Calculate how many pole barn posts you need and their even spacing from building size, plus the post length to order and concrete bags per hole.
Pole Barn Post Formula
The calculator has three quantity-planning modes. The posts mode counts the evenly spaced bays on each wall of a rectangular plan and shares corner posts around the perimeter:
Each wall is divided into equal bays rather than spacing posts by s with one short final bay. The resulting spacing is at or below the specified maximum. Optional interior rows are evenly spaced across the width and use the same lengthwise bay grid; coincident end-wall posts are counted only once:
spacing = wall length / ceil(wall length / s)
To find the post length to order, it adds the below-grade embedment to the above-grade height:
Post length = H + d + extra d (one-third geometry) = H / 2
To find the concrete for a straight cylindrical hole, it uses the volume of a cylinder and multiplies by the number of holes:
Variables:
- P is the perimeter post count with corners shared; R is the number of uniform interior rows, bL = ceil(L/s), and bW = ceil(W/s)
- L and W are the building length and width
- s is the maximum post spacing from your approved design; the calculator does not choose it
- ceil rounds up to the next whole number, so the spacing never exceeds your target
- H is the above-grade post height, measured from finished grade to the top of the post
- d is the specified embedment or H/2 under the optional one-third geometry. That fraction applies before extra length is added and is not a safe-depth design rule. Use consistent length units in the formula.
- D is the hole diameter and h is the hole depth, both used to size the concrete per hole
Pick a mode at the top. Posts mode returns the distinct count, bay counts and even spacing; adding or changing uniform interior rows can reposition them and change shared end posts. Length mode uses a specified depth or the geometric H/2 comparison, adds any allowance, and rounds under editable stock settings. The initial 2 ft step and 8 ft minimum are ordering assumptions to confirm with the supplier. Concrete mode uses required hole dimensions, a stated yield and optional overage; blank overage adds 0%. Optional actual post dimensions subtract a rectangular volume across the full hole depth.
Post Length and Spacing Reference
The table is an arithmetic example using 2 ft stock increments, an 8 ft minimum, no extra allowance and the two stated embedment depths. These are ordering and geometry assumptions, not universally available lengths or recommended depths. Confirm actual stock and the approved embedment before ordering.
| Above-grade wall height | Buried 42 in, order length | Buried 48 in, order length |
|---|---|---|
| 8 ft | 12 ft | 12 ft |
| 10 ft | 14 ft | 14 ft |
| 12 ft | 16 ft | 16 ft |
| 14 ft | 18 ft | 18 ft |
| 16 ft | 20 ft | 20 ft |
Post spacing, section, connections and foundations interact with the complete frame and site loads. The following checklist does not specify a safe spacing or post size. ASABE EP 486.3 addresses post/pier foundation adequacy under structural loads and soil conditions; 2024 IBC Section 1807.3 provides or references design procedures. Confirm the locally adopted requirements and a project-specific design.
| Situation | Spacing checks | Post and foundation checks |
|---|---|---|
| Light agricultural or open shelter | Wind, uplift and bracing plan | Designed section and foundation |
| Enclosed barn | Wall, roof and bracing plan | Designed axial and lateral capacity |
| Occupied space or overhead doors | Opening and frame design | Designed posts and connections |
| Snow or wind exposure | Site loads and combinations | Designed frame and foundations |
| Tall walls or wide clear span | Geometry and deflection checks | Designed section and bracing |
Example Problems
Example 1: Count the posts for a 40 by 24 ft barn at 8 ft spacing.
The length has ceil(40/8) = 5 bays and the width has ceil(24/8) = 3 bays. The perimeter total is P = 2*5 + 2*3 = 16 posts. Both walls divide evenly, so the actual spacing stays at 8 ft on center, with 6 posts on each 40 ft wall and 4 on each 24 ft wall once corners are shared.
Example 2: Find the post length for 12 ft walls buried 42 in.
For this stated-depth example, 42 in is 3.5 ft. With no extra allowance, the total is 12 + 3.5 = 15.5 ft. Under the illustrative 2 ft step and 8 ft minimum, the modeled stock length is 16 ft. This does not establish that 42 in embedment is adequate.
Frequently Asked Questions
How far apart should pole barn posts be?
Use the spacing specified for the actual frame, connections, materials, openings and site loads. The calculator distributes regular bays at or below the maximum you enter and counts perimeter corners once. Meeting that numerical maximum does not prove that the frame is safe.
How deep should pole barn posts be set?
Use the embedment and foundation details required by the approved design and applicable local code. Soil, frost, uplift, axial and lateral resistance can all matter. The one-third option is only the identity d = H/2 before extra allowance; it does not determine an adequate foundation depth.
How much concrete does each post hole need?
For an illustrative straight 12 in diameter, 48 in deep hole, the full cylinder holds about 3.1416 ft³ (π ft³). At the approximate QUIKRETE Concrete Mix No. 1101 yields, rounding up gives 6 bags of 80 lb mix, 7 bags of 60 lb mix or 11 bags of 40 lb mix before overage and without post subtraction. Use the actual product yield and hole shape. Optional subtraction requires the actual post diagonal to fit the hole and assumes the post occupies the full hole depth.
