Post Hole Concrete Calculator

Concrete = posts × ( (π ÷ 4) × hole diameter² − post width² ) × hole depth × (1 + wastage ÷ 100)

posts

How many holes you are digging — the page multiplies one hole by this

d

Diameter of the hole. The hole is treated as a cylinder, so π ÷ 4 × d² is its cross-section

p

Width of the post, treated as a square: a 4 inch post displaces 4 × 4 = 16 square inches, not the 12.6 of a 4 inch circle

h

Depth of the hole, which is also the length of post that sits inside the concrete

w

Wastage allowance, as a percentage — the concrete that spills over the edge or goes into a hole dug wider than planned

Use it before you buy bags, and dig one hole first so the diameter and the depth you enter are the ones you are actually getting rather than the ones on the plan — a hole dug with a clamshell or a bar is rarely round, and an oversized hole is the usual reason a job runs short. It also answers the reverse question: if you already have a pallet of mix, divide by the per-post figure to see how many posts it will set. For concrete going into a slab or a footing rather than a hole, use the concrete calculator; for the cores of a block wall, the block fill calculator.

1. Four fence posts: 10 inch holes, 30 inches deep, 4 inch posts, 5% allowed

   1. Hole cross-section: π ÷ 4 × 10² = 78.54 in²
   2. Hole volume: 78.54 × 30 = 2,356 in³
   3. Post displaces 4 × 4 × 30 = 480 in³
   4. Net per hole: 2,356 − 480 = 1,876 in³ = 1.085 ft³
   5. Four holes with 5% allowed: 1.085 × 4 × 1.05 = 4.56 ft³ = 0.129 m³
   6. In 80 lb bags: 4.56 ÷ 0.6 = 7.6, rounded up to 8

   Two bags a post, which is the answer to the question almost everyone arrives with. The post only takes 480 of the 2,356 cubic inches — a fifth of the hole — so the concrete, not the post, is what you are buying. The commonest way to get this wrong is to leave the post out of the calculation: filling the hole solid would be 5.73 cubic feet instead of 4.56, and that is an extra two bags for the same four posts.

2. Twelve deck posts: 12 inch holes, 36 inches deep, 6 inch posts, 10% allowed

   1. Hole cross-section: π ÷ 4 × 12² = 113.10 in²
   2. Hole volume: 113.10 × 36 = 4,072 in³
   3. Post displaces 6 × 6 × 36 = 1,296 in³
   4. Net per hole: 4,072 − 1,296 = 2,776 in³ = 1.607 ft³
   5. Twelve holes with 10% allowed: 1.607 × 12 × 1.1 = 21.2 ft³ = 0.6 m³
   6. In 80 lb bags: 21.2 ÷ 0.6 = 35.3, rounded up to 36

   Three bags a post here rather than two, and the extra is entirely the bigger hole: the post went from 4 to 6 inches square but the hole went from 10 to 12 inches across, and the hole is the term that grows with the square of its size. Twelve posts at 36 bags is where mixing by hand starts to hurt — it is over a tonne of mix, and the deck will not wait while you mix it one bag at a time.

3. Six holes 4 inches across and 24 inches deep, with no post in them

   1. Hole cross-section: π ÷ 4 × 4² = 12.57 in²
   2. Hole volume: 12.57 × 24 = 302 in³ = 0.175 ft³
   3. Post width 0, so nothing is displaced and the whole hole is concrete
   4. Six holes with 5% allowed: 0.175 × 6 × 1.05 = 1.10 ft³ = 0.031 m³
   5. In 80 lb bags: 1.10 ÷ 0.6 = 1.8, rounded up to 2

   Setting the post width to 0 models a hole with nothing in it — an anchor, a bollard, a socket for a removable post, or simply the check that the page is doing what you think it is. It is worth running once on a job like this: six small holes come to two bags, and the temptation to buy a pallet for a morning's work is how bags get left over and go hard in the shed.

The post is modelled as a square of the width you enter, so a 4 inch post displaces 16 square inches while a round post of the same diameter would displace 12.6 — the concrete figure for a round post is therefore a little on the high side, which is the safe direction for a bag count. The hole is modelled as a clean cylinder of the diameter you enter; a hole dug by hand or with a bar is usually wider than the tool, wider again in soft ground, and the failure mode is running short, not long, which is what the wastage field is for. The depth is taken as the length of post inside the concrete, so if you are bedding the post on gravel for drainage, enter the depth of the concrete rather than the depth of the hole — the gravel does not consume mix. One hole size is applied to every post: a run with two hole sizes or two post sizes needs two calculations. And the page counts bags, not setting: it does not place the concrete, slope the top away from the post, brace the post plumb while it cures, or allow for the fact that a post in a wet hole wants a different mix from a post in dry ground.

How many bags of concrete do I need per fence post?

One to three for a normal post, and the two answers above are typical: a 10 inch hole 30 inches deep with a 4 inch post takes 8 bags for four posts, so two each, while a 12 inch hole 36 inches deep with a 6 inch post takes three. The hole diameter is what moves it, because the volume grows with its square: going from 8 to 10 inches is nearly twice as much concrete once the post is taken out. Buy a bag more than the arithmetic says if you have never dug in that ground before.

Should I fill the post hole right to the top?

No — the concrete should finish a little above the surrounding ground and slope away from the post, so water runs off rather than standing against it. That crown is why the depth you enter should be the depth of the concrete rather than the depth of the hole: if the hole is 30 inches and you want 30 inches of concrete, enter 30, and if you are bedding the post on gravel the concrete starts above the gravel, not at the bottom of the hole.

Why does the calculator ask for the post width?

Because the post occupies part of the hole and only the rest is concrete. Leaving it out is the commonest mistake here: a 4 inch post in a 10 inch hole 30 inches deep accounts for 480 of the hole's 2,356 cubic inches, so ignoring it overstates the concrete by about a quarter. The post is treated as a square of that width, which is right for the sawn posts most fences and decks use and slightly generous for a round one.

Can I use this for a deck or a fence on a slope?

The concrete is the same per hole for every post — a hole of a given diameter and depth holds the same whether the ground around it is level or not, and the depth is measured down the hole rather than as a fall on the plan. What changes on a slope is the post length, not the concrete, so treat footing depth and post length as two separate questions.

What if my holes end up a different size?

Dig one and measure it before you order, because ground rarely takes the size of the tool and the error is always in the same direction — a hole dug with a bar or a clamshell is wider than the auger that started it. Measure the hole across the middle rather than at the rim, and remember that widening the diameter by an inch takes a quarter more concrete. That is what the wastage field is for.

Is there a faster way than mixing bags?

For a dozen posts or more, a mixer and two people beats a wheelbarrow and a hoe by a wide margin, and some suppliers will deliver a small load for a run of deck footings. What does not work well is mixing a whole pallet in one go: concrete sets on its own timetable, so the bags have to be mixed and placed post by post, which is an argument for ordering the right number rather than a few spare.

• Approximate Conversions from U.S. Customary Measures to Metric — the chart behind the inch, cubic foot and pound figures used here — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the metre and its multiples, the units the hole and post dimensions are defined in — Bureau International des Poids et Mesures (BIPM)