How to Calculate Concrete for Fence Posts (Step-by-Step)
Every fence post calculation, no matter the size or hole, comes down to the same nine steps done in the same order. Learn the sequence once, and you can work out the bag count for any post — not just the sizes covered in a reference chart.
Quick answer
Measure the post, pick a hole diameter and depth, calculate the hole’s volume, subtract the post’s volume, add 10% waste, then divide by your bag’s yield and multiply by how many posts you’re setting. Nine steps, always in the same order — walked through in full below with one running example.
The Fence Post Concrete Calculator runs all nine steps below instantly once you enter your numbers. This guide shows exactly what it’s doing behind the scenes, so you can run the same math by hand or double-check the result.
What you’ll need before you start
- A tape measureFor the post’s actual dimensions and your planned hole size.
- Your post countIncluding which posts, if any, will get a wider gate or corner hole.
- A bag size in mind50, 60, or 80 lb — check the yield printed on the bag.
- A calculator or the tool aboveThe math is simple, but there are enough steps that a calculator helps avoid an arithmetic slip.
Step 1: Measure your post’s actual dimensions
Lumber is sold by a nominal size that isn’t its real size — a “4×4” post actually measures about 3.5×3.5 inches. Always measure the post itself rather than trusting the label name, since this number gets used later to subtract the post’s volume from the hole.
Running example: a 4×4 post, measuring 3.5×3.5 in (0.292×0.292 ft).
Step 2: Decide your hole diameter
The standard rule is about 3 times the post’s width. For a 3.5-inch post, that’s roughly a 10-12 inch hole.
Running example: a 10 in hole, or 0.833 ft diameter — 0.417 ft radius.
Step 3: Decide your hole depth
A third to half of the post’s above-ground height is the standard guideline, adjusted upward if your local frost line requires going deeper.
Running example: a 6 ft fence post, buried 2 ft deep.
Step 4: Calculate the hole’s volume
Running example: 3.14 × 0.417² × 2 = 1.09 cu ft.
Step 5: Calculate the post’s volume
Running example: 0.292 × 0.292 × 2 = 0.17 cu ft.
Step 6: Subtract the post’s volume from the hole’s volume
This is the step most people skip, and it’s the one that separates a rough guess from an accurate number — the post physically takes up space the concrete won’t fill.
Running example: 1.09 − 0.17 = 0.92 cu ft.
Step 7: Add your waste allowance
10% covers the normal spillage and minor irregularity of a hand-dug hole. Use 15% if the ground is rocky or uneven.
Running example: 0.92 × 1.10 = 1.01 cu ft.
Step 8: Divide by your bag’s yield
Check the yield printed on your specific bag — 0.60 cu ft for an 80 lb bag is standard, but always confirm against the label.
Running example: 1.01 ÷ 0.60 = 1.69, rounded up to 2 bags per post.
Step 9: Multiply by your total post count
Repeat the calculation separately for any posts using a different hole size — gate and corner posts especially — then add every post’s total together.
Running example: 8 posts × 2 bags = 16 bags of 80 lb concrete for the whole fence line.
Skip the nine steps.
Enter your post size, hole dimensions, and post count for an instant bag count.
Use the free Fence Post Concrete CalculatorFill in your own numbers
Use this as a worksheet — copy your own measurements into the middle column and follow the same nine steps above.
| Step | Your value | Formula |
|---|---|---|
| 1. Post width (actual, ft) | _____ | — |
| 2. Hole radius (ft) | _____ | diameter ÷ 2 |
| 3. Depth (ft) | _____ | — |
| 4. Hole volume | _____ | 3.14 × radius² × depth |
| 5. Post volume | _____ | width × width × depth |
| 6. Net volume | _____ | hole volume − post volume |
| 7. With waste (×1.10) | _____ | net volume × 1.10 |
| 8. Bags per post | _____ | ÷ bag yield, round up |
| 9. Total bags | _____ | × post count |
Where people go wrong in this process
- Step 1 — using nominal size instead of actualA “4×4” measured as exactly 4 inches slightly overstates how much volume the post displaces.
- Step 4 — mixing up diameter and radiusThe formula needs radius (half the diameter), and using the full diameter by mistake roughly quadruples the calculated hole volume.
- Step 6 — skipping the subtraction entirelyLeads to a consistent over-order across every post in the project.
- Step 7 — forgetting waste, or adding it twiceAdd it once, after the subtraction — not before, and not skipped altogether.
- Step 8 — using the wrong bag’s yield50, 60, and 80 lb bags all have different yields — check the one you’re actually buying.
- Step 9 — applying one number to every postGate and corner posts usually need a separate calculation with a wider hole.
What changes the result at each step
- Post sizeBigger posts displace more volume, reducing the concrete needed per hole.
- Hole diameterThe single biggest lever in the whole calculation — small changes here have an outsized effect.
- DepthSet by fence height and local frost depth, whichever requires more.
- Bag yieldVaries by size and sometimes by brand — always confirm against the actual label.
The nine steps, condensed
- Measure the post’s actual size and decide hole diameter and depth.
- Calculate hole volume, then post volume.
- Subtract post volume from hole volume.
- Add 10% waste, then divide by your bag’s yield and round up.
- Multiply by post count, calculating gate and corner posts separately.
Frequently asked questions
What’s the first step in calculating fence post concrete?
Measuring the post’s actual dimensions — not its nominal name. A “4×4” is really 3.5×3.5 inches, and that real number is what the rest of the calculation uses.
Do I really need to subtract the post’s volume?
Yes, for an accurate number. Skipping it means calculating as if the hole were empty, which overstates how much concrete each post actually needs.
What if my post hole isn’t a perfect circle?
Most hand-dug or augered holes are close enough to circular for this formula to work well. For a clearly square or rectangular hole, use width × length × depth instead of the circle formula.
Can I use this method for a square post hole too?
Yes — just swap the circle formula (3.14 × radius² × depth) for a rectangle formula (length × width × depth) in Step 4, and the rest of the process stays the same.
How do I calculate gate posts differently from line posts?
Run the same nine steps separately using the gate post’s wider hole diameter, then add its bag total to the rest of the fence line rather than using one number for every post.
What waste percentage should I use in Step 7?
10% for a clean, formed hole. Bump it to 15% for rocky or uneven ground, where hand-digging tends to leave a less regular shape.
Does this process work for round or metal posts too?
Yes. For a round post, use the same circle formula for Step 5’s post volume, just with the post’s own radius instead of a squared width.
How is this different from just using a bag-count chart?
A chart gives you an answer for common, pre-listed sizes. This process gets you an accurate number for any post, hole, or depth combination, including ones a chart wouldn’t cover.
Related reading
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