Post Hole Concrete Calculator
Concrete for fence posts, mailbox posts and deck posts — by hole size, depth and post width.
This post hole concrete calculator works out the concrete for setting posts — fence posts, a mailbox post, deck and pergola posts — from the hole diameter, the hole depth, and the size of the post that displaces some of that volume.
Choose the circle shape, enter the hole diameter and depth, set the number of posts, and the tool returns the total concrete in bags and cubic feet. Subtract the post volume for a wood or steel post that stays in the concrete.
How deep and how wide
Bury at least a third of the post's above-ground height, and never less than 2 feet — deeper for corner and gate posts, and below the frost line in cold climates. The hole should be about three times the post width: a 4×4 post wants a 10 to 12 inch hole.
A 10-inch hole 2 feet deep holds about 1.1 cubic feet. Subtract roughly 0.2 cubic feet for the 4×4 post and you need about 0.9 cubic feet per post — one and a half 60 lb bags, or one 80 lb bag.
Dry-set or mixed
For a fence, many builders dry-set: pour dry pre-mix around the plumbed post, then soak it with water. It is faster over dozens of posts and cures hard within a day. For gate posts and structural posts, mix the concrete properly and crown the top so water sheds away from the wood.
Put 3 to 6 inches of gravel in the bottom of the hole first so the post end can drain and does not sit in trapped water.
Buying for a whole fence
Multiply the per-post figure by the number of holes and add 10%. A 40-post fence at one 60 lb bag per post is about 44 bags — a full pallet. At that scale, price a ready-mix trailer or bulk bag delivery against the hardware-store bag price.
Common post-setting mistakes
Hole too narrow. Aim for about three times the post width so there is a real concrete collar — a 10 to 12 inch hole for a 4×4.
Not deep enough. At least a third of the above-ground height, never under 2 feet, and below the frost line for gate and corner posts.
No gravel at the base. A few inches of stone lets the post end drain instead of sitting in trapped water.
Flat or dished top. Crown the concrete above grade so water runs off the post.
| Product | Yield | Per cubic yard | Best for | Notes |
|---|---|---|---|---|
| Ready-mix (delivered) | 1 yd³ per yd³ | — | Slabs, driveways, footings > ½ yd³ | Short-load fee under ~1 yd³ |
| 80 lb bag | ~0.60 ft³ | 45 bags | Small pads, posts, repairs | ~4,000 psi mixed |
| 60 lb bag | ~0.45 ft³ | 60 bags | Post setting, small footings | Common fence-post size |
| 50 lb bag | ~0.375 ft³ | 72 bags | Small repairs | |
| 40 lb bag | ~0.30 ft³ | 90 bags | Patching, setting small posts | |
| Cured concrete weight | — | ~4,000 lb (2 tons) | — | 145–150 lb/ft³ |
Every material calculator
Each of these uses the same measured-drawing tool with the right density and bag sizes built in: gravel calculator , mulch calculator , concrete calculator , concrete slab calculator , sand calculator , topsoil calculator , paver sand calculator , crushed stone calculator , sonotube concrete calculator , pea gravel calculator , river rock calculator , decomposed granite calculator . Or start from the material calculator homepage and pick a material there.
Frequently asked questions
How much concrete do I need per fence post?
A 4×4 post in a 10-inch hole 2 feet deep needs about 0.9 cubic feet after subtracting the post — roughly one and a half 60 lb bags or one 80 lb bag.
How many bags of concrete for a 6 foot fence post?
A 6-foot-tall fence post set 2 feet deep in a 10 to 12 inch hole takes one to two 60 lb bags depending on hole width. Corner and gate posts go deeper and wider.
How deep should a fence post be?
At least one-third of the above-ground height and never less than 2 feet, going below the frost line in cold regions and deeper for gate and corner posts.
How wide should a post hole be?
About three times the post width — a 10 to 12 inch hole for a 4×4, 12 inches for a 6×6.
Can I just pour dry concrete in a post hole?
For non-structural fence posts, yes — set the post plumb, pour dry pre-mix around it, then saturate with water. It cures hard in a day. Mix properly for gate and structural posts.
Should I put gravel at the bottom of a post hole?
Yes, 3 to 6 inches, so water drains away from the post end instead of pooling and rotting the wood.
How many 60 lb bags of concrete make a cubic yard?
About 60. Each 60 lb bag yields roughly 0.45 cubic feet.
Should the concrete be above ground level?
Crown it an inch or two above grade and slope it away from the post so rainwater sheds off rather than collecting against the wood.
How many bags of concrete for one fence post?
A 4×4 in a 10-inch hole 2 feet deep needs about 0.9 cubic feet after the post is subtracted — roughly one and a half 60 lb bags, or one 80 lb bag.
Is it OK to set fence posts with dry concrete?
For ordinary fence line posts, yes — set the post plumb, pour dry pre-mix around it, then saturate with water. Mix properly for gate and corner posts.
How deep should a 6-foot fence post be?
About 2 feet for a line post, 2.5 to 3 feet for gate and corner posts, and always below the frost line in cold regions.
How much concrete for a mailbox post?
A single 4×4 mailbox post in an 8-inch hole 18 inches deep needs about 0.5 cubic feet — one 60 lb bag.
Should concrete go all the way to the bottom of the hole?
Put 3 to 6 inches of gravel in first for drainage, then fill with concrete from the gravel up to just above grade.