Cubic Yards Of Concrete For A Fence Post

Before you order a truck or stack bags in the driveway, you need a realistic quantity for cubic yards of concrete for a fence post. Too little concrete means a cold joint halfway through the pour, a second trip, and a weak seam; too much means bags or a short-load fee you did not need. This guide walks through four fence post holes, each 12 in across and 30 in deep with the volume formula, the typical bag yields from our material data, and a 10% waste allowance, and it ends with a range you can actually order against. Everything here is an estimate — measure twice, check the bag label, and buy a little extra for the parts of the job you cannot see until the forms come off.

The calculator on this page asks for the same inputs the worked example uses: the dimensions of four fence post holes, each 12 in across and 30 in deep, the depth, and the bag size. It returns a range, because the honest answer is a range — the low end assumes a flat base and no spillage, and the high end assumes the usual 10% waste. Plan your order on the high end. A spare bag costs little; a short pour costs a second delivery, a cold joint, and a visible repair.

How the calculation works

The calculator needs the shape of the pour, the depth, and the bag size you plan to buy, and each one changes the answer. For a rectangular pour the volume is length times width times depth, all in feet, divided by 27 to get cubic yards — one cubic yard is 27 cubic feet (0.76 cubic metres). Depth is where most mistakes happen: 4 in (10 cm) is 0.333 ft, 6 in (15 cm) is 0.5 ft, so always convert inches to feet before multiplying. A slab that is 10 ft by 10 ft at 4 in thick is 100 times 0.333, or 33.3 cubic feet, which is about 1.2 cubic yards before waste. The same slab at 6 in thick is half as much again, about 1.9 cubic yards, which is why the thickness in the keyword matters as much as the footprint.

Once the volume is known, it is converted to bags. Our material data gives typical yields of about 45 80 lb (36.3 kg) bags per cubic yard and about 60 60 lb (27.2 kg) bags per cubic yard, because an 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet — those are typical values, and the bag label on the exact product you buy governs. On top of that, add the typical waste allowance of about 10% for spillage, an uneven subgrade, and the concrete that stays in the mixer and the wheelbarrow. The worked example below shows the low end with no waste and the high end with the full 10% allowance, so you can see the range instead of a single number that pretends the pour will be perfect.

A post hole is a cylinder, so the formula is pi times radius squared times depth, with every dimension in feet. For a 12 in hole the radius is 6 in, or 0.5 ft. The post itself displaces a little concrete — a 4x4 post takes up about 0.11 square feet of the cross-section — but the worked example ignores that, because the hole is usually dug a little wider than planned and the difference is inside the 10% waste allowance. If your holes are dug with an auger, measure the real diameter; an auger often cuts wider than its nominal size.

Worked example: four fence post holes, each 12 in across and 30 in deep

Here is the full arithmetic for four fence post holes, each 12 in (30 cm) across and 30 in (76 cm) deep.

Step 1 — radius in feet. Diameter 12 in, so the radius is 6 in, or 0.5 ft.

Step 2 — one hole. Pi × 0.5² × 2.5 ft deep = 3.1416 × 0.25 × 2.5 = 1.96 cubic feet per hole.

Step 3 — four holes. 1.96 × 4 = 7.85 cubic feet, or 7.85 ÷ 27 = 0.29 cubic yards.

Step 4 — bags, low end. At the typical yield of about 45 80 lb bags per cubic yard: 0.29 × 45 = 13.1 bags, rounded up to 14 bags. With 60 lb bags: 0.29 × 60 = 17.5, rounded up to 18 bags.

Step 5 — bags, high end with the 10% waste allowance. 0.29 × 1.10 = 0.32 cubic yards, or 15 80 lb bags (20 60 lb bags). Order 14–15 80 lb bags for the four posts — and for a full fence, multiply the per-hole figure by the number of posts before you order, because a 20-post fence needs five times this amount.

Cost guidance

Budgeting for cubic yards of concrete for a fence post means pricing the range, not a single number. Take the worked-example quantity (0.29–0.32 cubic yards, or about 14–15 80 lb bags) and price it twice: once at the low end of the range and once at the high end, using the current shelf price per bag at the store where you will actually buy, or the per-cubic-yard ready-mix quote from a local supplier. The difference between the two totals is your contingency, and it is usually smaller than the cost of running short mid-pour. Two cost traps catch first-timers: ordering the low end and then paying a second delivery charge for the few bags that finish the job, and forgetting the extras — formwork, stakes, rebar or mesh, a float, and curing compound or plastic sheeting. Compare the total delivered cost, not the per-bag price, and keep one or two unopened bags for patching; an opened bag that gets damp is wasted money.

If the job grows while you measure — a footing that turns out longer than the plan, a sidewalk that needs an extra section — redo the volume from the new dimensions rather than guessing a percentage on top of the old total. Concrete quantities scale with volume, not with length alone, so a small change in depth changes the total more than most people expect. And if you are comparing bagged mix with ready-mix, remember the comparison only works when both quotes cover the same volume including waste, the same delivery distance, and the same extras; a cheap per-bag price with an expensive delivery is not a cheap job.

Step by step: measure, calculate, order

1. Measure twice. Measure the hole diameter in inches, the depth in inches, and the number of posts. Measure each run separately — a footing that turns a corner is two runs, not one — and write every dimension down in feet and inches before you convert anything.

2. Convert and calculate. Convert inches to feet (divide by 12), then apply the volume formula for your shape: length times width times depth divided by 27 for a rectangle, or pi times radius squared times depth for a round tube or post hole. Use the calculator on this page to check your arithmetic, and run it twice with the low and high depth if your excavation is uneven.

3. Add waste and round up. Add the typical 10% waste allowance for spillage and an uneven subgrade, then round up to whole bags or to the nearest quarter cubic yard for a truck. A bag cannot be bought in fractions, and a pour that stops short is far more expensive than one spare bag.

4. Order and check the delivery. Order the high end of the bag range below, all the same mix, all the same product and mix, and check the bag labels against the yields the calculator assumed. If you are ordering ready-mix, confirm the slump, the mix strength and the delivery window, and make sure the truck can reach the pour.

5. Before you pour. Set and brace the forms, check the depth with a straightedge in several places, wet the subgrade lightly so it does not steal water from the mix, and have your screed, float and curing cover within reach. Concrete waits for nobody once it is mixed.

Common mistakes to avoid

Three mistakes ruin more small concrete jobs than anything else. First, using the nominal depth instead of the real one — a slab formed with 2x4s is often poured a little deeper than 4 in once the base settles, so measure after the base is compacted. Second, mixing bag counts from two different bag sizes: 80 lb and 60 lb bags yield different volumes, so pick one size and do all the arithmetic in that size. Third, pouring without a plan for finishing and curing — concrete that dries too fast cracks, and concrete that is not screeded level cannot be fixed later. Have the tools, the help and the curing cover ready before the first bag is mixed, and never add extra water to make a stiff mix easier to place; it weakens the finished pour.

Frequently asked questions

How much concrete do I need for a fence post?

Use pi times radius squared times depth. One 12 in hole, 30 in deep, holds about 1.96 cubic feet; four posts need about 0.29 cubic yards, or 14–15 80 lb bags including the typical 10% waste allowance. Multiply the per-hole volume by the number of posts for the whole fence.

How deep should fence posts be set in concrete?

The depth is set by local code and by the frost line — in cold climates the concrete should reach below frost depth so the post is not lifted by freezing ground. About 30 in is common for a 6 ft fence in many areas, with at least a third of the post below ground. Check the code for your area before you dig.

Should I use concrete or gravel for fence posts?

Concrete holds the post firmly and resists leaning, which is why it is the usual choice for a fence that must stay straight. Gravel drains better and lets a post be replaced more easily, but it holds less firmly in soft soil. In wet ground, concrete with the top sloped away from the post keeps water from pooling against the wood.

How many bags of concrete per fence post?

For a 12 in hole 30 in deep, about 3.5 80 lb bags per post on average — but buy by the total, not per post, because you cannot buy part of a bag. Four posts need 14–15 bags. A 20-post fence at the same hole size needs roughly 70–75 bags.

How long before I can hang fence panels on new posts?

In normal conditions, about 24–48 hours before the posts take a load, and longer in cold or wet weather. Brace the posts plumb while the concrete sets and do not hang panels or gates until the concrete has had at least a day to gain strength. A gate post takes more load than a line post, so give it the full two days.

Related calculators

Results are estimates — verify with the product label. Coverage, pack sizes, yields and prices vary by brand, product line, surface and region; the label on the product you buy governs. Cost figures are typical retail ranges — check the current shelf price before budgeting.