Fence line, gate posts or deck project
How many bags of concrete do your post holes need? Enter the hole dimensions and the actual post size, then compare bag counts and keep a purchase worksheet. Add separate groups for larger gate posts or different hole depths.
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Enter hole and post measurements to estimate concrete.
Enter the round hole size and the post inside it. Your result subtracts the post, adds your chosen allowance and rounds up using your bag’s actual yield.
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Get the inputs right
For each group of matching holes, measure the round hole’s diameter, the depth that will contain concrete, the actual post cross-section and the part of the post inside that concrete. Count the matching holes, then add another group for a different size.
Measure across the hole, through its center. This calculator models a cylinder with a constant diameter. A tapered or widened base is not the same shape; measure its volume separately rather than using a narrow top measurement for the entire excavation.
Count the vertical height of concrete only. If an excavation includes a gravel base, do not enter the gravel as concrete. A project’s hole depth and its concrete-filled depth can be different.
Use a tape measure, not just the lumber name. A nominal 4 × 4 post is often 3.5 × 3.5 inches, but actual dimensions can vary. For rectangular posts, enter both width and thickness. For a round post, enter its diameter.
Subtract only the post portion within the concrete. If there is a layer of concrete beneath the post, the post’s embedded depth is smaller than the concrete-filled depth. The diagram distinguishes these two measurements.
Start with dimensions appropriate for your project. This is a concrete quantity calculator; it does not choose structural hole dimensions, frost depth or an installation method.
See where the answer comes from
Find the volume of each round hole, subtract the part occupied by the post, and multiply by the number of matching holes. Add your chosen allowance once. Divide that concrete volume by the mixed yield printed on your bag, then round up according to how you plan to allocate the bags.
Calculate the round hole volume, then subtract the post volume. The hole and post can have different depths.
Multiply by each group’s hole count, combine the groups, then add your chosen allowance once.
Use the finished yield on your exact bag. Pool the project and round once, or round each hole up before totaling.
When working in inches, divide cubic inches by 1,728 to get cubic feet. One cubic yard is 27 cubic feet. One cubic foot is 28.316846592 liters. The calculator converts measurements for you and keeps the chosen bag-yield unit separate.
Suppose one hole needs 1.3810 cubic feet and your bag yields 0.60 cubic feet. That individual hole needs three whole bags. Ten matching holes need about 13.8104 cubic feet in total, which rounds to 24 bags when pooled. Allocating three whole bags to each hole gives 30 bags. The concrete requirement is the same; the rounding step is different.
Choose the allocation that fits how the work will be organized and the product used. The surplus shown is purchased mixed yield beyond your calculated requirement, including any allowance. It is not a promise of recoverable leftover concrete.
Check a familiar project
Ten round holes, each 10 inches wide and filled with concrete to 36 inches, contain 3.5 × 3.5-inch posts embedded through that full depth. Net concrete is about 13.8104 cubic feet. With no added allowance and a 0.60-cubic-foot bag yield, buy 24 bags when pooling the concrete, or 30 if allocating whole bags separately to every hole.
Four 12-inch-diameter holes with 24 inches of concrete and 3.5 × 3.5-inch posts through that depth need about 5.6026 cubic feet. At 0.60 cubic feet per bag, that is 10 pooled bags or 12 when allocating three whole bags to each hole. These dimensions illustrate the calculation; they are not a sizing recommendation.
One 30-centimeter-diameter hole with 90 centimeters of concrete and a 10 × 10-centimeter post embedded through that depth needs approximately 54.6173 liters. If your bag label states a 20-liter yield, the pooled purchase is three bags before any added allowance.
All three examples use zero extra allowance. Replace every sample measurement and bag yield with your own project details. Select Try example in the calculator to load the ten-post example and see its worksheet.
Read the yield, not only the weight
Bag weight tells you what you carry. Mixed yield tells you how much space the finished concrete fills. These are approximate yields listed for QUIKRETE Concrete Mix No. 1101; they are labeled examples, not a universal rule for every brand or mix.
| Bag weight | Mixed yield | Approx. liters |
|---|---|---|
| 40 lb | 0.30 ft³ | 8.50 L |
| 50 lb | 0.375 ft³ | 10.62 L |
| 60 lb | 0.45 ft³ | 12.74 L |
| 80 lb | 0.60 ft³ | 16.99 L |
| 90 lb | 0.675 ft³ | 19.11 L |
Source: QUIKRETE Concrete Mix No. 1101 data sheet. Product instructions and yield checked October 30, 2026.
Standard concrete mix and fast-setting post products can have different handling instructions. A quantity worksheet does not replace those instructions. The calculator lets you type the yield for the actual product you plan to buy; it does not choose a mix or tell you whether to use a dry-pour method.
Practical answers
It depends on the concrete-filled hole dimensions, the actual post size and the yield of the bag. For the worked example of a 10-inch hole filled to 36 inches with a 3.5 × 3.5-inch post, net concrete is about 1.3810 cubic feet. At 0.60 cubic feet per bag, allocate three whole bags to that individual hole. Several holes can need fewer bags when the concrete is pooled.
Yes. The post occupies space that concrete cannot fill. Enter its actual cross-section and only the depth embedded in concrete. For a rectangular post, the width and thickness both matter. Select “No post to subtract” if you are estimating an empty cylindrical fill.
Enter the concrete-filled depth. Leave out gravel or other base material that is not concrete. If the post stops above the bottom of the concrete, enter its shorter depth inside concrete. Do not subtract that lower concrete layer as though the post occupies it.
QUIKRETE Concrete Mix No. 1101 lists an approximate yield of 0.60 cubic feet for an 80 lb bag. That is about 17 liters. Other products can differ. Use the yield on the specific bag or manufacturer data sheet, even when the weight matches.
Rounding each hole separately can leave usable yield allocated to that hole. Pooled mode adds all required volume before rounding; per-hole mode rounds each hole to a whole-bag count first. Choose the allocation that fits your work plan, product handling and crew instructions.
Yes. Choose a round post and enter its measured diameter, or use a square or rectangular cross-section. Add a different hole group for another hole diameter, concrete depth or post size. All groups use the same bag product and allowance within one worksheet.
This calculator does not determine safe hole dimensions. Those depend on the project, local requirements, loads, soil and other site conditions. Get the required dimensions for your installation first, then use them here to estimate material.
Use an allowance you have chosen for the actual project. It starts at zero rather than silently adding extra material. An allowance can account for some variation or handling loss, but it does not replace measurement of an enlarged or irregular hole. Avoid counting the same allowance twice.
QUIKRETE Concrete Mix No. 1101 — product data sheet — Primary source for the explicitly labeled approximate 40, 50, 60, 80 and 90 lb bag-yield examples and product-specific installation instructions. Checked October 30, 2026.