How Much Concrete for Fence Posts?
Most residential fence posts need about one to three 60–80 lb bags of concrete each, depending on hole diameter and depth. An 8-inch-wide, 2-foot-deep hole holds roughly 0.70 cubic feet—about two bags of standard mix after waste. Multiply by your post count, and dig below the local frost line.
How it's calculated
Treat each fence-post hole as a cylinder. Convert the hole diameter to feet, then:
Volume (cu ft) ≈ π × (diameter ÷ 2)² × depth, with diameter and depth in feet.
Example conversion: an 8-inch diameter is 8÷12 = 0.667 ft, so the radius is 0.333 ft. For a 2-foot depth, volume ≈ 3.1416 × 0.333² × 2 ≈ 0.70 cu ft. Bags ≈ ceil(volume ÷ bag_yield). YardTally’s typical yields match common bagged concrete coverage: about 0.30 cu ft (40 lb), 0.45 cu ft (60 lb), and 0.60 cu ft (80 lb). Multiply by the number of posts, then apply a waste factor (often about 10%) for over-excavation and spillage.
Post count comes from fence length and spacing. A 96 ft run with 8 ft bays needs 13 posts if you count both end posts (12 bays + 1). Use the fence calculator for picket or panel layouts, then feed hole size into the post hole concrete calculator—not the rectangular slab calculator.
Quick chart (approx., before waste)
| Hole diameter | Depth | Volume | 80 lb bags |
|---|---|---|---|
| 8" | 2 ft | 0.70 cu ft | 2 |
| 10" | 2.5 ft | 1.36 cu ft | 3 |
| 12" | 3 ft | 2.36 cu ft | 4 |
Bag counts in the chart round up per hole using a 0.60 cu ft yield. Your brand’s fast-setting mix may list a different yield—always check the bag.
Worked example: 12 posts
Twelve 8" × 2 ft holes ≈ 12 × 0.70 = 8.4 cu ft. With 10% waste → about 9.2 cu ft. At 0.60 cu ft per 80 lb bag that is ceil(9.2 ÷ 0.60) = 16 bags. At 0.45 cu ft per 60 lb bag, plan on 21 bags. That matches the same cylinder math the post-hole calculator uses.
Worked example: 48 ft privacy fence
At 8 ft post spacing you need 7 posts (6 sections + end). If each hole is 10" × 2.5 ft, one hole is ~1.36 cu ft. Seven holes ≈ 9.55 cu ft before waste, about 10.5 cu ft with 10% waste—roughly eighteen 60 lb bags or fifteen 80 lb bags. Brace posts plumb before you place mix, and slope the finished concrete so water drains away from the wood.
Step-by-step ordering
- Count posts with the fence calculator (include corners and gates).
- Decide hole diameter (often about 3× post width as a rule of thumb) and depth (commonly ~⅓ above-ground height, below frost where required).
- Run the post hole concrete calculator for total cubic feet and bag counts.
- Add ~10% waste and buy matching outdoor concrete or fast-setting post mix—or order ready-mix for very large runs.
- Optional: place a few inches of gravel under the post for drainage, then set, brace, and fill per the bag instructions.
Tips
- Call before you dig—utility locate services exist for a reason.
- Keep a spare bag or two on site; running short mid-set is expensive.
- Fast-setting mixes are convenient for posts; structural deck footings may need a different product and forms.
- For cardboard tube forms (deck piers), switch to the Sonotube workflow covered in the bags of concrete guide.
- Compare bag math with the general 10×10 slab guide only when you are pouring pads—not for round holes.
Common mistakes
- Setting posts too shallow for wind load or frost heave.
- Filling to the top without sloping for drainage away from the post.
- Using interior drywall or mortar mix instead of outdoor concrete/setting mix.
- Using the slab tool for round holes (use the post-hole tool instead).
- Ignoring gate posts, which often need larger diameter or deeper embedment than line posts.
- Forgetting that gravel in the hole bottom displaces some concrete—measure the concrete column you actually place.
Choosing the mix and setting method
Most bagged post-setting products fall into two camps. General-purpose concrete is mixed with water to a stiff, workable consistency and shoveled around a braced post. Fast-setting post mixes are formulated so the dry mix can be poured around the post and then wetted, which saves a mixing tub but still demands that you follow the bag directions for the water amount. Neither style changes the volume math on this page; the label simply tells you how many bags a cubic foot requires and how long you have before the mix stiffens.
Posts that carry heavy loads—gates, corner posts, or tall privacy panels that catch wind—often justify a larger hole diameter and a deeper embedment than a line post, not just more bags in the same hole. Treat the examples here as planning math for common residential line posts, and check the fence manufacturer or your local building department for gate-post and height requirements. Soft or sandy soil can also call for wider holes so the concrete collar has enough surface contact with undisturbed ground.
A practical way to sanity-check your order: count the holes you dug, note the average diameter and depth with a tape, and rerun the calculator with those real numbers before you leave the store. Holes dug with a power auger are often slightly larger than the auger bit suggests, especially in loose soil, so measuring beats assuming. If you find the holes average an inch wider than planned, the extra concrete adds up quickly across a long fence line.
When to call a pro / safety / codes
Hire a fencing or concrete contractor when soil is unstable, the fence is tall or on a steep grade, property lines are disputed, or local code requires engineered footings. Wear eye and hand protection when mixing cement products; avoid skin contact with wet concrete. Depth below frost, setback rules, and HOA limits are local—this page is a planning estimate, not a building-code opinion. Confirm requirements with your building department before you dig.
Sources
- QUIKRETE – Setting posts in concrete
- QUIKRETE calculator & yields
- Sakrete concrete calculators
- Clemson HGIC landscape resources
FAQ
How many bags of concrete per fence post?
It depends on hole diameter and depth, not post height alone. An 8-inch-wide, 2-foot-deep hole is about 0.70 cu ft—about two 80 lb bags of standard mix after 10% waste. A 12-inch × 3-foot hole is about 2.36 cu ft (four 80 lb bags before waste). Fast-Setting dry-pour products may list fewer bags—follow that label.
Should fence post holes be three times the post width?
That is a common rule of thumb (for example, a 4-inch post in a roughly 12-inch hole), and manufacturer post-setting guides often use similar proportions. Treat it as a starting point—loose soil, tall privacy panels, and frost heave may call for a wider or deeper hole or a tube form. Confirm local practice before you dig.
Do I mix concrete or pour dry mix into the hole?
Fast-setting bagged mixes are often placed dry in the hole and then watered in place for fence posts, following the bag instructions. Structural footings and deck posts usually need mixed concrete and may require forms. Read the product label; post mixes and general concrete mixes are not interchangeable for every job.
How deep should fence posts be set?
A frequent DIY rule of thumb is about one-third of the above-ground post height in the ground, and below the local frost line where frost heave is a concern. Manufacturer guides sometimes describe one-third to one-half of the above-ground length plus gravel. Always check frost maps and local code for your climate.
Can I use the slab concrete calculator for post holes?
No. Slabs are rectangular volume (length × width × thickness). Post holes are cylinders. Use the post hole concrete calculator so diameter and depth convert correctly, then convert volume to bags.
Related calculators
- Concrete Calculator for Post Holes — Fence and sign post hole concrete by diameter, depth, and count.
- Fence Calculator — Posts, rails, pickets, and post concrete bags.
- Concrete Calculator — Slabs, footings, and solid steps with cubic yards and bag counts.