Concrete · updated 2026
Fence Post Concrete Calculator
Setting fence posts in concrete keeps them straight, solid, and standing for decades — but guessing the bag count means a second trip to the store mid-project. This fence post concrete calculator tells you exactly how much concrete you need: enter your hole diameter, depth, and number of posts, and it returns the total volume plus the number of 60-lb and 80-lb bags to buy, with current material costs. The math assumes standard round holes; if you're using square holes dug with a shovel, add about 15% for the irregular shape.
A 4×4 post needs a 10–12 in hole — roughly 3× the post width
Minimum 24 in; go below your local frost line — 30–36 in in most northern states
Show the math
Radius = 12 ÷ 24 = 0.50 ft
Volume per hole = π × 0.50² × (36 ÷ 12) = 2.36 ft³
Total volume = 2.36 × 10 holes = 23.56 ft³
80-lb bags = 23.56 ÷ 0.6 = 40 (rounded up — an 80-lb bag yields 0.6 cu ft)
What the numbers mean
The total volume is the space inside all of your round holes. Because holes are cylindrical, a small change in diameter can change the concrete quantity quickly.
Bag counts are rounded up because concrete is sold in whole bags. The 80-lb estimate uses a 0.6 cu ft yield; the 60-lb estimate uses a 0.45 cu ft yield.
The price range is for the seeded 80-lb bag price multiplied by the rounded bag count. Add posts, delivery, and any local tax separately.
2026 material prices
| Material | 2026 average price | Note |
|---|---|---|
| 80-lb concrete mix bag | $6.47 – $7.58 / bag | Quikrete #110180 at Home Depot, Sep 2026 |
| 4×4×8 pressure-treated post | $9.00 – $14.00 / post | If you still need posts |
Material prices: Home Depot, Sep 2026. National averages; local prices vary. 2026 U.S. national averages. Prices vary by region and retailer — confirm locally before buying.
Pro tips
- Dig the hole 3× the post width: a 4×4 post (3.5 in actual) wants a 10–12 in hole.
- Set posts 1/3 of their above-ground height deep, minimum 24 in — deeper where the ground freezes.
- Bell out the bottom of the hole slightly wider than the top; it locks the footing against frost heave.
- Don't fill concrete to the top — stop 3–4 in below grade and cover with soil so grass grows over.
- For 1–2 posts, fast-setting mix saves a day; for 10+ posts, standard mix is cheaper and more forgiving.
- Slope the concrete surface away from the post so water drains instead of pooling.
Common questions
Frequently asked questions
How many bags of concrete per fence post?
A typical 4×4 post in a 12-in diameter, 36-in deep hole needs about 4 eighty-lb bags. A 10-in hole at 30 in deep needs about 2–3 bags. Enter your exact dimensions above for your number.
How deep should fence post holes be?
One-third of the post's above-ground height, with a 24-in minimum. In cold climates, go below the frost line — usually 30–36 in.
Can I set fence posts without concrete?
Yes — tamped crushed stone or packed soil works for light fences in mild climates, but posts last roughly half as long and shift sooner. Use concrete for gates, corners, and privacy fences.
Fast-setting vs regular concrete for posts?
Fast-setting costs more per bag but lets you hang rails the same day. Regular mix is cheaper for big runs and gives you time to plumb every post before it sets.
How long before I can build the fence on new posts?
Wait 24–48 hours for standard mix (4 hours for fast-setting) before hanging rails or panels.
Should the concrete go above ground level?
No — crown it slightly above the hole but below grade, sloping away from the post, then cover with soil.
Related calculators
Planning estimates only. Prices are 2026 U.S. national averages and vary by region — confirm with your local supplier before buying.