Post Hole Concrete Calculator
Works for fence posts, deck footings and sonotubes. The calculator subtracts the post itself and the gravel at the bottom, so you don't over-buy.
- Tested formulas
- Feet or metric
- Printable shopping list
2 What to buy
Updates as you type| Concrete, all holes (with waste) | 10.13 cu ft · 0.375 yd³ · 0.287 m³ |
|---|---|
| 40 lb bag | 34 bags |
| 60 lb bag | 23 bags |
| 80 lb bag | 17 bags |
| 20 kg bag | 31 bags |
| 25 kg bag | 25 bags |
| 30 kg bag | 21 bags |
| Gravel for drainage | 2.73 cu ft (6 × 0.5 cu ft bags) |
Fast-setting mix (like Quikrete Fast-Setting) is sold in 50 lb bags that yield about 0.375 cu ft. If you use it, buy 28 bags instead.
Shopping list
Quick answer
A 10-inch hole, 30 inches deep, with 6 inches of gravel and a 4×4 post takes 0.92 cu ft of concrete, about 1.5 bags of 80 lb mix. Ten posts need 17 bags with 10% extra.
80 lb bags per post hole
| Hole width | 24" deep | 30" deep | 36" deep | 42" deep |
|---|---|---|---|---|
| 8" | 0.7 bags | 0.9 bags | 1.1 bags | 1.3 bags |
| 10" | 1.2 bags | 1.5 bags | 1.9 bags | 2.3 bags |
| 12" | 1.8 bags | 2.3 bags | 2.9 bags | 3.5 bags |
One hole, with a 4×4 post and 6" of gravel at the bottom, no waste allowance. Use the calculator above for your exact size.
How much concrete per post hole?
A post hole is a cylinder, so its volume is π × radius² × depth. Two things reduce how much concrete you actually pour. The post takes up space: a 4×4 is really 3½" × 3½". And the gravel layer at the bottom isn't concrete either. Most online calculators ignore both, which is why people end up with leftover bags.
Worked example: 10 fence posts
A 10" wide hole, 30" deep, with 6" of gravel and a 4×4 post:
- Concrete depth: 30 − 6 = 24"
- Per hole: (π × 5² − 3.5²) × 24 ÷ 1728 = 0.92 cu ft, about 1.5 × 80 lb bags
- 10 holes + 10% waste: 17 × 80 lb bags (10.1 cu ft)
How deep should post holes be?
The rule of thumb is one-third of the post's total length in the ground. A 6 ft fence on 8 ft posts should be set 24–32" deep. In cold climates, footings must also go below the frost line, which runs from 12" in the southern US to 48" or more in the northern states. Otherwise frost heave pushes posts up. Your local building department publishes the required depth.
Hole diameter guide
| Post | Hole diameter | Typical use |
|---|---|---|
| 4×4 | 10–12" | Fences, mailboxes, light deck posts |
| 6×6 | 12–16" | Deck and pergola posts, gate posts |
| Sonotube 8–12" | Tube size | Deck footings and piers |
Dry-pour or mixed?
Fast-setting mix poured dry into the hole and soaked with water is popular for fence posts. It works, but mixing gives a stronger, more consistent footing. For deck footings that carry structural loads, always mix the concrete properly.
Frequently asked questions
How many bags of concrete per fence post?
A typical 4×4 fence post in a 10" hole, 30" deep, takes about 1.5–2 bags of 80 lb mix, or 2–3 bags of 50 lb fast-setting mix. Use the calculator for your exact hole size.
How many 50 lb bags of fast-setting concrete per post?
Fast-setting mix yields about 0.375 cu ft per 50 lb bag. A 10"×24" concrete column around a 4×4 needs about 0.94 cu ft, which is roughly 2.5 bags, so plan on 3.
Should I put gravel at the bottom of a post hole?
Yes, usually 4–6 inches. It lets water drain away from the end grain of the post and slows rot.
Last reviewed 2026-10-21. See how we calculate for our sources and assumptions.