Fence post calculator

Bags of concrete per fence post — the hole volume minus the post that sits in it. Fast-set 50 lb or standard 60/80 lb, imperial or metric, with the two bag counts most calculators collapse into one shown side by side.

01 How the math works

The concrete in a post hole is the hole minus the post. Dig a round hole, drop the post in, and the concrete fills what's left around it. That subtraction is the part most fence-post calculators skip — they hand you the hole volume and quietly forget the post takes up a share of it.

Everything in consistent units. Two numbers set the hole. Diameter runs about three times the post width, so a 4-inch post takes a 12-inch hole — Quikrete's stated starting figure, not a structural value. Depth runs one-third to one-half of the post's height above ground, which puts a 6 ft (1.83 m) fence post at least 2 ft (0.61 m) down.

The depth rule has a hard override. The bottom of the hole must also reach the greater of the 12 in (305 mm) minimum footing depth and your local frost line, and that frost line runs from 0 in in the frost-free far south to more than 60 in (1,524 mm) in the coldest northern states. North of the frost-free zone, the frost depth usually governs — not the one-third rule.

02 Worked example

Take a 96 ft run of 6 ft privacy fence, 4×4 posts (actual 3.5 in / 89 mm) at 8 ft on-center: 96 ÷ 8 + 1 = 13 posts. Each post sits in a 12 in (305 mm) hole, 2 ft (0.61 m) deep.

Per hole, the hole holds 1.57 ft³ and the post below grade takes up 0.17 ft³, so the concrete is 1.40 ft³ (0.0397 m³). Across 13 posts that's 18.21 ft³ (0.516 m³).

In 50 lb fast-setting bags at 0.375 ft³ each, the pooled math is 18.21 ÷ 0.375 = 49 bags. Set hole-by-hole, each hole takes ⌈1.40 ÷ 0.375⌉ = 4 bags — 52 bags — because the leftover of the fourth bag at one hole doesn't ride along to the next. Add a 10% margin to the pooled figure and it rounds to 54. Buy the larger of the two.

03 When this calculator is wrong

Two things move the honest number away from a single pooled bag count, and both cut in the direction of ordering more. The right calibration on any material estimate is what to buy, not what the math gives you — round up one more increment before you round down.

• You buy per hole, not per pour. The pooled count and the count you actually open are different numbers. You mix and set one hole at a time, and the partial bag left at each hole doesn't carry — a 1.40 ft³ hole burns four fast-set bags and wastes most of the fourth. Across 13 posts that's 52 bags opened against the 49 the pooled math predicts. The bag has a picture of a plumb fence post on it; it does not have a picture of the three-quarters of a bag left over at every hole. Buy for the per-hole count.
• Frost line overrides the one-third rule. In a jurisdiction with a 42 in (1,067 mm) frost line, a 2 ft hole is too shallow and the post can heave in the first hard winter. Set the depth to the greater of the 12 in minimum and the local frost figure first, then check that it also clears one-third of the post height. Deeper holes hold more concrete — a 4×4 in a 12 in hole needs about 0.70 ft³ more per extra foot of depth.
• The yield is the mixed figure. The 0.375 ft³ per 50 lb bag is Quikrete's yield for mix-and-place. Fast-set poured in dry and hydrated in the hole packs differently, and real yield from any bag drifts with how much water goes in. Treat the bag count as a floor, not a promise.

04 What to do with the result

Order the larger of the two bag counts, and if you're setting in cold or wet ground, add one more bag for every five posts — a heaved or under-filled post is a redo, and the extra bag is a rounding error against that. Fast-set is worth it when you need to plumb and brace in one pass; standard 60 or 80 lb mix is cheaper per cubic foot when you can let the posts sit braced overnight.

For a long run — past about 20 posts, or holes deeper than 3 ft — price the bags against a small ready-mix load or a rented mixer and bulk mix. Somewhere in that range the bag count stops being a convenience and starts being a Saturday. Confirm the required depth with your local building department before you dig; the frost figure is theirs to set, not the calculator's.

05 Common questions

How many bags of concrete per fence post?

For a 4×4 post in a standard 12 in (305 mm) hole 2 ft (0.61 m) deep, the net concrete is 1.40 ft³ per hole — about four 50 lb fast-set bags (0.375 ft³ each), or a shade over two 80 lb or three 60 lb standard bags of mixed concrete. Deeper or wider holes need proportionally more; set the numbers above and read the per-hole count.

How deep should a fence post be?

One-third to one-half of the post's height above ground is the rule of thumb — 2 ft for a 6 ft fence. But the hole must also reach below your local frost line (0 to 60+ in depending on the jurisdiction) and no less than the 12 in code-minimum footing depth. Use whichever is deepest.

How wide should the post hole be?

About three times the post width — a 4 in post takes a 12 in (305 mm) hole. A wider hole holds more concrete and resists frost heave and lateral load better, which matters most on gate posts and end posts carrying the pull of the fence.

Do you need gravel at the bottom of a post hole?

A few inches of compacted gravel under the post gives water somewhere to drain instead of pooling against the end grain. It also lifts the post off the bottom, which reduces the concrete slightly — the displacement math above assumes the post runs the full depth, so gravel makes the real concrete a touch less than calculated, not more.

Can you pour dry concrete in a post hole?

Fast-setting mix is made for it: brace the post, pour the dry mix around it, add water per the bag, and it sets in 20–40 minutes. The 0.375 ft³ yield is the mixed figure, so dry-set holes can take marginally more or less depending on how the mix packs. Standard concrete mix should be mixed before placing.