How much concrete does a post hole take?
A post hole is a cylinder with the post standing in the middle, so the concrete is (hole area − post area) × concrete depth. Work in inches and divide by 1,728 to get cubic feet, then divide by the bag yield from the manufacturer's data sheet: 0.375 cu ft for a 50-lb bag of Fast-Setting, 0.45 for 60 lb, 0.60 for an 80-lb bag of Concrete Mix.
Worked example — a 6 ft fence on 4×4 posts: bury a third of the 9 ft post (36 in), dig a hole 3× the post width (10½ in) and put 6 in of gravel in the bottom. The concrete runs from the gravel to 3 in below grade, so it is 33 in deep. Hole area 86.6 sq in − post 12.25 sq in = 74.3 sq in × 33 in = 2,453 cu in = 1.42 cu ft. That is 3.8 bags of 50-lb Fast-Setting per post. For a whole fence, multiply by the number of posts, add 10% and round up once.
Hole size and depth rules
- Width: about 3× the post. Quikrete and Sakrete both say so: 10½ in for a 4×4 (3½ in actual), 16½ in for a 6×6 (5½ in actual). If your auger is a different size, enter the real hole diameter — the count changes a lot with it.
- Depth: one-third of the post's overall length. That is the same as half the height above ground — a 6 ft fence needs 3 ft in the ground and a 9 ft post.
- Bottom of the hole: Quikrete's how-to adds 6 in of gravel for drainage; its Concrete Mix sheet puts 6 in of dry mix there instead; its Fast-Setting sheet stands the post on the soil. Sakrete's sheets call for 3–6 in of gravel. The calculator handles all three.
- Top of the hole: stop the concrete 3–4 in below grade and finish with soil or sod, or bring it flush and slope it away from the post so water runs off.
- Frost: posts that hold up a structure (deck, pergola, carport) must reach at least 6 in below the frost line, and local code decides. Call 811 before you dig.
Bags per post for common fences
Hole 3× the post, 6 in gravel, concrete stopped 3 in below grade, no waste added:
| Post | Hole | 50-lb Fast-Setting | 80-lb Concrete Mix |
|---|---|---|---|
| 4×4, 4 ft fence (24 in buried) | 10.5 × 30 in | 2.4 | 1.5 |
| 4×4, 6 ft fence (36 in buried) | 10.5 × 42 in | 3.8 | 2.4 |
| 4×4, 48 in buried | 10.5 × 54 in | 5.2 | 3.2 |
| 5×5 vinyl, 36 in buried | 15 × 42 in | 7.7 | 4.8 |
| 6×6, 36 in buried | 16.5 × 42 in | 9.3 | 5.8 |
| 6×6, 48 in buried | 16.5 × 54 in | 12.7 | 8 |
Fast-Setting or regular Concrete Mix?
Fast-Setting goes into the hole dry: fill to 3–4 in below grade, then add about 1 gallon of water per 50-lb bag and let it soak in. For holes deeper than 30 in, fill and soak in lifts of 30 in or less. It sets in 20–40 minutes and the post can take rails after 4 hours. Concrete Mix costs less per cubic foot ($10.78 vs $21.25 at typical 2026 prices) but has to be mixed first, and the post must stay braced for 24 hours. On a long fence that difference adds up — the calculator shows both.
Why manufacturer numbers disagree
Quikrete's Fast-Setting bag says two 50-lb bags set a 3 in post in a 9 in hole 30 in deep. The geometry says that hole holds 2.53 bags filled to the top, and Quikrete's own online calculator rounds a 3 in round post at 30 in up to 3. Plan on three. Sakrete's table for a 10 in hole lists 1.25 bags at 12 in, 2 bags at 18 in, 2.5 bags at 24 in, 3 bags at 30 in — within a quarter bag of the same math for a 4×4. Quikrete's calculator only offers round posts:
| Post / hole | 24 in | 30 in | 36 in | 42 in | 48 in |
|---|---|---|---|---|---|
| 2 in post, 6 in hole | 1 | 2 | 2 | 2 | 2 |
| 3 in post, 9 in hole | 3 | 3 | 4 | 4 | 5 |
| 4 in post, 12 in hole | 4 | 5 | 6 | 7 | 8 |
| 6 in post, 18 in hole | 9 | 11 | 13 | 15 | 17 |
Those counts fill the whole hole with concrete; a gravel base and soil cap lower them, which is why this calculator usually comes in under the bag label.
Common mistakes
- Using the nominal size: a 4×4 is 3½ in, so the hole is 10½ in, not 12.
- Rounding up for every hole: pour the leftover half bag into the next hole and round once for the job.
- Mounding concrete above grade so water pools against the post — crown it away or stop below grade.
- Hanging gates or stretching wire before the concrete is ready: 4 hours for Fast-Setting, 24 hours for Concrete Mix.
Planning the whole fence? The fence calculator counts posts, rails and pickets. Setting deck or porch piers in tubes instead? Use the Sonotube calculator or the footing calculator.