Fence Post Concrete Calculator
How many bags of concrete to set fence posts, deck posts or mailbox posts, or to fill a Sonotube footing, for any hole diameter and depth.
Concrete = π × (hole diameter ÷ 2)² × depth − the post's volume. A 4×4 post in a 10 in hole, 24 in deep, takes about 0.92 cu ft: 3 bags of 50 lb fast-setting or 2 of 80 lb concrete per post. A 12 in Sonotube 48 in deep takes 3.1 cu ft (6 × 80 lb bags).
Bags per hole for a 4×4 post
| Hole | 24 in deep | 30 in deep | 36 in deep |
|---|---|---|---|
| 8 in wide | 2 × 50 lb fast-set or 1 × 80 lb | 2 × 50 lb fast-set or 2 × 80 lb | 3 × 50 lb fast-set or 2 × 80 lb |
| 10 in wide | 3 × 50 lb fast-set or 2 × 80 lb | 4 × 50 lb fast-set or 2 × 80 lb | 4 × 50 lb fast-set or 3 × 80 lb |
| 12 in wide | 4 × 50 lb fast-set or 3 × 80 lb | 5 × 50 lb fast-set or 3 × 80 lb | 6 × 50 lb fast-set or 4 × 80 lb |
Sonotube footings (no post)
| Tube | 36 in deep | 48 in deep |
|---|---|---|
| 8 in | 1.05 cu ft · 2 × 80 lb | 1.40 cu ft · 3 × 80 lb |
| 10 in | 1.64 cu ft · 3 × 80 lb | 2.18 cu ft · 4 × 80 lb |
| 12 in | 2.36 cu ft · 4 × 80 lb | 3.14 cu ft · 6 × 80 lb |
| 16 in | 4.19 cu ft · 7 × 80 lb | 5.59 cu ft · 10 × 80 lb |
| 18 in | 5.30 cu ft · 9 × 80 lb | 7.07 cu ft · 12 × 80 lb |
Frequently asked questions
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10 in hole 24 in deep, about 2 bags of 80 lb concrete or 3 bags of 50 lb fast-setting concrete. A 6×6 post in a 12 in hole 36 in deep takes about 3 bags of 80 lb.
How big should a fence post hole be?
About three times the post's width: 10–12 in across for a 4×4 and 12–14 in for a 6×6, and at least 24 in deep, or below the frost line.
How many bags of concrete for a Sonotube?
A 12 in tube 48 in deep holds about 3.1 cu ft, which is 6 bags of 80 lb or 7 bags of 60 lb concrete. Multiply diameter² × depth × 0.000454 for cubic feet.
Is fast-setting concrete good for fence posts?
Yes. You pour the dry mix into the hole and add water, and it sets in 20–40 minutes, so posts can be braced and the fence built the same day. Use regular mix for structural deck footings.
Should I put gravel at the bottom of a post hole?
A few inches of gravel under the post helps water drain away from the end grain, which slows rot. Include it in your hole depth.
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Hole volume is a cylinder (π r² h) minus the post; 4×4 posts are 3½ in square and 6×6 posts 5½ in. Bag yields: 50 lb fast-setting 0.375 cu ft, 60 lb 0.45 cu ft and 80 lb 0.6 cu ft, per common bag labels, with 10% added for spills. Footings for decks and structures must follow your local code.