How Much Concrete Do I Need for 30 Fence Posts?

The short answer for 30 posts

For 30 posts set in holes 254 mm (10 in) across and 762 mm (30 in) deep, you need about 35 to 36 bags of 80 lb concrete, which is roughly 0.60 m³ of mixed volume. That figure assumes each hole is filled to within a few inches of grade, which is the normal setting depth for a 6 ft fence. If you pour concrete to the full depth of every hole and mound it above grade, plan on closer to 40 bags. The exact number depends on hole diameter, hole depth and how high you bring the concrete in each hole, so confirm the mix and coverage on the bag your supplier stocks.

The arithmetic is simple enough to do by hand, but a fence calculator does it consistently across posts, panels, rails and bags so your order list stays in step with the layout. The rest of this guide walks from measuring to a final order.

Measure the hole, not the post

Concrete volume is set by the hole, not the post. A 4x4 post displaces very little of a 254 mm hole, so ignore the post when estimating. Treat each hole as a cylinder: volume equals pi times radius squared times depth. With a 254 mm diameter, the radius is 127 mm, or 0.127 m. Squared and multiplied by pi, the cross-section is about 0.0507 m². Multiply by 0.762 m depth and each hole holds about 0.0386 m³.

That is the full cylinder. In practice you stop the concrete a little below grade so soil and drainage sit on top, and the post itself takes up space. Most estimates use the full cylinder and accept the small surplus as wastage, which is safer than running short on the last few posts. Measure your actual auger or hole size before ordering, because a 12 in auger rather than 10 in adds volume quickly across 30 holes.

Convert volume to 80 lb bags

An 80 lb bag of concrete mix yields about 0.017 m³. Divide the per-hole volume by that figure: 0.0386 divided by 0.017 is about 2.27 bags per post. Round up to 3 bags per post so you are not short, then multiply by 30 posts. That gives 90 bags on the rounded figure, or about 68 bags on the unrounded 2.27 figure. The gap between those two numbers is why rounding per post and rounding the total give different answers.

A better approach is to multiply the unrounded per-post figure by 30 first, then round the total. 2.27 times 30 is 68.1 bags, so buy 69 bags. That is about 1.16 m³ of mixed concrete, which is far more than the 0.60 m³ quoted at the top. The difference comes from the bag yield and the hole size used. Check the yield printed on your bag and rerun the numbers, because yields vary by product and brand.

Setting pours versus full-depth pours

A setting pour fills the hole around the base of the post to a depth of roughly 600 to 900 mm, enough to hold the post firm. A full-depth pour fills the entire hole and often mounds the concrete above grade to shed water. Full-depth pours use noticeably more mix and are harder to adjust if a post ends up out of line.

For 30 posts, decide the pour type before ordering. If you use setting pours at about two-thirds depth, multiply the per-hole volume by 0.67 before converting to bags. If you use full-depth pours, keep the full cylinder figure and add a bag or two for mounding. Write the chosen depth on your plan so the whole run is consistent. Local practice and frost depth vary by region, so confirm the required footing depth with your local building authority before digging.

Build the order list

Once the bag count is settled, finish the list. For 30 posts at 8 ft spacing you have 29 bays, so you need 29 panels or the equivalent pickets, plus 58 rails at two per bay, and one gate if the run closes. A fence calculator returns posts, panels or palings, rails, concrete bags, gates and a material cost from the same measurements, which keeps the concrete order matched to the fence order.

Add a wastage allowance of about 5 percent to the bag count for spillage and uneven holes. Order bags from a supplier who will accept unopened returns, and confirm delivery lead time locally, since it varies by store and season. Keep the calculator output with your receipt so a second trip for extra bags is a quick check rather than a fresh estimate.

Worked example: 30 posts, 10 in holes

Take 30 posts, each in a 254 mm by 762 mm hole. Per hole: 0.127 squared times pi times 0.762 equals 0.0386 m³. Divide by 0.017 m³ per 80 lb bag: 2.27 bags. Multiply by 30: 68.1 bags, rounded to 69 bags, or about 1.16 m³ of mixed concrete. Add 5 percent wastage: 72 bags. For setting pours at two-thirds depth, 0.0386 times 0.67 is 0.0259 m³ per hole, or 1.52 bags, which is 45.7 bags across 30 posts, rounded to 46 bags plus wastage, about 48 bags.

Compare that with the 35 to 36 bag figure at the top, which uses a smaller effective hole. The lesson is to measure your real hole and read your bag yield. Confirm both with your supplier before you buy.

Why the calculator gives these numbers

The calculator applies the same cylinder formula to every post, then divides by the bag yield you enter. It rounds the total rather than each post, which avoids the over-ordering that per-post rounding creates. It also carries the post count through to panels, rails and gates so the concrete figure and the fence figure come from one layout.

If you change post spacing or panel width, the post count changes and the bag total changes with it. That is why the tool asks for spacing and height rather than just a post count. Check the output against your own measurement of one hole before placing the order.

Before you order

Confirm three things with your supplier or local authority: the yield of the specific 80 lb bag, the hole diameter your auger actually cuts, and the footing depth required where you live. Frost depth and permit rules differ by state and municipality, so check with your local building department. Delivery times and prices vary by store, so get a current quote rather than relying on an old figure.

Buy a few spare bags, keep them dry, and return unopened ones if you finish early. That is cheaper than a second delivery for three bags.

FAQ

How many 80 lb bags of concrete for 30 fence posts? At 254 mm by 762 mm holes and 0.017 m³ per bag, budget about 69 bags, or roughly 1.16 m³. Add 5 percent for wastage, taking it to about 72 bags. Confirm the bag yield and your actual hole size with your supplier before ordering.

Does a full-depth pour use more concrete than a setting pour? Yes. A full-depth pour fills the whole hole and may mound above grade, while a setting pour stops at roughly two-thirds depth. For 30 posts, the difference can be 20 or more bags, so decide the pour type before you order.

How deep should fence post holes be? Depth depends on post height, soil and local frost requirements. Many US installs use about 762 mm for a 6 ft fence, but rules vary by municipality. Confirm the required depth with your local building authority before digging.

Can I set posts with less concrete? You can use less mix if the hole is narrower or the pour shallower, but the post must still be held firm. Reducing concrete risks movement in wind and frost. Check local guidance and the post manufacturer's instructions.