Four posts get counted twice and nobody notices until the delivery
Take a 48 by 30 building at 8 ft sidewall spacing with the endwall posts on 10 ft centres. The sidewall divides into six bays, so each sidewall has seven post lines and the two sidewalls together take fourteen posts. The endwall divides into three bays, which is four post lines — but two of those four are the corners, and the corners are already in the sidewall count. So each endwall contributes two posts, not four, and the building takes eighteen posts.
Count each wall on its own and you get 7 + 7 + 4 + 4, which is twenty-two. That is four posts too many, one per corner. On an eighteen-post building the error is 22 percent, and it is the single most common arithmetic mistake in a post-frame take-off because each wall looks like a fence line when you draw it and a fence line genuinely does need spacing-plus-one.
| Building | Sidewall bays | Endwall bays | Posts | Counted per wall |
|---|---|---|---|---|
| 40 x 24, 8 ft and 8 ft | 5 | 3 | 16 | 20 |
| 48 x 30, 8 ft and 10 ft | 6 | 3 | 18 | 22 |
| 60 x 40, 10 ft and 10 ft | 6 | 4 | 20 | 24 |
| 80 x 40, 8 ft and 10 ft | 10 | 4 | 28 | 32 |
The overcount is always exactly four, which makes it easy to correct and easy to miss, because four extra posts on a delivery ticket look like somebody being careful rather than somebody being wrong.
Rounding up the bays shortens every bay, not one
Divide 48 ft by 8 ft and you get six bays exactly. Divide 50 ft by 8 and you get 6.25, which rounds up to seven bays — and seven bays across 50 ft is 7.14 ft each, not six bays of 8 ft plus one of 2 ft. This calculator distributes the shortfall evenly because that is what most post-frame plans do: the bents are laid out as equal divisions of the wall, so the trusses, the purlins and the panel joints all stay on one module.
The alternative, full bays plus one odd bay at the end, exists and gets built, usually when the building is a later addition matching an existing module. It changes the girt cut list completely — most pieces at one length and a handful at another — and the calculator does not produce it. If that is your layout, run the full bays as one building and the odd bay as a second.
The girts are where the offcuts hide
Seven girt rows on a 48 by 30 building is 7 x (48 + 48 + 30 + 30), or 1,092 lineal feet of girt in the wall. Cut between posts, the sidewall pieces are 8 ft and the endwall pieces are 10 ft. Two 8 ft pieces come out of a 16 ft stick with nothing left. One 10 ft piece comes out of a 16 ft stick and strands 6 ft, every single time, and there are 42 of those pieces.
So the sidewalls take 42 sticks and cover 672 ft with no waste, and the endwalls take 42 sticks to cover 420 ft. Together that is 84 sticks, 1,344 ft bought against 1,092 ft installed, and 252 ft — nearly nineteen percent — sitting in a pile. The fix is not in this calculator: it is buying 20 ft stock for the endwalls, where two 10 ft pieces come out clean. The page shows you the stranded figure so the question gets asked before the order goes in, and the exterior trim and corner board calculator does the same nesting arithmetic for trim.
What the hole volume does and does not include
An 18 inch hole is 1.767 square feet of bottom. Four feet deep that is 7.07 cubic feet of hole. Backfill it with concrete around a 6 by 6 post sitting on an 8 inch pad and the post displaces 0.25 sq ft over the 3.33 ft above the pad, so the concrete comes to 6.24 cubic feet per hole. Eighteen holes is 112 cubic feet, or 4.16 cubic yards, or 188 bags at 80 lb.
That is a volume, and volume is the easy part. Whether the hole should be 18 inches or 24, whether it should be backfilled at all, whether it wants a pad or a collar or a bolted-on foot, how deep it has to go for frost and for uplift, and what the soil under it will actually carry are none of them decided here. They come from the plan and from the local building department, and they vary between counties that share a border. For the pour itself the concrete slab calculator handles bags against ready-mix and short-load fees, and the fence post calculator does the same hole arithmetic for a straight run of fence.
What this hands off to
Once the grid is set, the panel take-off is a separate problem — see the pole barn wall panel and wainscot take-off for the elevations and the corrugated panel coverage and purlin calculator or the metal roof panel calculator for the roof. Openings are their own arithmetic, because they interrupt girt rows: the door opening framing take-off works out what one costs. For a stick-framed building instead of a post-frame one, the shed material takeoff calculator produces a stud-and-rafter bill, and the shed foundation options calculator compares bases under a light building.
Questions people ask
Why are there four fewer posts than adding up the walls suggests?
Because the corner posts belong to two walls at once. A sidewall with six bays has seven post lines, and the first and last of those seven are the corners. Work the endwall out separately and its outermost posts are the same two pieces of wood. The formula this page uses is 2 x (sidewall bays + 1) for both sidewalls, which includes all four corners, plus (endwall bays - 1) for each endwall, which is the posts strictly between the corners. On a 48 by 30 at 8 and 10 ft that is 14 + 4 = 18, against 22 if each wall is counted alone. If your plan shows more posts than this, it is usually because door bents, an interior partition or a lean-to are adding them, and those are not in this count.
Does the calculator tell me what post spacing to use?
No, and it will not. Post spacing is a structural decision that depends on the truss, the post section, the wall height, the snow and wind figures your jurisdiction has adopted, and the soil. It is a field you fill in from the plan, the post-frame supplier or the engineer who sealed the drawings. What this page does is take your spacing and tell you what it costs in posts, bays, girt rows and concrete, and show you the spacing the bays actually land at once the length is divided evenly. Those two numbers are usually not the same, which is the point of showing both.
Why do the bays come out shorter than the spacing I entered?
Because the number of bays has to be a whole number and the calculator rounds it up rather than down. A 50 ft wall at 8 ft spacing needs 6.25 bays; six bays would put the posts at 8.33 ft, which is wider than you asked for, so it uses seven bays at 7.14 ft. Rounding up guarantees no bay exceeds your figure. The output prints how much tighter each bay is than your entry so you can see whether the difference matters — going from 8 ft to 7.14 ft costs you one extra post per sidewall and about ten percent more girt pieces.
Are the endwall girts above the eave included?
No. The girt figures cover the rectangular part of each wall, from the splashboard up to the eave line. The gable triangle above the eave on each endwall needs its own framing, and how it is built varies too much to guess at — some are framed off the end truss, some carry their own posts up, some are sheeted vertically with no girts at all. Work out the triangle separately: its area is half the building width times the rise, and the rise is half the width times the pitch over twelve. On a 30 ft wide building at 4 in 12 that is a 5 ft rise and 75 square feet per end.
What is the splashboard field for?
A splashboard, or skirt board, is the horizontal member at the bottom of the wall that the cladding starts on and that keeps the panel out of the dirt. Where one is fitted, the girt run starts above it rather than at grade, so an 8 inch splashboard on a 14 ft wall leaves 160 inches of girt run instead of 168. That is enough to change the row count at some spacings and not at others, which is why it is a field rather than an assumption. Set it to zero if the cladding runs to grade or if the plan handles the bottom differently.