Concrete for post holes is the volume of the concrete-filled excavation minus the space occupied by the embedded post. Multiply by the number of matching holes, add a chosen allowance, divide by mixed yield per bag and round up. Pooling several holes before rounding can give a different purchase count from rounding each hole separately.
Use the concrete-filled dimensions
A cylindrical hole has a volume of pi multiplied by radius squared and depth. Use the depth that will actually contain concrete. If the installation includes a gravel layer, subtract that layer from total excavation depth before entering the value. An uneven real hole can require more material than a neat cylinder.
Subtract the space occupied by the post
A post takes up volume inside the hole. For a square post, multiply width by width to get the cross-sectional area. For a round post, use pi times radius squared. Multiply that area by concrete depth and subtract it from the gross hole volume. Use measured dimensions rather than a nominal product name.
Read the mixed yield on the bag
A dry bag’s weight is not its final volume. Look for the stated mixed yield, typically given in cubic feet or liters. Enter the yield for the exact product you plan to buy. Our example of 0.6 cubic feet per bag is editable and does not describe every bag on the market.
Pool the volume before rounding bags
For several identical holes, multiply the net volume of one hole by the hole count. Add the chosen extra allowance, divide by bag yield and round the final bag quantity up. This pooling assumes you can use mixed concrete across the project; if each hole is a separate purchase or batch, allow for the extra rounding.
An example you can check
A 12-inch-diameter hole with a 24-inch concrete depth holds about 1.571 cubic feet before the post is inserted. A 3.5-inch square post displaces about 0.170 cubic feet. That leaves about 1.401 cubic feet of concrete for one hole, before any allowance. The calculator shows net volume and the final rounded bag quantity.
Keep quantity separate from design
This calculation does not choose the correct foundation dimensions. Hole depth, footing size and concrete specification depend on the project, soil, frost conditions, loads and local requirements. Use an appropriate design first, then enter those dimensions. Follow the concrete label for mixing, handling, placing and curing.
Compare pooled purchases with bags rounded per hole
Using the example hole dimensions above, one hole contains about 1.4007 ft³ of concrete before extra allowance. At 10% extra and a 0.6-ft³ bag yield, the requirement is about 2.57 bags, rounded up to three. Eight identical holes together need about 20.54 bags, rounded up to 21. Buying three complete bags independently for every hole would instead mean 24 bags.
Neither number tells you how to organize mixing or placing. The difference is the rounding assumption: a pooled estimate uses the combined liquid volume before rounding the final purchase, while a per-hole plan repeats whole-bag rounding. Decide which assumption matches the way materials will be purchased and used. Keep the unrounded volume in your notes so the calculation can be adjusted.
Bag costs are also scenario inputs. With a $7 example price, 21 bags produce a $147 subtotal and 24 bags produce $168. That comparison excludes tools, delivery, labor and any unused mix. Do not read it as a recommendation to alter mixing instructions or add water to stretch the yield. Follow the chosen product’s directions.
Your final shopping-list checks
- Concrete-filled depth after excluding any gravel layer.
- Hole diameter and actual post width or round-post diameter.
- Count of identical holes, with different sizes grouped separately.
- Mixed yield and price from the exact bag product.
- A recorded allowance and a clear pooled or per-hole rounding method.
Check the post-hole concrete bag estimate with your measured dimensions. For a fence project, the fence component calculator and fence materials checklist help keep posts, concrete and other supplies on separate lines.
For the product context behind this planning guide, see QUIKRETE Concrete Mix product data sheet. The worked examples are our own calculations; use the instructions for your exact product.
Frequently asked questions
What changes if some holes are wider?
Calculate each dimension group separately and combine the unrounded volumes if using the same mix. Applying one average diameter can hide a meaningful volume difference because diameter is squared in the cylinder formula.
Can I enter a gravel base as concrete depth?
No. Enter only the part filled with concrete. The tool has no separate gravel field, so it will include the whole entered depth in the concrete calculation.
Does the bag estimate tell me the right concrete strength?
No. Quantity and specification are different questions. Choose the appropriate mix and foundation dimensions from the project requirements, then use its stated yield for the material estimate.