Concrete Fence Post & Hole Calculator: Bags to Set Each Post
This calculator finds the concrete needed to set round fence posts by taking the hole's diameter and depth, then subtracting the volume the post itself occupies. Enter your post count to get bags per hole and the total for the entire fence line.
Estimated concrete
Estimates only. Add ~5–10% for waste. Bag yields: 40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft. Concrete ≈ 150 lb per cu ft.
How to Use the Fence Post Calculator
- 1
Enter your dimensions
Type the length, width and thickness of your slab, footing or pad and choose your units, feet, inches or meters.
- 2
Get instant volume
See the result in cubic yards, cubic feet and cubic meters, plus weight, the moment you type, no page reloads.
- 3
Buy the right amount
We convert volume into the exact number of 40, 60 and 80 lb bags, truck loads and cost so you order without waste or shortfalls.
The formula
concrete = [π × (hole_dia_in ÷ 24)² × hole_depth_ft − post_volume_ft³] × number_of_posts
- ·hole_dia_in = auger/hole diameter (in)
- ·hole_depth_ft = hole depth (ft)
- ·post_volume_ft³ = volume the post occupies in the hole
- ·number_of_posts = total posts to set
- ·÷24 converts diameter in inches to a radius in feet
Each hole is a cylinder with the post's volume removed. Divide the diameter by 24 to get the radius in feet (÷2 for radius, ÷12 for inches to feet), and subtract the buried part of the post.
Worked examples
Example 1: One 4×4 post, 10 in hole, 2 ft deep
- Hole = π × (5÷12)² × 2 = π × 0.4167² × 2 = 1.09 ft³
- Post = (3.5÷12)² × 2 = 0.29² × 2 = 0.17 ft³
- Concrete = 1.09 − 0.17 = 0.92 ft³
- 60 lb bags = 0.92 ÷ 0.45 = 2.05 → 3 bags
Result: ≈0.92 ft³ per post (3 × 60 lb bags)
Example 2: Eight 4×4 posts, 8 in hole, 30 in deep
- Hole = π × (4÷12)² × 2.5 = π × 0.333² × 2.5 = 0.87 ft³
- Post = (3.5÷12)² × 2.5 = 0.213 ft³
- Per hole = 0.87 − 0.213 = 0.66 ft³ → ×8 = 5.28 ft³
- 80 lb bags = 0.66 ÷ 0.60 = 1.1 → 2 per post = 16 bags
Result: ≈5.28 ft³ total (16 × 80 lb bags)
Concrete calculators by project type
Concrete bag yield & coverage
How much mixed concrete a standard bag yields, and how far a cubic yard goes at common slab depths.
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
- 40 lb
- 60 lb
- 80 lb
Related guides
Fence Post Concrete FAQs
How much concrete do I need to set a fence post?
Most 4x4 posts in a 10-inch by 24-inch hole take about one to two 50-pound bags of fast-setting concrete each. Enter your hole diameter, depth, and post size above for an exact bag count.
How deep should a fence post hole be?
A good rule is to bury one-third of the post length, or at least below your local frost line. For a standard 6-foot fence, dig holes about 24 to 30 inches deep.
How wide should a fence post hole be?
The hole should be about three times the post width. A 4x4 post needs roughly a 10- to 12-inch diameter hole so there's enough concrete collar around it.
Do I subtract the post volume when calculating concrete?
Yes, the concrete only fills the space around the post, so this calculator subtracts the post's volume from the hole volume. Skipping that step overestimates how many bags you need.
Can I use fast-setting concrete for fence posts?
Yes, fast-setting mix is popular for posts because you can pour it dry into the hole and add water, with no mixing. It sets in 20 to 40 minutes, letting you brace and move on quickly.
How many bags of concrete per fence post?
For a typical 4x4 post in a 10-inch, 2-foot-deep hole, plan on one to two 50-pound bags of fast-setting concrete. Deeper or wider holes for gate and corner posts will need more.
Should I put gravel at the bottom of a fence post hole?
Yes, a few inches of gravel below the post improves drainage so water does not sit against the post base and rot or rust it. Set the post on the gravel, then pour concrete around the sides. The gravel layer slightly reduces the concrete volume, so measure from the top of the gravel.
Can I set a fence post with a dry pour instead of mixing?
You can pour fast-setting mix dry into the hole around the post and then add water, which is a common shortcut for fence posts. It works because the small hole volume hydrates fully. For structural posts or large holes, mixing first gives more reliable strength.
How long before concrete around a fence post sets?
Fast-setting mixes firm up in 20 to 40 minutes and hold the post within a few hours, while standard mixes need to be braced for 24 to 48 hours. Wait a full day before hanging gates or applying tension. Full cure still takes about 28 days.
What is the rule of thumb for post hole depth versus post height?
A common guideline is to bury about one-third of the post's above-ground height, with a minimum of two feet and below the local frost line. A 6-foot-tall fence post therefore sits in a hole roughly 2 to 3 feet deep. Deeper holes need more concrete, which the calculator accounts for.
Do corner and gate posts need more concrete?
Yes. Corner, end, and gate posts carry more lateral load, so they are usually set in wider, deeper holes with more concrete than line posts. Increase the hole diameter or depth for those posts and calculate them separately. Gate posts especially benefit from a larger footing.
Should the concrete around a fence post be sloped at the top?
Yes, mound and trowel the top of the concrete so it sheds water away from the post rather than pooling against it. This slows rot and rust at the most vulnerable spot. Finish the crown slightly above grade for the same reason.
Can I reuse leftover concrete from setting posts?
No. Once concrete starts hydrating it cannot be saved for later, so mix only what each hole needs. Ordering by the per-post volume above helps avoid waste. If you do have extra fresh mix, use it immediately on another hole.