How many posts does a 30x40 pole barn need?

It depends on the spacing your design calls for. With posts 8' apart down the 40' sides and 10' apart across the 30' ends, it is 6 posts a side and 2 between the corners on each end — 16 in all. Doors usually add posts of their own, and the spacing itself is a structural decision tied to the trusses and the headers.

How far apart should pole barn posts and trusses be?

Whatever the building is designed for — and that is the one number this calculator will not choose. Post-frame trusses are made and stamped for a spacing, and a truss that falls between posts sits on a header that has to be sized for it. Posts 8' apart with a truss on every one is a common layout; wider post spacing with trusses on headers between is another. The calculator counts either and flags a truss spacing that does not divide the post spacing.

How deep do pole barn posts go?

Below the frost line, and deep enough to resist the wind trying to lift and tip the building — both of which depend on where you are, the soil, and how the hole is backfilled or footed. Your building department or the building's designer sets it. The calculator asks for the depth and adds it to the eave height to give the post length, rather than guessing a figure that looks authoritative.

How much steel siding does a pole barn need?

For a 30' x 40' barn with 10' eaves at 4/12 in 36" panels: 28 sidewall panels at the eave height, and 20 endwall panels that step up the gable to the peak and back down — 1,550 sq ft of wall with the gables. Doors and windows are cut out of that, not subtracted from the order, because the panel still has to reach above and below them.

Pole barn materials

Enter the footprint, the eave height, the pitch and the spacings your design calls for, and get the posts, trusses, girts, purlins and the steel for the roof and walls. It counts the frame; post size, depth and truss spacing are yours to give.

Width

The truss span

Length

Along the sidewall

Eave height

Grade to the eave

Pitch

:12

Rise per foot of run

Post spacing

Down the sidewalls — your design

Truss spacing

From your truss drawing

Post size

Yours to choose — not checked

Post depth

Below grade, per your building department

Advanced settingsendwall posts 10 ft · girts 24" · purlins 24" · 12" eave, 12" rake · 36" steel

Endwall post spacing

Between the corners

Steel coverage

Net panel width, 36 usual

Girt spacing

On centre, up the walls

Purlin spacing

On centre, up the roof

Eave overhang

Level, past the sidewalls

Rake overhang

Past each endwall

6x6 posts, 14 ft each

16posts

Trusses

Roof panels

Wall panels

It counts the frame; it does not design it

Post size, how deep the posts go, the truss spacing and the trusses themselves decide whether this building stands up to your snow and wind, and none of them is this page's to choose. Post-frame trusses come stamped for a spacing; headers that carry trusses between posts need sizing; the post depth turns on frost line, soil and uplift. Ring your building department before you dig.

The frame

Posts, 6x66 down each sidewall 8 ft apart, 2 across each endwall between the corners; 10 ft of eave plus 4 ft in the ground: 16 × 14 ft
Trusses30 ft span, 8 ft apart — one at each gable end: 6
Skirt board, treatedOne row round the bottom, where the steel meets the ground: 140 ft
Wall girts5 rows round the walls, plus 2 shortening rows up each gable — openings not taken off: 748 ft
Purlins10 rows a side, one piece a truss bay — 840 ft with the rake overhangs: 100 × 8 ft

The steel

Roof panels14 a side, eave edge to ridge — the metal roofing calculator adds the overhang into the gutter, the trim and the screws: 28 × 16 ft 11 in
Sidewall panelsSkirt board to eave: 28 × 10 ft
Endwall panels4 at 15 ft, 4 at 14 ft, 4 at 13 ft, 4 at 12 ft, 4 at 11 ft — each at its taller edge, the rake cut on site: 20
Steel area1,417 of roof and 1,550 of wall, gables included: 2,967 sq ft

Checks

All 3 checks passShow the numbers

Trusses line up with the posts — passesEvery truss lands on a pair of posts.
Posts come in a stocked length — passesEach post is 14 ft, eave height plus embedment.
At least 3:12 for exposed-fastener panels — passesThis roof is 4:12, steep enough for lapped exposed-fastener panels without lap sealant (IRC R905.10.2).

These check the layout's arithmetic and the 2021 IRC's minimum slope for metal panels (R905.10.2) — defaults, not law, and nothing about whether a post or a truss is big enough.

Where to buy this

16 6x6 postsHome Depot Lowe's
6 trusses, 30 ftHome Depot Lowe's
140 ft skirt boardHome Depot Lowe's
748 ft girtsHome Depot Lowe's
100 purlinsHome Depot Lowe's
76 steel panelsHome Depot Lowe's

This searches the materials, not a post, girt or purlin size we worked out for you — your design or your building department decides those.

Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.

Worked example: a 30 × 40 ft pole barn with 10 ft eaves

With posts 8 ft apart down the sides and 10 ft across the ends, a 30 × 40 ft barn takes 16 6x6 posts — 6 a side and 2 between the corners on each end — each 14 ft with 4 ft in the ground.

Trusses 8 ft apart make 6, one at each gable end. Girts at 24" run 5 rows round the walls and 2 more up each gable, 748 ft in all, and purlins at 24" are 10 rows a side, 100 pieces.

At 4:12 the roof takes 28 panels at 16 ft 11 in and the walls 48 panels. The post size, depth and truss spacing are the builder's, not this page's.

Posts: 16 × 14 ft
Trusses: 6
Girts: 748 ft
Purlins: 100
Roof panels: 28
Wall panels: 48

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

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About pole barns

How many posts does a 30x40 pole barn need?: It depends on the spacing your design calls for. With posts 8' apart down the 40' sides and 10' apart across the 30' ends, it is 6 posts a side and 2 between the corners on each end — 16 in all. Doors usually add posts of their own, and the spacing itself is a structural decision tied to the trusses and the headers.
How far apart should pole barn posts and trusses be?: Whatever the building is designed for — and that is the one number this calculator will not choose. Post-frame trusses are made and stamped for a spacing, and a truss that falls between posts sits on a header that has to be sized for it. Posts 8' apart with a truss on every one is a common layout; wider post spacing with trusses on headers between is another. The calculator counts either and flags a truss spacing that does not divide the post spacing.
How deep do pole barn posts go?: Below the frost line, and deep enough to resist the wind trying to lift and tip the building — both of which depend on where you are, the soil, and how the hole is backfilled or footed. Your building department or the building's designer sets it. The calculator asks for the depth and adds it to the eave height to give the post length, rather than guessing a figure that looks authoritative.
How much steel siding does a pole barn need?: For a 30' x 40' barn with 10' eaves at 4/12 in 36" panels: 28 sidewall panels at the eave height, and 20 endwall panels that step up the gable to the peak and back down — 1,550 sq ft of wall with the gables. Doors and windows are cut out of that, not subtracted from the order, because the panel still has to reach above and below them.