Posts
Post Hole Concrete Calculator
Size one hole, or lay out a fence by length and spacing. The post itself can be subtracted so you do not buy concrete for the wood.
Your region
Feet, inches, cubic yards, and 40–80 lb bags
Concrete Calculator
Example measurements — replace them with yours.
Use 0 if the hole is all concrete. Many posts sit on a few inches of gravel.
Common sizes
Most orders add 5–10% for spillage, uneven ground, and the last bit left in the chute.
Cost, truck size, and concrete weight
Leave a price blank to skip that estimate. Use the number your supplier quoted.
This job
Add each pour — slab, piers, steps — and Concrete Calculator totals them.
How to use a post hole concrete calculator
A post hole is a short cylinder. Gravel in the bottom is not concrete — set that depth and the pour shortens. A 4×4 occupies a 3.5×3.5 in square; subtracting it avoids buying concrete for the wood. Fast-set mixes sometimes print different yields — the bag wins if the label disagrees with Concrete Calculator’s defaults.
Along a fence, posts = length ÷ spacing + 1 for the end. Gate posts are often larger — add them as a second pour. Full cardboard tubes that become the pier belong on the sonotube page, not here.
Worked example with the post hole concrete calculator
Gross ≈ 1.57 ft³. Post displaces ≈ 0.17 ft³. Concrete ≈ 1.40 ft³ → 3 bags of 80 lb after rounding. An 8 in hole of the same depth is closer to 1–2 bags.
Eighty feet of fence at 8 ft centers is 11 posts. Eleven × 3 bags is a pallet conversation, not a mystery — see bags on a pallet. Re-run the Post Hole Concrete Calculator after thickness changes — depth moves yardage faster than length.
Fence layout math
Posts = floor(run length ÷ spacing) + 1. Corners and gate openings break the rhythm — count them by hand. The fence post guide walks hole depth and frost rules without pretending one depth fits every climate.
Common mistakes
Using post length instead of hole depth.
- Counting spaces instead of posts (forget the +1).
- Fast-set bag yields differ — read the bag if you switch brands.
- Ignoring gravel depth at the bottom of the hole.
Bags, yards, or a truck?
Fence lines are bag jobs that add up. Mix in small batches so the post stays plumb.
The formula Concrete Calculator uses
πr² × (depth − gravel) − post area × same depth, times hole count.
Who this is for
Homeowners setting fence posts.
Deck builders on buried posts (not full tubes).
How many bags of concrete for a fence post — ranges
An 8 in × 2 ft hole without subtracting the post is about 0.70 ft³ — often 2 bags of 80 lb after rounding. A 12 in × 2 ft hole lands near 3 bags with a 4×4 subtracted. A 12 in × 3 ft hole climbs toward 4–5 bags. Depth and diameter both matter; “two bags per post” is folklore until you measure.
Concrete calculator for deck posts follows the same cylinder. Confirm whether the plan wants a full cardboard tube (sonotube page) or a dug hole with the post set in concrete (this page).
Related calculators and guides
Open the pages that match the next question on the job — volume is only the first number.
Frequently asked questions
How many bags per fence post?
A 12 in × 2 ft hole with a 4×4 ≈ 3 bags of 80 lb. Measure your hole.
How deep should the hole be?
Often ~⅓ of post length and below frost. Confirm locally.
Gravel at the bottom?
Yes for drainage on many fences — exclude it from the concrete depth.
Fence mode math?
Posts = floor(length/spacing) + 1.
Deck posts?
Same cylinder math — confirm diameter and depth on the plan.