bays has a post at both ends. Ten bays along a sidewall need eleven posts, and there are two sidewalls, so the building needs two more posts than bays × 2. It is the fencepost error, and it is the single most common miscount in post-frame takeoffs.
- What post spacing should I use?
- Eight feet is the traditional figure and remains common; ten and twelve foot spacings are widely used in modern engineered post-frame with heavier columns and headers. It is not a free choice — spacing, column size and header sizing are engineered together, so take the spacing from your building plan rather than picking it here.
- Does this size the posts or the headers?
- No, deliberately. This counts members and measures lengths. What size column, what header, how deep the embedment, what uplift connection — all of that is structural engineering that depends on wind and snow loads for your specific site, and it belongs in a stamped drawing.
- How does roof pitch change the material?
- A steeper pitch makes the rafter run longer than half the building width, which increases both the roof area and the number of purlin rows. At 4/12 on a 40-foot building the run is about 21 feet rather than 20 — a five percent addition that carries through to purlins and sheeting.
- What about trusses?
- Not counted here. Post-frame buildings are usually roofed with trusses bearing on the columns, and trusses are engineered and quoted per building. Ask your supplier for the truss count at your chosen spacing, and treat this calculator's purlin figures as what spans between them.