The per-hole answer, with the post’s own volume subtracted. Enter hole diameter and depth, post size and hole count for concrete per hole, bags per hole and the project total.
Post Hole Calculator
The post itself displaces concrete, a 4×4 in a 3 ft hole takes up about 0.26 ft³. This tool subtracts it; most rough estimates don’t, and over-order all day.
You need (with waste)
Add a few inches of gravel at the bottom of each hole for drainage before the post goes in, that gravel is not in these figures; it replaces concrete at the very bottom, which is the point of it.
A post hole calculator subtracts the post from the hole: concrete per hole equals the hole’s cylinder minus the post’s below-ground volume. A 12 in hole, 3 ft deep, around a 4×4 post needs 2.10 ft³, 4 bags per hole with allowance, 40 bags for ten holes. Enter your hole and post above.
How to Calculate Concrete for a Post Hole
Post holes are the most-poured small job in the country and the most casually estimated, ‘about two bags a hole’ is folklore that survives because nobody checks. The real figure depends on the hole’s diameter squared, the depth, and a subtlety almost everyone misses: the post occupies space, so the concrete is the hole minus the post. This post hole calculator does the subtraction and gives per-hole and per-project numbers that survive contact with the store.
The Formula
Post volume (ft³) = post cross-section (ft²) × depth
Concrete per hole = hole − post · Bags per hole = ft³ × (1 + waste) ÷ 0.60, rounded up
Post sizes are actual, not nominal: a ‘4×4’ is 3.5 × 3.5 in (0.085 ft²) and a ‘6×6’ is 5.5 × 5.5 in (0.210 ft²). Diameter dominates everything, going from a 10 in to a 12 in hole adds 44% more concrete per hole, which is why auger size is a budget decision.
Worked Example: 12 in Hole, 3 ft Deep, 4×4 Post, 10 Holes
Hole: π × 0.5² × 3 = 2.36 ft³. Post: 0.085 × 3 = 0.26 ft³. Concrete per hole = 2.10 ft³ → × 1.10 ÷ 0.60 = 4 bags per hole. Ten holes: 40 bags (0.86 yd³ with waste). The folklore said 20 bags. The folklore is why half the fence lines in America have a second store trip in their history.
Concrete per Hole by Diameter (3 ft Deep, 4×4 Post)
| Hole diameter | Concrete per hole | Bags per hole | Bags for 10 holes |
|---|---|---|---|
| 8 in | 0.79 ft³ | 2 | 20 |
| 10 in | 1.38 ft³ | 3 | 30 |
| 12 in | 2.10 ft³ | 4 | 40 |
| 14 in | 2.95 ft³ | 6 | 60 |
| 16 in | 3.93 ft³ | 8 | 80 |
| 18 in | 5.04 ft³ | 10 | 100 |
Depth scales linearly, a 4 ft hole takes a third more than the 3 ft figures. So does the post: swap in a 6×6 (0.63 ft³ displacement at 3 ft) and each 12 in hole drops to about 1.73 ft³ of concrete.
Why Hole Waste Is Real Waste
Holes belly at the bottom, augers drift oversize in soft soil, and the last lift of each hole gets crowned and spilled. Ten percent is a fair allowance for augered holes in decent ground; hand-dug holes in rocky or rooty soil deserve 15%. Bags are rounded up per hole, you cannot carry 0.7 of a bag to the next hole and call it even, so project totals from this page already carry a quiet extra margin.
How to Measure and Calculate: Step by Step
- Set hole diameter from the post size and code: typically 3× the post width, minimum.
- Set depth below the frost line per your local code.
- Select the actual post that will stand in the hole.
- Enter diameter, depth, post and count above; note bags per hole.
- Stage the per-hole bags at each hole before mixing, fence-line production depends on it.
Worked Example 2: Mailbox Post, 10 in Hole, 2.5 ft Deep, 4×4
Hole: π × (10/24)² × 2.5 = 1.36 ft³. Post: 0.085 × 2.5 = 0.21 ft³. Concrete = 1.15 ft³ → 3 bags with allowance (2.1 exact). The single-post job is the one case where ‘a couple of bags’ is nearly right, and even here the honest count is 3, with the third bag being the difference between a crowned, shedding top and a shy one.
Common Quantities at a Glance: Cheat Sheet
| Job / quantity | Result |
|---|---|
| 12 in × 3 ft, 4×4 post | 2.10 ft³ · 4 bags per hole |
| 10 in × 3 ft, 4×4 post | 1.38 ft³ · 3 bags |
| 12 in × 4 ft, 4×4 post | 2.80 ft³ · 6 bags |
| 12 in × 3 ft, 6×6 post | 1.73 ft³ · 4 bags |
| 8 in × 2 ft, 4×4 post | 0.53 ft³ · 1 bag |
| Per extra inch of diameter (12 in base) | ≈ +17% concrete |
Practical Tips From the Job
Put 4–6 in of gravel in the bottom before the post: the post end then drains instead of sitting in a concrete cup of water, and the post lasts years longer. Crown the top of the concrete above grade and slope it away from the post. Brace the post plumb until the concrete takes up, a post that leans at hour one leans forever.
Common Mistakes to Avoid
- Using the ‘two bags per hole’ folklore for every diameter and depth.
- Forgetting the post displaces concrete (over-ordering small jobs).
- Digging the hole wider ‘for working room’ and doubling the concrete.
- Setting the post end directly in wet concrete with no gravel drainage layer.
- Skipping bracing, then straightening against half-set concrete.
What It Costs and How to Order Smart
Bags dominate this scale: 40 bags for the example is about $240 at $6, near the point where a small ready-mix or mixer-truck visit competes, if access allows. The Bag Cost Calculator runs that comparison from the total yardage (0.86 yd³ here). Fast-setting mixes carry a premium per bag; on a 10-hole job the premium is real money, so reserve fast-set for the holes that need the fence up today.
Who Uses This Calculator?
Builders setting structural posts use the per-hole figure to stage material. Contractors price fence and deck post work per hole from these counts. Engineers care about diameter and depth, not bags, but their details drive them. Architects specify the footing sizes homeowners then shop for, and homeowners finally learn why the auger size on the rental counter decides the bag count in the truck.
Post Hole Calculator FAQs
How many bags of concrete do I need per post hole?
For the standard case, 12 in diameter, 3 ft deep, 4×4 post, it is 4 × 80 lb bags per hole with a working allowance (2.10 ft³ of concrete). A 10 in hole takes 3 bags; an 8 in hole, 2. Diameter changes everything because volume follows diameter squared, so measure your auger, not your memory.
How much concrete does a 12 inch post hole take?
The hole alone is 2.36 ft³ at 3 ft deep; minus a 4×4 post’s 0.26 ft³, the concrete is 2.10 ft³ per hole, 0.08 yd³. Ten holes are 0.78 yd³ exact, 0.86 with waste. With a 6×6 post the same hole takes 1.73 ft³, because the bigger post displaces more.
How deep and wide should a post hole be?
Depth: below your local frost line, a code figure that runs from under a foot in warm regions to 4 ft in cold ones. Diameter: commonly about 3× the post width (12 in for a 4×4) for fence posts, wider for load-bearing deck posts per the footing tables. This calculator takes the diameter and depth you or your code choose and returns the concrete.
Should I put gravel at the bottom of a post hole?
Yes, a few inches of compacted gravel under the post gives the end grain drainage instead of a concrete cup that holds water against the wood. It slightly reduces the concrete (the gravel occupies the bottom inches), which this page’s waste allowance absorbs. For structural posts, follow the footing detail: some designs bear the post on a concrete pad at the bottom instead.
Can I dry-pour concrete into post holes and add water?
The ‘dry pack and hose’ method is widely used for fence posts and widely argued about: it can work for non-structural posts when done exactly per the bag maker’s directions, but mixing properly gives a stronger, verifiable result and is the right call for deck, pergola or any load-bearing post. If the post holds something up, mix the concrete.
How long before a post set in concrete can be used?
Fast-setting mixes allow fence building in a few hours per their labels; standard mixes want a day before rails and tension, and several days before real loads like gates or deck framing. The concrete around a post is a small footing, treat its curing like one, especially in cold weather when everything slows down.
What if my hole is bigger than planned?
Recompute with the as-dug diameter before mixing, the cost of guessing is per-hole and multiplies down the line. A 14 in hole dug where 12 was planned raises concrete per hole by about 40% (2.95 vs 2.10 ft³). Soft soil bellies holes; the waste allowance covers a little of it, not an auger size.
Do gate posts need bigger holes?
Usually yes: gate posts carry a swinging, latching load at the top of a lever, so they commonly get a larger diameter and depth than line posts, sometimes with the two posts tied by a concrete grade beam in problem soils. Size the gate holes from the gate’s weight and your code’s guidance, then count them separately here.
How accurate is this post hole calculator?
The cylinder-minus-post geometry is exact; field accuracy depends on the dug hole matching the entered diameter. Augered holes in stable soil track the calculation within the waste allowance. Hand-dug and belled holes vary more, measure a representative hole’s actual width at depth if the count is large and the budget is tight.
Related Calculators
Sources used for standards and conversions: National Ready Mixed Concrete Association (NRMCA) guidance on ordering and yield; American Concrete Institute (ACI) practice for mix and placement; Portland Cement Association (PCA) design and control guidance; US manufacturers’ printed yields (80 lb bag = 0.60 ft³). Depths, sizes and spacings set by your local building code govern wherever they differ from the general figures here.
Disclaimer: Results are estimates for planning and ordering. Check the quantities with your supplier before you order. Footings, walls, columns, beams and any other load-bearing work need a licensed engineer to look them over.