How many bags of concrete do I need for a post hole?
Find the cylindrical hole volume, subtract the portion occupied by the post, multiply by the number of holes, then add your extra allowance. Divide by the bag’s mixed yield and round up. With the default inputs, eight holes need 21 bags at 0.6 ft³ per bag and 10% extra.
Enter the concrete-filled hole dimensions
Hole diameter and concrete depth determine the gross cylinder volume. Measure the portion that will actually be filled with concrete. If the installation includes a gravel layer, deduct it from the depth before entering the value. The calculator does not have a separate gravel-depth field and will not subtract one automatically.
Use actual post width for a square post or actual diameter for a round one. A nominal size is not a substitute for the measured cross-section. The model assumes the post extends through the entered concrete depth. Irregular holes, flared bases and partially embedded shapes need a more detailed volume estimate.
Why post displacement changes concrete volume
A square post occupies width × width of cross-sectional area. A round post occupies π × diameter² ÷ 4. The area of a round hole is also π × diameter² ÷ 4. Subtract the post area from the hole area before multiplying by concrete depth.
The fit check is important for square posts. A square’s diagonal, rather than its side alone, must fit within the circular hole. A 3.5-inch square post will not fit inside a 4-inch circular opening. The calculator rejects that combination instead of returning a misleading positive volume.
Post-hole concrete and bag calculation formula
Net concrete = holes × depth × (hole area − post area). Concrete with allowance = net concrete × (1 + extra percentage ÷ 100). Bags to buy = ceil(concrete with allowance ÷ mixed yield per bag). Each measurement in the volume equation must use a consistent length unit.
In imperial mode, the result is displayed in cubic feet and bag yield is entered in cubic feet per bag. In metric mode, the displayed volume and bag yield use liters. One cubic foot is approximately 28.3168 liters. The unit switch converts the entered yield along with the dimensions.
Worked example: eight holes with square posts
Take a 12-inch-diameter hole with a 24-inch concrete depth and a 3.5-inch square post. The hole radius is 0.5 ft and the depth is 2 ft. Gross volume is π × 0.5² × 2 ≈ 1.5708 ft³. Post displacement is (3.5 ÷ 12)² × 2 ≈ 0.1701 ft³.
Net volume is about 1.4007 ft³ per hole, or 11.2058 ft³ for eight holes. Adding 10% gives about 12.3264 ft³. Dividing by a 0.6-ft³ bag yield gives about 20.54 bags, so the final purchase quantity is 21. At an entered $7 per bag, that subtotal is $147.
| Identical holes | Net volume before extra | Bags with 10% extra |
|---|---|---|
| 1 | 1.40 ft³ | 3 |
| 4 | 5.60 ft³ | 11 |
| 8 | 11.21 ft³ | 21 |
Read bag yield instead of guessing from weight
Mixed yield describes the volume that one bag produces under its stated mixing instructions. The dry package weight is a different quantity. A product-specific yield is more useful for purchasing than a blanket rule that all bags of a given weight behave alike. Replace the default with the value on your chosen mix.
The final quantity pools concrete across all identical holes before rounding bags up. Mixing and buying whole bags independently for every hole can require more bags. The calculator also assumes that mixed material can be used efficiently across the project. Allow for your actual batching plan rather than treating the mathematical volume as a mixing schedule.
Use the bag estimate as one part of the plan
Download the estimate with its hole count, dimensions, post shape and yield. Check that the depth describes concrete only and that the bag label uses the same volume unit. An extra allowance is a way to model uneven excavation or material loss; it is not a substitute for measuring the holes.
The fence material calculator can help count an open run’s posts before you estimate their concrete. Read how post-hole concrete volume is calculated for the purchasing workflow. Footing size, frost depth, suitable mix and structural requirements must come from the project’s design, not from this quantity tool.
The QUIKRETE Concrete Mix data sheet lists an approximate 0.60 ft³ yield for its 80-lb bag. That is a product example, not a universal weight-to-volume conversion. Enter the yield printed on the bag you buy. QUIKRETE Concrete Mix product data sheet.
Frequently asked questions
Does this calculator choose how deep to dig?
No. It estimates quantity from the depth you enter. Required embedment and footing dimensions depend on the project and site. Enter the appropriate design dimensions rather than treating the default 24 inches as a recommendation.
Can I estimate holes with different sizes?
Calculate each size group separately. To combine them, add the unrounded concrete volumes, apply the planned allowance and then divide by bag yield. Adding separately rounded bag counts can overstate a pooled purchase.
Is concrete volume the same as the amount of cement?
No. This tool estimates mixed concrete volume and packaged concrete bags. It does not calculate separate cement, sand and aggregate quantities or provide a concrete mix design.
Written by Velour Trends · Updated 7 November 2026. Worked examples use the calculator’s stated formulas and rounding model. Our editorial approach · Report a calculation issue.