Quick answer
Post Hole Concrete Calculator: direct answer
Post Hole Concrete Calculator helps you estimate concrete for round post holes by diameter, depth, quantity, bag size, and waste factor. It is best for fence posts, deck posts, sign posts, and round holes and returns cubic feet, cubic yards, whole bag count for material planning.
Use this calculator when you know project shape such as slab, wall, footing, or post hole, length, width, thickness, height, depth, or diameter, quantity when the same shape repeats. The estimate uses this rule: cubic feet = pi x radius squared x hole depth x quantity.
Inputs
- Project shape such as slab, wall, footing, or post hole
- Length, width, thickness, height, depth, or diameter
- Quantity when the same shape repeats
- Waste percentage
- Bag size, bag cost, and ready-mix cost when pricing is needed
Outputs
- Cubic feet
- Cubic yards
- Whole bag count
- Estimated material weight
- Estimated material cost
Formula
How this estimate works
cubic feet = pi x radius squared x hole depth x quantity
In plain terms, calculate the concrete volume from the shape dimensions, convert cubic feet into cubic yards, then add waste before comparing ready-mix and bagged options.
Eight 12 in diameter holes at 3 ft deep need about 0.77 cubic yards with 10% waste.
Use cases
When to use this calculator
Estimate concrete for repeated round holes before buying bagged mix for a fence line.
Plan small batches where the hole diameter and depth are known before mixing.
Estimate volume and bag count before following the product label for placement and water.
Worked example
Estimate concrete for eight fence posts
Post holes are round cylinders, so small changes in hole diameter and depth can change the number of bags quickly.
- Enter the actual hole diameter, depth, and number of posts.
- Choose the bag size that matches the product you plan to buy.
- Add waste for loose soil, uneven digging, and slightly oversized holes.
Round hole volume
Post holes are estimated as cylinders. The calculator converts diameter to radius, calculates circular area, then multiplies by hole depth and quantity.
Why hole diameter matters
A small increase in diameter increases volume across the entire depth of the hole. Measure the actual auger or dug hole size, not only the post size.
Measurement tips for a better estimate
- Measure the actual dug hole diameter, not the post width.
- If holes are not uniform, estimate the larger holes separately or use a conservative average.
- Check whether the bottom of the hole is flat or belled because that changes volume.
Common estimating mistakes
- Entering post diameter instead of hole diameter.
- Forgetting to multiply by the number of holes.
- Ignoring collapsed soil or over-excavation that increases the concrete needed.
Assumptions used
- Normal-weight concrete is estimated at 150 lb per cubic foot.
- Premix bag yield is estimated from common 40, 50, 60, and 80 lb bag sizes.
- Waste factor is applied after the base volume is calculated.
Before you order materials
- Use actual hole diameter, not only post size.
- Account for over-digging and soil collapse.
- Follow product instructions for fast-setting mixes.
Frequently asked questions
How much concrete per fence post?
It depends on hole diameter and depth. A 12 inch diameter, 3 ft deep hole is much larger than a shallow small-diameter hole.
Should I add waste for post holes?
Yes. Soil collapse, uneven digging, and over-excavation can increase concrete use.
Can I use fast-setting concrete?
This calculator estimates volume. Follow the product label for water, placement, and curing instructions.