Find out how much concrete your fence posts need. Enter the number of posts, the hole size and the post size: the calculator subtracts the post and the gravel base and tells you the volume and how many 40, 50, 60 or 80 lb bags to buy, per hole and in total.
Last updated:
Units
in
About 3× the post width
cm
About 3× the post width
in
cm
in
Gravel under the post for drainage (0 for none)
cm
Gravel under the post for drainage (0 for none)
%
Feet (meters in metric) to suggest a hole depth
Enter your values to see the result.
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How to use the Fence Post Concrete Calculator
Choose inches or centimeters and enter the number of posts.
Pick the post size and enter the hole diameter and depth.
Enter the gravel base depth if you put gravel under the posts.
Choose your bag size and read the bags needed per hole and in total.
Concrete per hole = hole volume − post area × concrete depth
Bags = concrete per hole × number of posts × (1 + waste) ÷ bag yield
Example: 10 in hole, 30 in deep, 4 in gravel, 4×4 post (3.5 × 3.5 in): concrete depth 26 in, hole = π × 5² × 26 = 2,042 in³, post = 12.25 × 26 = 318.5 in³, so concrete = 1,723.6 in³ = 1.0 ft³ per hole, about 2.7 bags of 50 lb.
Bags per hole (4×4 post, no gravel)
Hole
Concrete
50 lb bags
80 lb bags
8 in × 24 in
0.53 ft³
1.4
0.9
10 in × 24 in
0.92 ft³
2.5
1.5
10 in × 30 in
1.15 ft³
3.1
1.9
12 in × 30 in
1.75 ft³
4.7
2.9
12 in × 36 in
2.10 ft³
5.6
3.5
Round up to whole bags and keep one spare: holes are rarely perfectly round, and the sides tend to widen as you dig.
Frequently asked questions
How many bags of concrete do I need per fence post?
A typical 10 in wide, 30 in deep hole for a 4×4 post with 4 in of gravel holds about 1.0 ft³ of concrete after subtracting the post, or roughly three 50 lb bags (0.375 ft³ each) or two 80 lb bags (0.6 ft³ each). Wider or deeper holes need more.
How much concrete does a bag make?
Standard premixed concrete yields about 0.30 ft³ per 40 lb bag, 0.375 ft³ per 50 lb bag, 0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag. Fast-setting post mix in 50 lb bags yields about the same as a 50 lb bag of regular mix. Check the yield printed on your brand's bag.
How big should a fence post hole be?
A common rule is a hole diameter about three times the width of the post (10–12 in for a 4×4) and a depth of one third to one half of the post height above ground, plus about 6 in for gravel. In cold climates the hole should go below the frost line.
Should I put gravel at the bottom of a post hole?
Many builders add 4–6 in of compacted gravel under the post so water drains away from the end grain, which helps wood posts last longer. The calculator subtracts that depth from the concrete and estimates the gravel separately.
How is the concrete volume calculated?
Volume = π × (diameter ÷ 2)² × concrete depth, minus the post's cross-section area × concrete depth. The result in cubic inches is divided by 1,728 to get cubic feet, multiplied by the number of posts and a waste allowance, and divided by the bag yield.
Is my data sent anywhere?
No. Everything is calculated in your browser.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.