Count every hole size separately
Fence lines often mix corner posts, gate posts, and line posts. Calculate each size group separately if the diameters or depths differ.
concrete
Post Hole Concrete Calculator
Calculate concrete bags for fence post holes, deck posts, and footings.
Post Hole Concrete CalculatorDiameter, depth or height, and quantity are required before the estimate appears.
Example12 in diameter x 4 ft deep = 0.12 yd³
Use the inside form diameter and include below-grade depth for piers.
Last reviewed: May 12, 2026 by the BuilderCalc editorial team.
SSR formula reference
Post Hole Concrete Calculator formulas
These formulas are rendered in the server HTML so crawlers, LLMs, and users can read the method without running JavaScript. The interactive calculator can change the inputs, but the estimating math below is visible in the raw page source.
Volume (cubic feet) = Length x width x thickness (in inches / 12).
Volume (cubic yards) = Cubic feet / 27.
Bags (80-lb) = Cubic feet / 0.60, rounded up.
80-lb bag yields about 0.60 ft3, or 0.022 yd3.
| Result | Formula |
|---|---|
| Radius | Diameter / 2 |
| Cylinder volume | 3.1416 x radius x radius x depth |
| Cubic yards | Cubic feet / 27 |
| Bags | Cubic feet / bag yield, rounded up |
Standard PSI by application
| Application | Recommended PSI |
|---|---|
| Patio or walkway | 3,000 PSI |
| Garage floor, residential | 3,500 PSI |
| Driveway, cars | 3,500 to 4,000 PSI |
| Driveway, trucks or RVs | 4,000 to 4,500 PSI |
| Foundations and footings | 3,000 to 4,000 PSI, per plan and code |
Bag yield reference
| Bag size | Common yield |
|---|---|
| 80-lb bag | About 0.60 ft3, or 0.022 yd3 |
| 60-lb bag | About 0.45 ft3, or 0.017 yd3 |
| 50-lb bag | About 0.375 ft3, or 0.0138 yd3 |
| 40-lb bag | About 0.30 ft3, or 0.011 yd3 |
Post hole planning
Post Hole Concrete Calculator planning notes
Post holes are usually bag-friendly, but depth, diameter, and the number of holes change bag count quickly. Confirm whether gravel drainage or a wider bell base is part of the design.
Do not ignore post displacement
A post placed in the hole displaces some concrete. For planning, small overage usually covers this, but tight orders should account for actual post size.
How to use it
Keep the estimate tied to field measurements
Group matching posts
Calculate line posts, corner posts, and gate posts separately if diameter or depth differs.
Enter concrete depth
If gravel is used at the bottom, enter only the depth that will be filled with concrete.
Round up bags
Post holes vary in shape, so round up and keep extra material available on site.
Related calculators
FAQ
Concrete estimating questions
How many concrete bags do I need for fence posts?
Bag count depends on hole diameter, depth, number of posts, and whether the post displaces part of the hole volume. Calculate each post size group separately.
Should I include gravel under post concrete?
If you place gravel in the bottom of the hole, do not count that gravel depth as concrete depth. Enter only the depth that will be filled with concrete.
What hole diameter should I use?
A common rule of thumb is a hole about three times the post width, but fence height, soil, wind, and local practice can change the design.
Do post holes need waste allowance?
Yes. Holes are rarely perfectly round or identical, so round bag counts up and keep extra material available.