Quick answer: a 4×4 post set in a 12 inch wide, 2 foot deep hole needs about 2.3 bags of 80 lb concrete mix, so plan on 3 bags per post. Hole size follows a simple rule from QUIKRETE’s data sheet: dig the hole about three times the post’s diameter and a depth of one third of the post’s overall height. Then subtract the volume of the post itself, because the concrete only fills the space around it.
For other sizes, use the Post Hole Concrete Calculator. This guide shows how it works and gives a chart you can scan.
The sizing rules
QUIKRETE’s method for setting fence posts, in its concrete mix data sheet, says:
- Dig the hole about 3 times the diameter of the post.
- Make the hole depth one third of the overall post height.
- Put 6 inches of dry concrete mix in the bottom, position the post, and check that it is level and plumb.
- Mix concrete with water, then fill the hole.
- When standing water has evaporated, smooth the surface and taper it away from the post so rain runs away.
- Wait 24 hours before the post takes any strain.
For load-bearing applications such as deck posts, the same data sheet says to follow local building codes for the footing. Frost depth can also require a deeper hole than the one-third rule, so ask your building department what applies where you live.
Because the hole is one third of the overall height, the part above ground is two thirds. A post with 6 ft above ground is 9 ft long overall and goes in a 3 ft deep hole. QUIKRETE’s setting-posts page gives a slightly different rule, a depth of one third to one half of the above-ground height plus 6 inches for gravel (about 30 to 42 inches for a 6 ft fence), so a 3 ft hole sits inside both versions.
The formula
Hole volume (ft³) = π × (hole diameter ÷ 2)² × depth
Post volume (ft³) = post width × post width × depth (square post, in feet)
Concrete (ft³) = hole volume − post volume
Bags = concrete ÷ yield per bag, rounded up
An 80 lb bag yields about 0.60 ft³ (QUIKRETE). A 4×4 post is really 3.5 inches square, which is 0.292 ft.
Worked example: one 4×4 post, 12 in hole, 2 ft deep
- Hole: π × (0.5)² × 2 = 1.571 ft³.
- Post: 0.292 × 0.292 × 2 = 0.170 ft³.
- Concrete: 1.571 − 0.170 = 1.40 ft³.
- Bags of 80 lb: 1.40 ÷ 0.60 = 2.33, so 3 bags if you buy for a single post.
Bags per hole for a 4×4 post
These numbers are 80 lb bags per hole, before rounding up. For a whole fence, add the figures for every hole and round up once, not per post.
| Hole depth | 10 in hole | 12 in hole |
|---|---|---|
| 2 ft | 1.5 | 2.3 |
| 2.5 ft | 1.9 | 2.9 |
| 3 ft | 2.3 | 3.5 |
| 3.5 ft | 2.7 | 4.1 |
| 4 ft | 3.1 | 4.7 |
Check the hole size rule against your post. A 4×4 post is 3.5 inches across, so three times that is about 10.5 inches, which is why 10 and 12 inch holes are the usual choices. A 6×6 post (actually 5.5 inches) calls for a hole of around 16 inches.
Example for a whole fence: 10 posts
Ten 4×4 posts in 12 inch holes, 2 feet deep:
- Concrete: 1.40 × 10 = 14.0 ft³
- Bags: 14.0 ÷ 0.60 = 23.3
- With 10% extra: 15.4 ft³, or 26 bags
Buying 26 bags is cheaper than a second store trip when the last post comes up short.
Mixing water
QUIKRETE gives about 6 pints of water per 80 lb bag as a starting point. The final amount is between about 6 and 9 pints per bag, depending on how workable the mix needs to be. For 26 bags that is roughly 20 to 29 gallons of water. If you are mixing in a wheelbarrow, mix only what you can place in a few minutes.
Dry-pour or wet mix?
The QUIKRETE data sheet describes mixing the concrete with water before placing it in the hole, after putting 6 inches of dry mix in the bottom. Some bagged products are made for pouring dry into the hole and adding water on site, but follow the instructions for the specific bag you buy. This guide uses the standard concrete mix method.
What this number doesn’t include
- A gravel base or drainage layer. If you add one, subtract it from the depth you fill with concrete.
- Gate posts, corners and end posts. These carry more load and are often set deeper or larger. Use your fence manufacturer’s instructions and local requirements.
- Rounding for waste. Spills, uneven holes and holes dug slightly oversized all use more than the formula.
Common mistakes
- Forgetting the post’s volume. The post displaces concrete, which is why the chart numbers are lower than a full cylinder.
- Using the nominal post size. A 4×4 is 3.5 inches wide, a 6×6 is 5.5 inches.
- Digging holes too narrow. If the hole is barely wider than the post, there is not enough concrete around it to hold the post against side loads. Follow the three times rule.
- Putting strain on posts too soon. Wait 24 hours, as QUIKRETE recommends, before hanging gates or tensioning wire.
Frequently asked questions
How many bags of concrete for one fence post?
For a 4×4 post in a 12 inch hole 2 feet deep, about 2.3 bags of 80 lb mix, so 3 bags. A 10 inch hole of the same depth needs about 1.5 bags.
How deep should a fence post hole be?
QUIKRETE’s rule is one third of the post’s overall height. Local frost depth or code may require more, so check with your building department.
How wide should a post hole be?
About three times the post’s width. For a 4×4, that is 10 to 12 inches.
How many bags of concrete for a 6 foot fence?
The count depends on the post spacing, not just the height. A 6 foot fence usually uses posts about 9 feet long, set 3 feet deep, which is about 3.5 bags per post in a 12 inch hole. Multiply by your number of posts and add 10%.
Should I put gravel at the bottom?
QUIKRETE’s method starts with 6 inches of dry concrete mix in the bottom of the hole. Other builders use gravel for drainage. Either way, count the gravel’s depth out of your concrete volume.
Sources
- QUIKRETE, >Concrete Mix Product No. 1101 data sheet: method for setting fence posts, bag yields, mixing water.
- NIST, >Guide to the SI, Appendix B.8: unit conversions.
Post size, spacing, frost depth and gate loads vary. Follow your fence manufacturer’s instructions and local code.