Fence Post Concrete Calculator

Before any concrete goes in, the hole itself needs to be sized correctly. Two rules of thumb do most of the work:

• Depth:bury roughly one third of the post's overall height, and make sure the bottom of the hole sits below your local frost line. In cold climates the frost line can be several feet down, which usually matters more than the one-third rule — check with your local building department for the number in your area.
• Width:a hole about 3 times the width of the post gives enough concrete around it to resist wobble and wind load. Much narrower and the post won't be held firmly; much wider just wastes concrete.

It's also worth adding a few inches of gravel at the bottom of the hole before setting the post. This gives water a place to drain to instead of sitting against the base of the post, which helps prevent wood rot and reduces how much frost heave can move the post over time. As always, treat these as general DIY guidance and check your local building code for anything structural, like a fence that also serves as a retaining structure or a gate post carrying extra load.

The concrete you need per post is the volume of the hole minus the volume the post itself takes up — you're not filling the whole hole with concrete, just the gap around the post:

Volume = [π × (hole diameter ÷ 2)² × depth] − [post width² × depth]

Working in inches keeps the geometry simple, then dividing by 1,728 converts cubic inches to cubic feet. For example, a 4x4 post (actually 3.5×3.5 inches) set in a 10-inch-diameter hole dug 24 inches deep:

(π × 5² × 24) − (3.5² × 24) = 1,884.96 − 294 = 1,590.96 in³ ≈ 0.92 ft³

That's about 0.92 cubic feet of concrete for one post — roughly 2 bags of 80 lb mix (0.60 ft³ each) or 3 bags of 50 lb mix (0.375 ft³ each) once you round up to a whole bag. This calculator does that math for every post size, hole size, and bag size combination automatically.

One tip worth knowing: for a multi-post job, total up the cubic feet across all posts first, then round up to whole bags once — don't round up per post and add those together. For 10 of these posts, rounding once needs 25 bags of 50 lb mix, but rounding each post individually and adding them up comes to 30 bags. That difference is five bags of concrete you didn't need to buy.

Most fence post projects use a fast-setting concrete mix (sold under names like Fast-Setting Concrete or similar), and the common method is a dry pour: set the post plumb in the hole, pour the dry mix straight from the bag into the gap around it, then add water on top and let it soak through — no mixing required. Fast-setting mixes are formulated for exactly this, typically holding the post in place within 20 to 40 minutes.

Regular concrete mix works too, but it's meant to be mixed with water to a proper consistency before pouring, either in a wheelbarrow or mixing tub. It takes longer to set — often a full 24 to 48 hours before the post can safely take side-load from attached rails or panels — but it's a reasonable choice if you're already mixing concrete for other parts of the same project. Either way, follow the water ratio and set times printed on your specific bag, since they vary by brand and formulation.

• Digging the hole too narrow. A hole barely wider than the post leaves almost no concrete around it, which makes for a weak, wobbly connection. Aim for roughly 3× the post width.
• Filling the hole flush or with a flat top. A flat-topped pour lets water pool right against the post and soak into the wood. Instead, crown the concrete slightly above grade around the post and slope it away, so rainwater runs off instead of collecting.
• Skipping the gravel base. Without a drainage layer at the bottom of the hole, water that gets in has nowhere to go and sits against the buried end of the post.
• Not checking plumb before the concrete sets. Once fast-setting mix grabs, the post is locked in whatever position it was in — recheck level and plumb every few minutes while it firms up, not just once at the start.