Post Hole Concrete Calculator: How Many Bags?
Calculate how many bags of concrete you need to set fence posts, deck posts, a mailbox or a flagpole — after subtracting the volume of the post itself.
A 4×4 post in a 10-inch hole 30 inches deep, with a 4-inch gravel base, takes about 1 cubic foot of concrete, or roughly 1.7 bags of 80-lb mix, so plan on 2 bags per post. Ten posts need about 19 bags including 10% extra.
Your numbers
Result
How much post hole concrete do you need?
Post holes are cylinders: π × radius² × depth. Subtract the volume of the post that sits in the concrete, and what remains is the concrete you need.
concrete = hole volume − post width² × concrete depth
The 80-lb bag yields about 0.60 cubic feet, the 60-lb bag 0.45 and the 50-lb bag 0.375. Divide the total volume by the yield and round up.
Bags per post (4×4 post, 4 in gravel base, 80-lb bags)
| Hole size | Concrete per post (ft³) | 80-lb bags per post |
|---|---|---|
| 8 in dia. × 24 in deep | 0.44 | 0.7 |
| 10 in dia. × 24 in deep | 0.77 | 1.3 |
| 10 in dia. × 30 in deep | 1.00 | 1.7 |
| 12 in dia. × 30 in deep | 1.52 | 2.5 |
| 12 in dia. × 36 in deep | 1.87 | 3.1 |
Hole size rules of thumb
- Diameter: about three times the post's width (10 in for a 4×4, 12–16 in for a 6×6).
- Depth: at least one-third of the post's total length, and below your local frost line for decks and anything that must not heave.
- Place 4–6 inches of gravel at the bottom for drainage, then slope the concrete away from the post at the surface.
Ready-mix for many posts
For more than about ten posts, ready-mix delivered by the cubic yard may be cheaper than bags — use the cubic yard calculator to compare. For the fence itself see the fence calculator, and for deck footings the deck calculator. Larger pours (slabs, walls) are covered by the concrete calculator.
Frequently asked questions
How many bags of concrete for a 4×4 fence post?
In a 10-inch diameter, 30-inch deep hole with a 4-inch gravel base, a 4×4 post needs about 1.7 80-lb bags, so plan on 2 bags per post.
How deep should a fence post hole be?
Typically one-third of the post's total length — roughly 24–30 inches for a 6-foot fence — and below the frost line where required.
Do I need gravel in the hole?
Yes. Gravel at the bottom helps water drain away from the post and extends its life.
Can I use fast-setting concrete for posts?
Yes. Fast-setting mix is designed for posts: pour it dry around a plumb post and add water on top, per the bag instructions.
How many bags of concrete for 10 fence posts?
About 17–20 bags of 80-lb mix for ten 4×4 posts in 10×30 in holes.
How many bags for a 6×6 deck post?
A 12-inch hole 36 inches deep with a 6×6 post takes roughly 2.6 80-lb bags per footing.
How long before I can attach the fence rails?
Fast-setting mixes allow work after about 20–40 minutes; regular mixes need 24–48 hours.
Should concrete go to the top of the hole?
Fill to a few inches below grade for soil cover, or finish just above grade and slope it away so water runs off the post.
Sources and further reading
- Call 811 before you dig — Common Ground Alliance
- Prescriptive Residential Wood Deck Construction Guide (DCA 6) — American Wood Council (post and footing guidance)
These references support the general guidance on this page. The calculator applies the standard formulas shown above to the numbers you enter.
Related calculators
. Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.