Fence Post Concrete Calculator

Background.

A 50 lb bag of fast-setting concrete makes about 0.375 cubic feet once it is in the ground. Poured around a 4×4 in a 10-inch hole, that fills the hole to a depth of less than ten inches. Set the post 30 inches deep and each hole swallows a little over three bags, which is why a fence that looked like a two-bags-a-post job empties the pallet with posts still standing in loose dirt.

The estimate starts from the shape of the hole. A post hole is a cylinder, so its volume is π times the radius squared times the depth. The post takes up part of that space — a square post its width squared times the depth, a round post its own small cylinder — and the concrete fills only what is left. Multiply the remainder by the number of posts, add a waste allowance for holes that come out wider or deeper than planned, and divide by the bag's yield. The result is rounded up, since you cannot buy part of a bag.

Diameter is the measurement to be careful with, because volume grows with its square. Widening 18 holes from 10 to 12 inches, with everything else unchanged, takes the order from 61 bags to 93. Depth acts in a straight line: the concrete column is the depth that matters, so leave out the gravel base and the few inches of soil that go back on top.

Yields differ by product and bag size, so use the figure printed on the bag you are buying. The arithmetic gives you a quantity of material; it does not decide how deep or wide a hole has to be to hold a gate, a tall privacy fence or a deck. Those depend on soil, frost depth, wind load and local code.

What is fence post concrete calculator?

A post concrete estimate is a volume takeoff for the material that fills the gap between a post and the sides of its hole. For one post that volume is the cylinder of the hole less the part of it occupied by the post, measured over the depth of the concrete column. Across a fence or deck it is that volume times the number of posts, increased by a waste allowance, and divided by the yield of one bag — the volume of placed concrete the bag makes, which manufacturers print on the bag and data sheet. Dimensions go in inches, volumes come out in cubic feet, and the answer that matters on the day is the whole number of bags to put on the truck. Gravel under the post and the soil cap on top are not concrete and stay out of the sum: QUIKRETE's guide puts 6 inches of gravel in the bottom of the hole to help drainage and finishes the last 3 to 4 inches with the soil dug out, so a 30-inch hole may hold a concrete column nearer 20 inches.

How to use this calculator.

1. 01Enter the number of posts that will be set in concrete. Gate posts count; posts set in gravel or driven into the ground do not.
2. 02Enter the diameter of the hole as you will actually dig it, and the depth of the concrete column: the hole depth less any gravel base and less the 3 to 4 inches of soil or sod that go back on top.
3. 03Choose square or round and give the post's actual size. A nominal 4×4 measures 3.5 inches; for metal or round posts, use the outside diameter.
4. 04Copy the yield from the bag you are buying, in cubic feet. QUIKRETE's 50 lb Fast-Setting Concrete Mix is about 0.375 and its 60 lb bag about 0.45.
5. 05Set a waste allowance — 10% suits neatly augered holes, more for hand-dug or crumbly ones — and the price per bag, then read the bag count and cost.

The formula.

For each hole, V_hole = π × r² × d, where r is half the hole diameter and d is the depth, both converted from inches to feet. The post's share is V_post = p² × d for a square post of width p, or π × (post diameter ÷ 2)² × d for a round one, both assuming the post runs the full depth of the concrete. Concrete per post is V_hole − V_post. The total is that figure times the number of posts N, then multiplied by (1 + w), where w is the waste percentage divided by 100. Bags = ⌈total ÷ y⌉, with y the yield of one bag in cubic feet, and the cost is the bag count times the price per bag.

The sensitivities follow from the formula. Doubling the depth or the number of posts doubles the concrete. Doubling the hole diameter roughly quadruples it, which is why an auger that cuts an inch wider than expected moves the order more than an extra few posts. The post's share is modest — for a 3.5-inch post in a 10-inch hole it is about 15.6% of the hole — but on a long fence it adds up to several bags.

The model treats every hole as a clean cylinder with the post centred and running to the bottom. Real holes flare at the top, belly out where stones come loose, and are sometimes dug dish-shaped on purpose in loose or sandy soil, as QUIKRETE recommends; the waste allowance is there to absorb that. A post's width has to be smaller than the hole's diameter or the calculation is refused. Yields are the manufacturer's approximate figures. Rounding is applied to the whole job rather than hole by hole: rounding each post up to 4 bags would buy 72 for the example, where 61 will do.

A worked example.

Frequently asked questions.

References& sources.

1. [1]QUIKRETE (n.d.). "Concrete Calculator - How Much Do I Need?" QUIKRETE.
2. [2]QUIKRETE (n.d.). "Setting Posts." QUIKRETE Project Guide.
3. [3]QUIKRETE (2026). "Fast-Setting Concrete Mix, Product No. 1004." Technical Data Sheet.
4. [4]Sakrete (n.d.). "Fast-Setting Concrete Mix." Sakrete.
5. [5]Sakrete (2021). "Fast Setting Concrete Mix." Product Data Sheet.
6. [6]National Institute of Standards and Technology (2023). "NIST Handbook 133, Appendix E: General Tables of Units of Measurement." U.S. Department of Commerce.