Checked against the manufacturers
The same formula, run against the published examples. The manufacturers don’t state every assumption, so ours are listed.
| Published example | Published | This calculator |
|---|---|---|
| Sakrete: 10 in hole, 12 in deep | 1¼ bags | 1.23 with a 4×4 post |
| Sakrete: 10 in hole, 18 in deep | 2 bags | 1.84 with a 4×4 post |
| Sakrete: 10 in hole, 24 in deep | 2½ bags | 2.46 with a 4×4 post |
| Sakrete: 10 in hole, 30 in deep | 3 bags | 3.07 with a 4×4 post |
| Quikrete: 3 in square post, 9 in hole, 30 in deep | 2 bags | 2.02 with a 6 in gravel base; 2.53 filled to the top |
All 50 lb fast-setting bags at 0.375 ft³. Sakrete’s chart doesn’t say what post it assumes; a 4×4 gives the closest match, and its rows are compared with no gravel in the hole. Quikrete’s data sheet doesn’t mention gravel, but its post-setting instructions put about 6 in in the bottom of the hole.
Manufacturers round to the nearest half or quarter bag; the calculator shows the exact figure and rounds the total up.
The formula
Post area is width × width for a square post, width × depth for a rectangular one, and π × (diameter ÷ 2)² for a round one. Dressed lumber is smaller than its name: a 4×4 measures about 3½ × 3½ in. Measure yours.
Sizing the hole
Quikrete’s post-setting instructions give three rules of thumb:
- Width: about three times the post’s width.
- Depth: a third of the post’s total length — equivalently, half the height that will stand above ground.
- Base: about 6 in of gravel in the bottom for drainage.
- Top: Quikrete’s fast-setting instructions leave the top 3–4 in of the hole for soil or sod. If you’ll do the same, enter a hole depth 3–4 in shallower.
Fill in the post height above ground in the calculator and it suggests a hole from these rules, with a button to use them.
Deck and structural posts
Posts that carry a deck, carport or porch roof sit on footings, and the IRC requires footings at least 12 in below undisturbed ground and below the frost line (R403.1.4). Your building department sets the frost depth and may require an inspection before you pour.
Per hole or in total?
If you mix one batch per hole, a partial bag left over from one hole is often wasted, so round each hole up. If you mix in a barrow and share batches across holes, buy for the total. The calculator shows both: the exact bags per hole and the total to buy.
Questions people ask
How many bags of concrete per fence post?
Sakrete’s chart for a 10 in hole gives 2 bags of 50 lb fast-setting mix at 18 in deep, 2½ at 24 in and 3 at 30 in. Subtracting a 4×4 post, with the gravel set to 0, the calculator gets 1.8, 2.5 and 3.1. With its default 6 in of gravel it’s less: 1.2, 1.8 and 2.5. For a different hole or post, enter your sizes.
How deep should a 6 ft fence post be?
Quikrete’s rule is a third of the post’s total length in the ground — the same as half the height above ground. A post standing 6 ft out of the ground goes 3 ft deep, so you’d buy a 9 ft post, then add about 6 in of gravel under it. In cold climates, posts that carry a structure go below the frost line.
How wide should the hole be?
About three times the post’s width, according to Quikrete: roughly 10–12 in for a 4×4, and 16–18 in for a 6×6. A narrow hole leaves a thin collar of concrete that cracks easily.
Why subtract the post?
Because it’s a real share of the hole. A 4×4 takes up 16% of a 10 in hole, so ignoring it overestimates by a bag every few posts. The calculator subtracts the post over the full height of the concrete.
Sources for the numbers on this page
- Quikrete — Setting posts — project instructions · checked October 1, 2026
- Quikrete — Fast-Setting Concrete Mix No. 1004 — technical data sheet · checked October 1, 2026
- Sakrete — Fast-Setting Concrete Mix — technical data sheet (bags per post hole) · checked October 1, 2026
- State of Ohio (adopted IRC text) — Residential Code of Ohio 4101:8-4-01 — Table R402.2 minimum compressive strength; R403.1.4 footing depth · checked October 1, 2026
Corrections and updates are logged on the sources page.