How many bags of concrete do I need per fence post?
A 12 in hole that is 30 in deep takes 1.96 cu ft, which is 3.3 bags of 80-lb mix. With the 10% allowance, rounded up, plan on 4 bags per hole. For a 36 in deep hole plan on 5 bags.
Fence Post Concrete Calculator: Bags per Hole and per Fence
Enter the diameter and depth of the post holes and how many posts the fence has. The calculator returns the concrete for all holes in 80-lb bags and cubic yards, with an allowance, and compares the bags with a ready-mix delivery.
1. Material Grade & Project Dimensions
Area: 11.0 sq ft (1.02 m²)Net Vol: 1.02 cu yd (0.78 m³)
2. Material Takeoff, Blueprint & Cost
Total Concrete Volume (incl. waste)
1.12 cu yd (0.86 m³)
30.2 cu ft · 4,475 lbs (2.24 US tons)
80-lb Concrete Bags (0.60 ft³)
51 bags
1.2 pallets (@ 42 bags/pallet)
60-lb or 40-lb Bag Option
68 × 60-lb
or 101 × 40-lb (0.30 cu ft)
Rebar Grid (#4 @ 18" O.C.)
16 lin ft
1 × 20-ft bars recommended
4" Compacted Gravel Sub-Base
0.19 US tons
Crushed stone base under slab
Cost Break-Even: Ready-Mix Truck vs. 80-lb Bags
Ready-Mix Truck saves $60
Ready-Mix Delivery (1.12 cu yd + $140 short-load fee)
$194
51 × 80-lb Pre-Mix Bags
$254
Show Step-by-Step Formula & Takeoff Math
Net Area = 11.00 sq ft (1.02 m²)
Net Volume = 27.49 cu ft ÷ 27 = 1.02 cu yd
Total Volume (+10% waste) = 30.24 cu ft = 1.12 cu yd (0.86 m³)
80-lb Bags = ceil(30.24 ÷ 0.60) = 51 bags
How much concrete fence posts need
Count the posts first. With posts every 8 ft, a 100 ft fence line needs 14 posts, one more than the number of sections. Then enter one circle zone with the hole diameter in feet, the hole depth in inches and the number of posts in the count field. The example uses 12 in holes that are 30 in deep, which is 2.16 cu ft or 4 bags per hole.
For the 100 ft fence above, 14 holes need 30.24 cu ft with a 10% allowance. That is 1.12 cu yd, 51 bags of 80-lb mix, or about 1.2 pallets. Bags are the sensible choice for a fence; the comparison box shows how that compares with ready-mix at the default prices.
The calculator fills the entire hole. A 4×4 post (3.5 in × 3.5 in actual) takes up about 11% of a 12 in hole, so the real volume is a little lower and the 10% allowance is a sensible reserve. Hole depth depends on fence height and frost depth; a common rule is to set one third of the post length in the ground.
Concrete for a fence line with posts every 8 ft (12 in holes, 30 in deep)
| Fence length | Posts | Concrete | 80-lb bags |
|---|---|---|---|
| 50 ft | 8 | 0.64 cu yd | 29 |
| 100 ft | 14 | 1.12 cu yd | 51 |
| 150 ft | 20 | 1.60 cu yd | 72 |
| 200 ft | 26 | 2.08 cu yd | 94 |
Frequently Asked Questions & Field Guide
How many posts do I need for a 100 ft fence?
With posts every 8 ft, 100 ft ÷ 8 ft = 12.5, which rounds up to 13 sections and 14 posts, one more post than sections. Closer spacing means more posts and more bags, so change the count field to match your plan.
How many bags of concrete for 14 fence posts?
Fourteen 12 in × 30 in holes need 51 bags of 80-lb mix with the 10% allowance, or 1.12 cu yd. At the default $6.45 a bag that is $329.
Does the calculation subtract the volume of the post?
No. The calculator treats each hole as full, so it overestimates by the space the post occupies, about 11% of a 12 in hole for a 4×4. Keep the 10% allowance as reserve rather than removing it.
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