Post hole concrete calculator
A 10 in hole 36 in deep for a fence post holds 1.64 cubic feet, which is 3 bags of 80 lb concrete with 10% waste. Enter the hole diameter, the depth, and how many holes, and this gives the bags and cubic yards for the whole job, rounded once on the total. For slabs, footings, and walls, use the concrete calculator.
Pouring a slab or a footing? Concrete calculator
Updated
Enter the hole the way your tape reads it: 10, 10 in, or 3 ft 6 in. Each hole is a cylinder, diameter by depth, taken as solid with no deduction for the post. The bag count rounds up once on the total, not hole by hole.
- Bags for all the holes
- 70 × 80 lb
- Bags for one hole
- 4
- Cubic feet with waste
- 42.00
- Cubic yards with waste
- 1.56
- Cubic feet per hole, before waste
- 1.909
Rounding each hole up on its own would buy 4 x 20 = 80 bags.
At 1 cu yd or more, ready-mix by the truck or a short-load usually beats bagging it.
How the math works
Each hole is a cylinder: pi times the radius squared times the depth. The calculator converts the diameter and depth from inches to feet, so a hole's volume comes out in cubic feet, then multiplies by the number of holes. Waste is added last, as a percentage of the total.
Bags are the total over the bag's yield, rounded up: an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag 0.45, a 50 lb bag 0.375, and a 40 lb bag 0.3. Rounding happens once, on the whole job. Rounding hole by hole buys a partial bag for every hole, which adds up fast on a long fence.
The hole is taken as solid, with no deduction for the post. A 4x4 (3-1/2 in square) set 36 in deep takes up about 0.26 cubic feet, roughly 16% of a 10 in hole, so the count leaves a little extra in the bucket. The CutOnce app uses the same cylinder math.
A worked example: a 20-post fence
Twenty fence posts in 10 in holes, 42 in deep, filled from 80 lb bags with 10% waste. This is one of the app's own test cases.
| One hole | 1.909 cu ft | π × 5² × 42 in, over 1,728 |
| All 20 holes | 38.18 cu ft | one hole × 20 |
| With 10% waste | 42.00 cu ft | total × 1.10 |
| In cubic yards | 1.56 cu yd | cubic feet over 27 |
| 80 lb bags | 70 bags | with waste, over 0.6, rounded up once |
| Rounding hole by hole | 80 bags | each hole rounded up to whole bags, times 20 |
At more than a cubic yard, a short-load of ready-mix is worth pricing against the bags, especially if the holes are close to the drive.
A second example: deck footings in tube forms
Six 12 in tube forms 48 in deep for deck piers hold 18.85 cubic feet. With 10% waste that is 35 bags of 80 lb mix, against 36 if each tube were rounded up on its own. IRC R403.1.4 puts the bottom of an exterior footing at least 12 in below undisturbed ground, and below the local frost depth; the frost depth comes from your jurisdiction.
Bags per hole for common sizes
One hole, 10% waste, rounded up to whole bags. Over many holes, enter the count above instead of multiplying this column, so the rounding happens once.
| Hole (diameter × depth) | Cu ft per hole | 80 lb | 60 lb | 50 lb |
|---|---|---|---|---|
| 6 in × 24 in | 0.39 | 1 | 1 | 2 |
| 8 in × 24 in | 0.70 | 2 | 2 | 3 |
| 8 in × 36 in | 1.05 | 2 | 3 | 4 |
| 10 in × 36 in | 1.64 | 3 | 4 | 5 |
| 10 in × 42 in | 1.91 | 4 | 5 | 6 |
| 12 in × 36 in | 2.36 | 5 | 6 | 7 |
| 12 in × 48 in | 3.14 | 6 | 8 | 10 |
| 16 in × 48 in | 5.59 | 11 | 14 | 17 |
Concrete per tube form
Volume per foot of a round tube form (sonotube), and the 80 lb bags for a 4 ft tube with 10% waste.
| Tube diameter | Cu ft per foot | 80 lb bags, 4 ft tube |
|---|---|---|
| 8 in | 0.349 | 3 |
| 10 in | 0.545 | 4 |
| 12 in | 0.785 | 6 |
| 14 in | 1.069 | 8 |
| 16 in | 1.396 | 11 |
| 18 in | 1.767 | 13 |
| 24 in | 3.142 | 24 |
What the app adds
This page sizes round holes in inches. The app also takes metric holes, carries the post count straight over from the fence calculator, sizes slabs, footings, and stepped pours, adds rebar and weight, and saves the job with a printable PDF. It works on the jobsite with no signal.
Common questions
- How many bags of concrete per fence post?
- For a 4x4 in a 10 in hole 36 in deep, 3 bags of 80 lb mix or 4 bags of 60 lb, with 10% waste. That hole holds 1.64 cubic feet. A wider or deeper hole changes the count, so enter yours above. On a whole fence, enter the post count rather than multiplying: 20 posts in 10 in by 42 in holes take 70 bags, not 80.
- How much concrete does a 6x6 post need?
- A 6x6 is 5-1/2 in square, and the usual hole is about three times the post width, so 16 in or more across. A 16 in hole 42 in deep takes 9 bags of 80 lb mix with 10% waste. Check the depth against your frost line and local code.
- How many bags of concrete do I need for a sonotube?
- Multiply the tube's cubic feet per foot by its height, add waste, and divide by the bag yield. Six 12 in tubes 48 in deep take 35 bags of 80 lb mix with 10% waste. The tube table above gives the volume per foot for common diameters.
- Should I subtract the post from the hole volume?
- The calculator does not, and the CutOnce app does not either. A 4x4 set 36 in deep takes up about 16% of a 10 in hole, which covers spillage and an uneven hole bottom. If you are pouring many holes and want to cut it close, lower the waste percentage instead of deducting the post.
- When should I order ready-mix instead of bags?
- Once the job passes about one cubic yard (27 cubic feet), a ready-mix truck or a short-load is usually worth pricing. Below that, bags are simpler, and post holes are often spread out enough that a truck chute cannot reach them all. The calculator flags the 1 cu yd point.
Count the bags once, for the whole job
CutOnce sizes post holes like this page, then hands the fence's post count to the concrete calculator, sizes slabs and footings, and saves the job as a PDF, with no signal needed.
Free to download, with a 7-day trial of everything. This page gives the bag count; the app keeps it with the rest of the job.
See also