Boneyard Tools

Setting fence posts in concrete the right way

How to size a post hole, why depth and diameter drive the concrete count, and the pour and curing steps that keep a set post from heaving or leaning.

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Sizing the hole before you buy concrete

The strength of a set post comes mostly from depth, not from a fat collar of concrete at the surface. A common rule sets the hole depth at about a third of the post's finished above-ground height and the diameter at roughly three times the post width. Digging below the frost line matters just as much, because water in the soil expands as it freezes and can lift a shallow post over a winter. Getting these two numbers right first is what makes the bag estimate meaningful, since the calculator simply follows the hole you describe.

Why the ring of concrete is smaller than the hole

It is easy to overbuy by pricing the full hole, but the post itself sits in the middle and takes up real space. The calculator models the hole as a cylinder and the post as a square column inside it, then fills only the difference. A 4 in post in a 10 in hole 30 in deep leaves about 1.09 cubic feet to fill, which is three 60 lb bags, whereas pricing the whole cylinder would have suggested four. Subtracting the post is what keeps the order honest.

Mixing, pouring and setting plumb

You can mix the bags in a tub or wheelbarrow, or use a fast-setting product that is poured in dry and soaked in place. Either way, brace the post plumb with temporary stakes before the concrete goes in, and check two adjacent faces with a level rather than eyeballing it. Slope the top of the concrete slightly away from the post so rain sheds off instead of pooling against the wood. Do not hang gates or heavy rails until the mix has cured enough to bear load.

Curing, drainage and common mistakes

Standard premix reaches most of its strength over several days, so leave the braces on for at least a day and avoid stressing the post early. A few inches of gravel under the post gives water somewhere to drain instead of sitting against the end grain, though remember to shorten the concrete depth you enter if you add it. The frequent mistakes are digging too shallow, skipping the frost-line check, and forgetting that a wider hole than planned quietly raises the bag count, which is why rounding up and keeping a spare bag on hand pays off.

Frequently asked questions

Do I really need to go below the frost line?

In climates that freeze, yes. Water in the soil expands when it freezes and can push a shallow post upward year after year. Setting the base below the frost line keeps the post anchored in soil that stays stable through winter.

Can I pour fast-setting concrete dry into the hole?

Many fast-setting products are designed to be poured in dry around a braced post and then wetted, which suits single posts. Enter that product's cubic-foot yield in the Bag yield box so the count matches, since it can differ from standard premix.

How long before I can hang a gate or attach rails?

Wait until the concrete has cured enough to carry load, which for standard premix is typically a few days rather than a few hours. Fast-setting mixes firm up sooner, but check the bag, and leave the bracing on until then.