Fencing guide
How Much Concrete Do I Need per Fence Post?
Estimate concrete for fence-post holes with a cylinder formula, bag-yield checks, and links to the fence and concrete calculators.
- By
- EstimatorPad Editorial Team
- Published
- July 22, 2026
- Reading time
- 5 min
Direct answer
Start with the useful estimate.
Concrete per fence post depends on hole diameter, hole depth, and post size. Calculate the round hole as a cylinder, subtract the buried post volume if you need a tighter estimate, divide by the yield printed on the concrete bag, and round up. A 12-inch diameter hole that is 36 inches deep is about 2.36 cubic feet before subtracting a post or adding waste.
Measurement steps
How to measure
- Count the posts that will be set in concrete.
- Measure or specify the hole diameter, not just the post width.
- Measure the concrete depth separately from any gravel base under the post.
- Record the buried post dimensions if you want to subtract displacement.
- Choose the bag size and use the yield printed on that product.
- Call 811 or use the local one-call system before digging post holes.
Formula explanation
The formulas to use
Hole_radius_ft = hole_diameter_in / 24
Hole_depth_ft = hole_depth_in / 12
Hole_volume_cu_ft = pi x radius_ft x radius_ft x depth_ft
Post_displacement_cu_ft = post_width_ft x post_thickness_ft x buried_depth_ft
Concrete_volume = hole_volume - post_displacement
Bags = ceiling(concrete_volume_with_waste / bag_yield_cu_ft)
Use with this article
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Worked example
One 12-inch hole at 36 inches deep
- 1Hole radius = 12 / 24 = 0.5 feet.
- 2Hole depth = 36 / 12 = 3 feet.
- 3Hole volume = pi x 0.5 x 0.5 x 3 = 2.36 cubic feet.
- 4If using a 60-lb bag that yields about 0.45 cubic feet, 2.36 / 0.45 = 5.24 bags before post displacement and waste.
- 5Round the final project total up after applying the product yield and any allowance.
The exact bag count depends on whether you subtract post displacement and which bag yield your product lists.
Use the concrete calculator in post-hole mode for the number of holes, diameter, depth, bag size, and waste allowance. The calculator uses the full cylindrical hole volume and does not subtract post displacement, so treat its result as a conservative planning quantity before checking the product label.
Hole diameter changes the answer quickly
Post-hole concrete is a cylinder calculation. The radius is squared, so increasing a hole from 10 inches to 12 inches has a larger effect than many quick tables make obvious. This is why the best estimate starts with the actual hole diameter, not a generic bags-per-post rule.
If the post takes up a meaningful amount of the hole, subtract the buried post volume. For small planning estimates, some users skip the subtraction and keep the extra material as a safety margin. Either approach should be documented so the order can be checked.
Use the yield printed on the bag
Common concrete mix bags publish approximate yields, and manufacturer calculators usually round up to full bags. The yield is the mixed volume, not the dry bag size. Standard 40-lb, 60-lb, and 80-lb mixes often appear around 0.30, 0.45, and 0.60 cubic feet respectively, but the product label controls your order.
Fast-setting mixes, specialty mixes, and regional products can differ. If the bag is not in your hand yet, use the calculator result as a planning number and verify the product before buying a full pallet.
Concrete per post is not the full fence estimate
A concrete-per-post guide answers one part of a fence project. You still need the number of line posts, terminal posts, corner posts, gate posts, and any bracing posts. Use the fence calculator or the fence-post count guide first when the number of posts is not known.
Gate locations can change post sizes and concrete needs. Sloped ground, soft soil, frost depth, wind exposure, and local requirements can also change the hole specification. Treat the formula as a material estimate, not a structural design.
Before digging post holes
Fence projects involve digging, even when the hole is small. Use 811 or the local one-call system before excavation so underground utilities can be marked. Also check property lines, easements, HOA rules, and permit requirements before setting permanent posts.
The calculator cannot know soil conditions or local frost requirements. If a fence must resist high wind, retain soil, hold a gate, or meet pool-barrier rules, get local guidance before relying on a generic hole size.
Concrete bag yield checks
Use the product label for the final yield.
| Bag size | Typical mixed yield | Bags for 2.36 cu ft before waste |
|---|---|---|
| 40 lb | about 0.30 cu ft | 8 bags |
| 60 lb | about 0.45 cu ft | 6 bags |
| 80 lb | about 0.60 cu ft | 4 bags |
Ordering margin
Waste and ordering margin
- Add a small waste allowance when holes are irregular, the soil caves in, or mixed concrete is spilled during placement.
- Do not assume every 60-lb or 80-lb bag from every product line has the same yield; check the package or product sheet.
- Round up to whole bags because partial-bag math is not useful at the store.
- Keep gravel base material separate from concrete volume if the hole includes both.
- Buy a little more when stopping mid-run would delay setting the remaining posts.
Avoid these
Common mistakes
- Using post spacing to estimate concrete before counting the actual posts.
- Using hole radius and diameter interchangeably in the cylinder formula.
- Counting the gravel base as concrete depth.
- Forgetting that a larger hole diameter increases volume quickly.
- Assuming a bag-count table applies to every post size and soil condition.
Project checklist
Before you use the result
- Count every post that will be set in concrete.
- Confirm hole diameter and concrete depth.
- Keep gravel base depth separate from concrete depth.
- Use the concrete calculator for cylinder volume.
- Check bag yield on the exact product.
- Round up to full bags for the complete project.
- Use 811 or the local one-call service before digging.
Estimate disclaimer
Verify before buying or building.
This guide estimates concrete material for fence-post holes. It does not determine structural footing design, frost depth, utility location, property-line requirements, pool-barrier compliance, or local code requirements.
FAQ
Common questions
01How many bags of concrete do I need per fence post?+
It depends on the hole diameter, depth, post size, and bag yield. Calculate the cylindrical hole volume and divide by the product's mixed yield.
02Should I subtract the post from the concrete volume?+
For a tighter estimate, yes. For a quick small-project estimate, some users skip the subtraction and keep the extra as a margin. Document the assumption either way.
03Is concrete depth the same as hole depth?+
Not always. If the bottom of the hole includes a gravel base or drainage layer, subtract that depth from the concrete portion.