How Many Bags of Concrete Do You Need Per Fence Post?
The short answer
A standard 4x4 fence post set in a 10 in wide, 24 in deep hole (25 x 60 cm) needs about 0.9 cubic feet (26 L) of concrete. Per post, that is:
- 50 lb fast-setting mix - about 2.5 bags (Quikrete’s own spec sheet says two bags set a 4 in post in exactly this hole, because their method stops the pour 3-4 in below grade).
- 60 lb concrete mix - about 2 bags.
- 80 lb concrete mix - about 1.5 bags, so buy 2 for a single post.
Bigger holes and 6x6 posts run 3 to 6 bags of 50 lb mix each (2 to 4 of the 80 lb). The exact count depends on one thing only: the volume of the hole minus the volume of the post. The table below has it worked out.
Bags per post by hole size
Bag yields are Quikrete’s published figures: a 50 lb (22.7 kg) bag of Fast-Setting Concrete Mix makes 0.375 ft³ (10.6 L); a 60 lb (27 kg) bag of standard Concrete Mix makes 0.45 ft³ (12.7 L); an 80 lb (36 kg) bag makes 0.60 ft³ (17 L). The table fills the hole to grade and subtracts the post’s actual size (a 4x4 is really 3.5 in square, a 6x6 is 5.5 in).
| Hole (dia x depth) | Post | Concrete needed | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|---|
| 8 in x 24 in | 4x4 | 0.53 ft³ (15 L) | 1.4 | 1.2 | 0.9 |
| 10 in x 24 in | 4x4 | 0.92 ft³ (26 L) | 2.5 | 2.0 | 1.5 |
| 10 in x 30 in | 4x4 | 1.15 ft³ (33 L) | 3.1 | 2.6 | 1.9 |
| 10 in x 36 in | 4x4 | 1.38 ft³ (39 L) | 3.7 | 3.1 | 2.3 |
| 12 in x 24 in | 4x4 | 1.40 ft³ (40 L) | 3.7 | 3.1 | 2.3 |
| 12 in x 36 in | 4x4 | 2.10 ft³ (59 L) | 5.6 | 4.7 | 3.5 |
| 12 in x 24 in | 6x6 | 1.15 ft³ (33 L) | 3.1 | 2.6 | 1.9 |
| 12 in x 36 in | 6x6 | 1.73 ft³ (49 L) | 4.6 | 3.8 | 2.9 |
The bag columns are exact fractions on purpose. Setting one post? Round up. Setting a whole fence? Multiply the ft³ by your post count first, then divide by the bag yield and round up once. Rounding up at every post overbuys by 20-30% - the worked example below shows the difference.
How big should the hole be?
Quikrete’s post-setting guidance, which matches standard fence practice:
- Depth: bury one-third of the above-ground post height (Quikrete says 1/3 to 1/2). A 6 ft (1.8 m) fence gets a 24 in (60 cm) hole. Treat 24 in as the floor for any full-height fence, and go below the local frost line where the ground freezes.
- Diameter: 3 times the post width. A 4x4 (3.5 in wide) gets a 10-12 in (25-30 cm) hole.
- Add 6 in (15 cm) of gravel under the post for drainage, so dig 6 in deeper than the concrete depth. Posts rot from the wet bottom first; the gravel bed is cheap insurance.
Post length follows from the same rule: above-ground height plus bury depth. A 6 ft fence on 4x4s means 8 ft posts.
Worked example: a 100 ft privacy fence
Say you’re building 100 ft (30 m) of 6 ft privacy fence with posts every 8 ft (2.4 m):
- Posts: 100 ÷ 8 = 12.5, round up to 13 sections, add one end post = 14 posts.
- Concrete per post: 4x4 posts in 10 in x 24 in holes = 0.92 ft³ each.
- Total: 14 × 0.92 = 12.9 ft³ (0.48 yd³, about 365 L).
- Bags: 12.9 ÷ 0.60 = 21.5, so 22 bags of 80 lb mix. Or 29 bags of 60 lb, or 35 bags of 50 lb fast-setting.
That’s 1,760 lb (800 kg) of mix in the 80 lb option - plan two pickup trips or delivery.
Now the rounding trap: buy “3 bags of 50 lb per post” (2.5 rounded up) and you’d cart home 42 bags. Totaling first needs 35. Same fence, seven fewer bags.
👉 The Fence Calculator does step 1 instantly - enter your length and post spacing and it counts the posts, rails and pickets (100 ft at 8 ft spacing returns exactly those 14 posts). For the bag math, model one hole as a column in the Concrete Calculator (10 in diameter, 2 ft high) and it converts the volume straight into 80, 60 and 40 lb bag counts. It fills the whole cylinder without subtracting the post, so it reads slightly high - the safe side when you’re buying.
Fast-setting vs regular mix
- Fast-setting (50 lb): pour the dry mix straight into the hole around the braced post, then add water on top - no mixing tub. Per Quikrete’s data sheet it sets in 20-40 minutes and the post can take strain after 4 hours. This is the easiest DIY route for fences.
- Regular concrete mix (60/80 lb): cheaper per cubic foot, but you mix it with water first and the post must stay braced plumb at least overnight while it hardens.
- Strength is not the tiebreaker: both cure to about 4,000 psi at 28 days per Quikrete’s data sheets. Pick fast-setting for convenience, regular mix for price on big runs.
Watch-outs
- Frost heave. In cold climates the hole must reach below the frost line, often 36-48 in (90-120 cm). Your local building department publishes the depth; a heaved gate post never closes right again.
- Shed the water. Slope the top of the concrete away from the post so rain runs off instead of pooling at the wood. Quikrete’s alternative: stop the pour 3-4 in below grade and backfill with soil.
- Gates and corners. Gate posts carry swing load; go up a size on hole diameter or depth. And the calculator counts a straight run - every corner adds a post.
- Call 811 before you dig (US) to have buried utilities marked. It’s free and it’s the law in most states.
Related
- Fence Calculator - posts, sections, rails and pickets from your fence length and spacing.
- Landscaping tools - the full landscaping toolset.
- How much gravel goes under a concrete slab - the same base-and-drainage logic, slab-sized.
- How thick should a concrete slab be for a shed - more bag math for small pours.
- What does a 10x10 concrete slab cost - bags vs ready-mix money, worked out.