How Many Bags of Concrete Per Fence Post?

A 10-inch post hole with 24 inches of concrete holds about 1.1 cubic feet. That is two 80 lb bags or three 60 lb bags per post. Fourteen posts on a 100 ft run is about 28 bags of 80 lb.

The short count shows up on the last three holes. You did the math once for the whole run, divided, and rounded down to a tidy pallet number. Posts one to eleven got their two bags. Twelve, thirteen, and fourteen sit in braces while someone drives back to the yard with the truck half empty.

The concrete-per-post formula is:

cu ft per hole = π × (hole diameter ÷ 2)² × concrete depth ÷ 1,728

Measure in inches. Concrete depth is the hole depth minus any gravel base. Divide by your bag yield and round up per hole, not per job. Estimates only — not engineering or building-code advice.

How much concrete for one fence post?

Treat the hole as a cylinder. Radius is half the diameter. Height is the concrete column only — if you put 6 inches of crushed stone in the bottom, a 30-inch hole is a 24-inch concrete pour.

Example: 10 in hole, 24 in of concrete. Radius = 5 in. Volume = π × 5² × 24 ≈ 1,885 cubic inches. Divide by 1,728 → 1.09 cu ft. At 0.60 cu ft per 80 lb bag that is 1.8 bags, so two bags. At 0.45 cu ft per 60 lb bag it is 2.4, so three bags.

Diameter matters more than depth. Going from a 10 in to a 12 in auger adds about 44% more concrete per hole, because the radius is squared.

Should you subtract the post?

A 4x4 is about 3.5 × 3.5 inches actual. Over a 24-inch column it displaces about 0.17 cu ft, which drops a 10 × 24 hole from 1.09 to about 0.92 cu ft. In 80 lb bags that is still two per hole. A 6x6 takes out more, but the rounding usually lands the same.

Sizing the full hole is the conservative call: holes come out wider than the bit, and the top gets a crowned cap to shed water. Fence Takeoff Master sizes the full cylinder for the same reason.

The formula, on a tape

• Radius (in) = hole diameter ÷ 2
• Cu in per hole = π × radius² × concrete depth (in)
• Cu ft per hole = cu in ÷ 1,728
• Bags per hole = ceil(cu ft per hole ÷ bag yield)
• Total bags = bags per hole × number of posts (gate and corner posts separately if they are bigger)

Metric bags work the same way in litres: a 250 mm × 600 mm hole is about 29 L, so around four 20 kg bags at ~9 L each, or three 25 kg bags at ~10 L. Check the yield printed on your bag; brands differ.

Common mistakes

Rounding once for the whole job. Pooled math says 14 holes × 1.09 cu ft = 15.3 cu ft ÷ 0.60 = 26 bags. But you mix per hole, and each hole wants 1.8 bags. Two per hole is 28. The two-bag gap is the trip back.

Using the auger size as the hole size. Rocky or sandy soil blows a 10 in bit out to 11 or 12 in. Measure one dug hole before you order. A 12 in hole at 24 in is three 80 lb bags, not two.

Forgetting gate posts. Gate and corner posts often go wider and deeper. Count them as their own row — a 12 × 36 gate hole is four 80 lb bags by itself.

Worked example for a 100 ft run

About 14 posts at 8 ft spacing (see how many fence posts do I need). Holes are 10 in, 30 in deep with a 6 in gravel base, so 24 in of concrete. Two 80 lb bags per hole × 14 = 28 bags. Add the gravel from how much gravel do I need for fence posts: about 9 bags of crushed stone.

If you quote fences for a living, that bag count is one line on the estimate, next to posts, rails, pickets, gates, and gravel. Fence Quote Pro turns that takeoff into a PDF quote with your logo you can share from your phone. For the free material count, open Fence Takeoff Master. Related: how many bags of concrete do I need (slabs), Fence Takeoff Master review.

Estimates only — not engineering or building-code advice. Frost depth and post embedment follow your local code. Measure the hole you dug. Round up per hole.

Estimate only — not engineering or building-code advice.