For a standard 4×4 fence post set in a 10-inch diameter hole, 2 feet deep, you need about 1 bag of 80lb concrete. For a larger 6×6 post in a 12-inch diameter hole, 2.5 feet deep, you'll want 2 bags of 80lb concrete. The exact amount depends on hole diameter, depth, and post size.
A lot of fence posts, a lot of bags. If you're installing a full fence line, even being off by half a bag per post adds up fast. Here's how experienced fence contractors figure this out before they head to the hardware store.
The Formula
The volume of a cylindrical post hole = π × r² × depth
Where r = radius of the hole (half the diameter) and depth = how deep you're digging.
Then subtract the volume of the post itself (usually negligible for wood posts), convert to cubic feet, and divide by the yield of your bag size.
Practical Quick Reference
| Post Size | Hole Diameter | Hole Depth | 80lb Bags | 60lb Bags |
|---|---|---|---|---|
| 4×4 wood | 10 inches | 2 feet | 1 bag | 1–2 bags |
| 4×4 wood | 10 inches | 2.5 feet | 1–2 bags | 2 bags |
| 6×6 wood | 12 inches | 2.5 feet | 2 bags | 3 bags |
| 4" metal post | 8 inches | 2 feet | 1 bag | 1 bag |
| 6" metal post | 12 inches | 3 feet | 2–3 bags | 3–4 bags |
Values rounded up. Always buy one extra bag as insurance — concrete doesn't hold well in a partial bag.
Should You Use Fast-Setting Concrete for Fence Posts?
For fence posts specifically, fast-setting concrete (like Quikrete Fast-Setting Concrete Mix) is actually a great choice. You pour dry granules into the hole around the post, add water, and it sets in 20–40 minutes. No mixing required. The yield is slightly lower (about 0.51 cu ft per 50lb bag), so adjust your count accordingly.
The regular mix holds up just as well long-term. Fast-setting just gets you back to stringing the fence line sooner.
Depth Rules for Fence Posts
A commonly followed rule of thumb: bury the post to a depth of one-third its total length, or below the frost line (whichever is deeper). In most of the continental U.S., that means:
- 6-foot fence → 2-foot post depth minimum
- 8-foot fence → 2.5–3-foot post depth
- Cold climates → add depth to get below frost line
Step-by-Step Example
Installing 20 fence posts (4×4) in a 10-inch diameter × 24-inch deep hole. How many bags?
- Hole volume: π × (5/12)² × (24/12) = π × 0.174 × 2 = 1.09 cubic feet
- Each 80lb bag yields: 0.60 cubic feet
- Bags per hole: 1.09 ÷ 0.60 = 1.82 → round up to 2 bags
- Total for 20 posts: 20 × 2 = 40 bags of 80lb concrete
Add a few extra bags for any holes that run deeper than expected.