Fence post guide
How many bags of concrete per fence post?
A 4×4 post in a 10-inch hole, 3 ft deep, takes about 3 bags of 80 lb concrete (1.38 cubic feet). The chart below covers other hole sizes, depths and 6×6 posts.
By the EstimateCement team ·
Bags per post
Concrete per post, by hole size and depth.
Each cell shows bags of 60 lb / 80 lb mix to fill the hole around the post, rounded up to whole bags. The post's own volume is subtracted.
| Hole depth | 4×4 post, 8″ hole | 4×4 post, 10″ hole | 6×6 post, 12″ hole |
|---|---|---|---|
| 24″ (2 ft) | 2 / 1 (0.53 ft³) | 3 / 2 (0.92 ft³) | 3 / 2 (1.15 ft³) |
| 30″ (2.5 ft) | 2 / 2 (0.66 ft³) | 3 / 2 (1.15 ft³) | 4 / 3 (1.44 ft³) |
| 36″ (3 ft) | 2 / 2 (0.79 ft³) | 4 / 3 (1.38 ft³) | 4 / 3 (1.73 ft³) |
| 42″ (3.5 ft) | 3 / 2 (0.92 ft³) | 4 / 3 (1.61 ft³) | 5 / 4 (2.01 ft³) |
| 48″ (4 ft) | 3 / 2 (1.06 ft³) | 5 / 4 (1.84 ft³) | 6 / 4 (2.30 ft³) |
Uses a yield of 0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag. Round holes dug with a post-hole digger or auger; a hole that flares at the bottom takes more.
Sizing the hole
About 3× the post's width, and a third of its height deep.
A common rule of thumb is a hole diameter about three times the post's width: 10–12 inches for a 4×4 (which actually measures 3½ inches) and 12–16 inches for a 6×6. Line posts on a short fence can get by with 8 inches. Gate and corner posts carry more load, so give them the bigger hole.
For depth, bury a third to half of the post's above-ground height. For a 6 ft privacy fence, that's 2 to 3 ft. In cold climates, go below the frost line.
Our fence post calculator assumes 2 bags of 60 lb mix per post, which fits an 8-inch hole about 2 ft deep. For bigger or deeper holes, use the chart above to adjust.
The math
Hole volume minus post volume.
The hole is a cylinder: π × (diameter ÷ 2)² × depth, in cubic inches. Subtract the post (width × width × depth), then divide by 1,728 to convert to cubic feet. For a 10-inch hole, 36 inches deep, with a 4×4: (3.1416 × 25 × 36 − 3.5 × 3.5 × 36) ÷ 1,728 = 1.38 ft³. Divide by the yield per bag and round up.
FAQ
Common questions
How many bags of concrete do I need for a 4×4 fence post?
For a typical 10-inch hole, 3 ft deep, about 3 bags of 80 lb mix or 4 bags of 60 lb. A smaller 8-inch hole, 2 ft deep, takes 1 bags of 80 lb or 2 of 60 lb.
How many bags of concrete do I need for a 6×6 post?
In a 12-inch hole, 3 ft deep, about 3 bags of 80 lb mix or 4 bags of 60 lb. Gate and corner posts are often 6×6 and set deeper, so check the chart for your depth.
How deep should a fence post hole be?
A common rule is one-third to one-half of the post's height above ground, so about 2 to 3 ft for a 6 ft fence. In cold climates, go below your local frost line so the post doesn't heave. Your local code or fence permit may set a minimum.
Should I put gravel in the bottom of the post hole?
Yes. 4–6 inches of compacted gravel under the post helps water drain away from the end grain. It also slightly reduces the concrete you need, so the chart's full-depth numbers leave you a little spare.
Can I use fast-setting concrete for fence posts?
Yes. Fast-setting post mix is poured dry into the hole and then watered, and it sets in 20–40 minutes. Check the bag for its yield, which can differ from standard mix, and divide your hole volume by it.
Planning the whole fence?