Fence post concrete calculator
Enter the post, the hole and how many posts you are setting to see the bags of concrete per post and in total.
Enter your measurements
More options: Gravel · Top · Extra …
- Bags per post
- 3.2 bags
- If you share batches between holes
- 32 bags · 50 lb bag
- Concrete for all posts
- 11.8 cu ft (0.44 cu yd)
- Gravel for the hole bottoms
- 1.82 cu ft (0.0673 cu yd)
- Quikrete’s rule of thumb for this post
- Hole about 10-1/2 in wide and 32 in deep for an 8 ft post
Hole 3.1416 × (10 in ÷ 2)² minus the post, × 28 in of concrete (after gravel and soil cap)
+ 10% = 1.18 cu ft per post ÷ 0.375 cu ft per 50 lb bag = 3.2 bags per post
Round up per post, × 10 posts = 40 bags (sharing batches: 32 bags)
- 40 bags rounds up hole by hole (4 a post) — the safe number if you mix each hole on its own. If you wet-mix in a wheelbarrow and share the last batch of each mix between holes, 32 is enough.
- Dry-pouring fast-setting mix into a hole deeper than 30 in? Quikrete says fill to 30 in or less, let the water soak all the way in, then add the rest of the mix and water.
- Call 811 (in the US) or your local utility locate service a few days before you dig.
Quick answers for common sizes
| Example | Answer |
|---|---|
| How much concrete for a 4×4 post | 3 bags of 80 lb (or 5 of 50 lb fast-setting) for a 10 in hole 3 ft deep |
| How much concrete for a 6×6 post | 4 bags of 80 lb for a 12 in hole 3 ft deep |
| How many bags of concrete for a mailbox post | 3 bags of 50 lb fast-setting (or 2 of 80 lb), 2 ft deep |
| How much concrete for a metal fence post | 3 bags of 50 lb fast-setting for an 8 in hole 30 in deep |
Bags per post by hole diameter (post, depth and bag as above)
| Hole diameter | Bags per post | Bags to buy | Concrete for all posts |
|---|---|---|---|
| 6 in | 0.8 bags | 10 bags | 2.86 cu ft |
| 8 in | 1.8 bags | 20 bags | 6.78 cu ft |
| 10 in | 3.2 bags | 40 bags | 11.8 cu ft |
| 12 in | 4.8 bags | 50 bags | 18 cu ft |
| 14 in | 6.7 bags | 70 bags | 25.3 cu ft |
| 16 in | 9 bags | 90 bags | 33.7 cu ft |
Post concrete fills the hole around the post, so the calculator subtracts the post itself — and any gravel in the bottom or soil on top — before dividing by the bag yield. A 4×4 in a 10 in hole 30 in deep takes about 1.3 cu ft with 10% extra: roughly 3½ bags of 50 lb fast-setting mix, or just over 2 bags of 80 lb.
The formula
concrete depth = hole depth − gravel − soil cap concrete per post = (3.1416 × (hole ÷ 2)² − post cross-section) × concrete depth bags per post = concrete × (1 + extra) ÷ bag yield
| Post | Cross-section | Typical hole (3× width) |
|---|---|---|
| 4×4 wood | 12.25 sq in (3½ × 3½) | 10–12 in |
| 6×6 wood | 30.25 sq in (5½ × 5½) | 16–18 in |
| 1⅝ in steel line post | 2.1 sq in | 6–8 in |
| 2⅜ in steel terminal post | 4.4 sq in | 8–10 in |
Fast-setting or regular concrete?
Quikrete’s Fast-Setting Concrete Mix (50 lb ≈ 0.375 cu ft) is poured dry into the hole around a braced post, then soaked with about a gallon of water per bag; it sets in 20–40 minutes and Quikrete says to wait 4 hours before putting strain on the post. Its standard Concrete Mix (80 lb ≈ 0.60 cu ft) is mixed wet first, and needs 24 hours before the post takes a load. Fast-setting costs more per cubic foot but saves bracing time on a long fence line.
How deep and how wide?
Quikrete’s instructions use a hole about three times the post width and one-third of the post’s total length deep — about 32 in for an 8 ft post. Where the ground freezes, local rules and good practice often want the bottom below the frost line, which can be deeper. Fence height, setbacks and post depth for gates and tall fences are local matters; check before you dig.
Worked examples
- How much concrete for a 4×4 post — the everyday wood fence post, 2 and 3 ft deep.
- How much concrete for a 6×6 post — gate, pergola and deck posts.
- How many bags of concrete for a mailbox post — one post, done in an afternoon.
- How much concrete for a metal fence post — chain-link line and terminal posts.
Tips
- Crown the top of the concrete so water runs away from the post instead of pooling against it.
- Use posts rated for ground contact. Some builders put gravel under wood posts for drainage; Quikrete’s standard method puts 6 in of dry mix in the bottom instead. The calculator starts at 4 in of gravel, a common choice; set it to 0 if you follow Quikrete’s method.
- Set corner and end posts first, string a line between them, then set the line posts to the string.
- Buy a spare bag or two. An open hole with half a bag missing is a bad place to be when the store has closed.
What you’ll need
- Post hole diggerMid-range price
Digs neat, straight-sided holes. For more than a dozen holes, rent a powered auger and use this to clean them out.
See options on Amazon (opens in a new tab) - Post levelBudget price
Straps to a post or tube form and reads plumb in both directions at once, so you can set it while the concrete is still soft.
See options on Amazon (opens in a new tab) - Mortar and concrete mixing tubBudget price
A wide, shallow tub for one or two bags at a time — easier to clean and to pull a hoe through than a wheelbarrow.
See options on Amazon (opens in a new tab) - Nitrile-coated work glovesBudget price
Wet concrete and mortar are caustic. Keep the cement off your skin with waterproof gloves, and wash off any that gets in.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 in a 10 in hole 30 in deep, about 3.4 bags of 50 lb fast-setting or 2.1 bags of 80 lb standard mix, with 10% extra. Deeper or wider holes take more — enter yours above.
Do you mix fast-setting concrete before pouring it in the hole?
No — Quikrete’s fast-setting method pours the dry mix into the hole around the post and adds about a gallon of water per 50 lb bag. Its standard Concrete Mix is mixed with water first.
How long before I can attach rails to a post set in concrete?
Quikrete says 4 hours for its fast-setting mix and 24 hours for standard Concrete Mix before the post is put under strain. The cure time calculator covers slabs and other pours.
Should the concrete go all the way to the top of the hole?
Either works. Quikrete’s fast-setting method stops 3–4 in below the surface and fills the rest with soil or sod; its standard method fills to the surface and slopes the top away from the post. Use the soil-cap field to match.
Sources
- Quikrete Fast-Setting Concrete Mix (No. 1004) data sheet (opens in a new tab) — 50 lb ≈ 0.375 cu ft, 60 lb ≈ 0.45 cu ft, 25 kg ≈ 11.6 L, 30 kg ≈ 14 L; sets in 20–40 minutes; post holes 3× the post width and ⅓ of the post length deep; wait 4 hours before loading a post.
- Quikrete Concrete Mix (No. 1101) data sheet (opens in a new tab) — Yields: 40 lb ≈ 0.30, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60, 90 lb ≈ 0.675 cu ft; unit weight ≈ 140 lb/cu ft; for pours 2 in thick or more; 6–9 pints of water per 80 lb bag; control joints every 6–8 ft; cure 5 days at 70 °F or above, 7 days at 50–70 °F; protect from freezing for 48 hours.
- 811 — Before You Dig (opens in a new tab) — The free national before-you-dig service; contact it a few business days before digging.