A 4x4 post in a 10-inch hole with 30 inches of concrete takes about 1.9 bags of 80 lb mix or 3.1 bags of 50 lb mix. Hole diameter changes the answer more than anything else, so a 12-inch hole needs roughly 50 percent more. The chart below covers common posts and holes, and the formula lets you work out any other size.
The formula
Concrete fills the hole minus the space the post takes up, from the top of the gravel base to the top of the hole.
V = [π × (D ÷ 2)² − post area] × concrete depth
- D is the hole diameter in inches.
- Post area is the cross section of the post in square inches. Use actual sizes: a 4x4 is 3½ × 3½ = 12.25 in², a 6x6 is 5½ × 5½ = 30.25 in², and a round post is π × (OD ÷ 2)².
- Concrete depth is the hole depth minus the gravel base. A 36-inch hole with 6 inches of gravel has 30 inches of concrete.
Divide the result by 1,728 to convert cubic inches to cubic feet, then divide by the bag yield.
For a 4x4 in a 10-inch hole with 30 inches of concrete: π × 5² = 78.5 in², minus 12.25 in² for the post = 66.3 in². Multiply by 30 inches = 1,989 in³, which is 1.15 ft³. At 0.60 ft³ per 80 lb bag, that is 1.9 bags.
Bag yields
Yields from Quikrete's Concrete Mix data sheet and Fast-Setting Concrete Mix data sheet:
| Bag | Yield |
|---|---|
| 40 lb Concrete Mix | 0.30 ft³ |
| 50 lb Concrete Mix or Fast-Setting | 0.375 ft³ |
| 60 lb Concrete Mix | 0.45 ft³ |
| 80 lb Concrete Mix | 0.60 ft³ |
| 90 lb Concrete Mix | 0.675 ft³ |
Other brands of bagged concrete land close to the same yield per pound, but check the bag.
Quikrete's Fast-Setting data sheet also states that two 50 lb bags will set a 3-inch square post in a 9-inch diameter hole 30 inches deep. By the formula that hole holds about 0.95 ft³ around the post, or 2.5 bags filled to the top. Quikrete's own instructions stop the concrete 3 to 4 inches below grade, which brings it to about 2.2 bags. Treat two bags as the least you will need for that hole, not a comfortable number.
Bags per post chart
All figures are bags per post before waste. Depth is the concrete depth, so subtract your gravel base from the hole depth first. Multiply by your post count, add 10 percent, and round up.
4x4 wood posts
| Hole diameter | Concrete depth | Cubic feet | 50 lb bags | 80 lb bags |
|---|---|---|---|---|
| 8" | 24" | 0.53 | 1.4 | 0.9 |
| 8" | 30" | 0.66 | 1.8 | 1.1 |
| 8" | 36" | 0.79 | 2.1 | 1.3 |
| 10" | 24" | 0.92 | 2.5 | 1.5 |
| 10" | 30" | 1.15 | 3.1 | 1.9 |
| 10" | 36" | 1.38 | 3.7 | 2.3 |
| 12" | 24" | 1.40 | 3.7 | 2.3 |
| 12" | 30" | 1.75 | 4.7 | 2.9 |
| 12" | 36" | 2.10 | 5.6 | 3.5 |
6x6 wood posts
| Hole diameter | Concrete depth | Cubic feet | 50 lb bags | 80 lb bags |
|---|---|---|---|---|
| 12" | 30" | 1.44 | 3.8 | 2.4 |
| 12" | 36" | 1.73 | 4.6 | 2.9 |
| 14" | 30" | 2.15 | 5.7 | 3.6 |
| 14" | 36" | 2.58 | 6.9 | 4.3 |
| 16" | 30" | 2.97 | 7.9 | 4.9 |
| 16" | 36" | 3.56 | 9.5 | 5.9 |
Chain link and vinyl posts
| Post | Hole | Cubic feet | 50 lb bags | 80 lb bags |
|---|---|---|---|---|
| Chain link line, 1⅝" OD | 6" × 24" | 0.36 | 1.0 | 0.6 |
| Chain link terminal, 2⅜" OD | 8" × 30" | 0.80 | 2.1 | 1.3 |
| Chain link gate, 2⅞" OD | 12" × 36" | 2.22 | 5.9 | 3.7 |
| Vinyl, 5x5 | 10" × 30" | 0.93 | 2.5 | 1.5 |
| Vinyl, 5x5 | 12" × 30" | 1.53 | 4.1 | 2.5 |
Chain link holes follow ASTM F567, which sets the diameter at four times the post's outside diameter for posts up to 4 inches and a minimum depth of 24 inches. ASTM F567 does not call for a gravel base, so the chain link figures assume concrete to the bottom of the hole. The vinyl figures assume you fill around the post, not inside it. If your vinyl manufacturer tells you to fill end, corner or gate posts with concrete, add the inside volume of the post as well.
CalculatorGet exact bag counts with the post concrete calculatorWhy "one bag per post" only works for 8-inch holes
You will hear that a fence takes one bag of concrete per post. That rule comes from narrow holes. One 80 lb bag fills an 8-inch hole around a 4x4 to about 27 inches deep, so it roughly works there.
Most guides, including Quikrete's data sheets, call for a hole three times the post width, which is 10 inches or more for a 4x4. At 10 inches the same depth takes about 1.7 bags of 80 lb mix, and at 12 inches it takes about 2.6. Volume grows with the square of the diameter, so a couple of extra inches on the auger adds up quickly across a whole fence. Buy for the hole you will actually dig.
Fast-setting vs standard concrete mix
Fast-setting mix is made for posts. Per Quikrete's data sheet, you set and brace the post, pour the dry mix into the hole to 3 to 4 inches below the top, and add about 1 gallon of water per 50 lb bag. For holes deeper than 30 inches, fill to 30 inches or less, let the water soak in, then add the rest. It sets in 20 to 40 minutes, and you wait 4 hours before putting strain on the post.
Standard concrete mix is mixed with water first, in a wheelbarrow or mixer, then placed around the post. Quikrete's Concrete Mix data sheet says to wait 24 hours before the post takes any strain. It costs less per cubic foot and comes in 80 and 90 lb bags, which means fewer bags to handle on a big job.
Use fast-setting mix when you are setting a handful of posts, working alone, or need to hang rails the same day. Use standard mix for large jobs, gate posts where you want a thorough mix, or when you have a mixer on site.
Buying: waste, hole classes and returns
- Add 10 percent. Holes come out oversized, bell out at the bottom, or cave in. A 10 percent allowance covers most of it.
- Buy per hole class. Line posts, terminal posts and gate posts usually get different hole sizes. Work out each class separately, add the waste, and round up each one. Rounding once at the end hides a shortage on the big holes.
- Keep unopened bags. Most home centers and yards accept unopened bags back, so buying a few extra costs little. Check the store's return policy before you count on it, and keep the bags dry.
Gravel under the posts
A 6-inch gravel base under each wood or vinyl post drains water away from the bottom of the post and gives it a firm seat. Quikrete All-Purpose Gravel comes in 50 lb bags that hold about 0.5 ft³.
| Hole diameter | Gravel for a 6" base | 50 lb bags per hole |
|---|---|---|
| 8" | 0.17 ft³ | 0.35 |
| 10" | 0.27 ft³ | 0.55 |
| 12" | 0.39 ft³ | 0.8 |
| 14" | 0.53 ft³ | 1.1 |
That is about half a bag per 10-inch hole. Remember to dig the extra 6 inches for the gravel, or you will lose that depth from the post's embed.
Alternatives to concrete
Concrete is the default, not the only option.
- Gravel-set posts. Compacted crushed stone, tamped in lifts of a few inches, holds a post firmly and drains well. The post can be pulled and replaced later without breaking out a footing. It suits wood posts in well-drained soil, and it works best with a slightly wider hole.
- Expanding post foam. Two-part polyurethane foam sets in minutes and weighs a fraction of concrete. Follow the manufacturer's limits on post size and fence type. It is not a substitute for concrete on gate posts or on tall solid fences in windy areas.
- Tamped earth. Split rail fences are traditionally set in the soil that came out of the hole, tamped back in firmly. The Split Rail Fence Store recommends against concrete for split rail because it traps water against the wood.
Chain link under ASTM F567 is set in concrete, and gate posts on any fence should be set in concrete.
Worked example: 24 posts
A 6-foot wood privacy fence with 20 line posts and 4 terminal posts (2 corners and 2 gate posts), all 4x4.
| Item | Line posts | Terminal posts |
|---|---|---|
| Count | 20 | 4 |
| Hole | 10" × 36" | 12" × 42" (6" deeper) |
| Gravel base | 6" | 6" |
| Concrete depth | 30" | 36" |
| Concrete per post | 1.15 ft³ | 2.10 ft³ |
| Total concrete | 23.0 ft³ | 8.4 ft³ |
| Plus 10% | 25.3 ft³ | 9.2 ft³ |
| 80 lb bags (0.60 ft³) | 42.2, buy 43 | 15.4, buy 16 |
| 50 lb bags (0.375 ft³) | 67.5, buy 68 | 24.6, buy 25 |
| Gravel, 50 lb bags | 5.5 ft³ ÷ 0.5 = 11 | 1.6 ft³ ÷ 0.5 = 3.2 |
Total: 59 bags of 80 lb mix or 93 bags of 50 lb fast-setting mix, plus 15 bags of gravel (14.1 rounded up). That is more than two tons of concrete, so arrange delivery or plan several trips, and stack the bags close to the fence line.
CalculatorEstimate the whole fence, including concrete, in the fence calculatorQuestions people ask
How many bags of concrete do I need per fence post?
For a 4x4 in a 10-inch hole with 30 inches of concrete, plan on about 1.9 bags of 80 lb mix or 3.1 bags of 50 lb mix. Smaller holes need less and larger holes need much more, so check the chart for your hole size.
Is one bag of concrete enough for a fence post?
Only for small holes. One 80 lb bag fills an 8-inch hole around a 4x4 to about 27 inches deep. A 10-inch hole of the same depth needs about 1.7 bags.
How many 50 lb bags of Quikrete per fence post?
Quikrete's data sheet says two 50 lb bags of Fast-Setting Concrete Mix set a 3-inch square post in a 9-inch by 30-inch hole. A standard 4x4 in a 10-inch hole with 30 inches of concrete takes about 3.1 bags.
How much gravel goes under a fence post?
About 6 inches. For a 10-inch hole that is roughly 0.27 cubic feet, a little over half of a 50 lb bag of all-purpose gravel.
Should I use fast-setting or regular concrete for fence posts?
Fast-setting mix is poured in dry and soaked with water, sets in 20 to 40 minutes, and is easier for a few posts. Regular concrete mix is mixed first, costs less per cubic foot, and makes sense for large jobs or a mixer.
How much extra concrete should I buy?
Add about 10 percent for irregular holes and spillage, then round up for each hole size separately. Holes are rarely the exact diameter you planned.
Use the calculators
Written by the My Fence Calculator editorial team. Quantities are estimates; verify with your supplier and local code.