Fence Post Concrete Calculator: How Much Per Post

Setting fence posts is the most common small concrete job there is, and the one where ordering is most often wrong — because the post itself occupies a good share of the hole. On a 4×4 in a 10 in hole, the timber displaces roughly 16% of the concrete you would calculate from the hole alone. Over a forty-post run, that difference is about six bags.

How much concrete per post

Concrete needed per post, after subtracting the post:

And in metric:

Round up to whole bags on every post, not on the run total — you cannot spread the rounding across holes that are already filled.

Sizing the hole

Two rules cover almost every residential fence:

• Width: three times the post. A 4×4 (89 mm actual) wants a 250-300 mm hole. Going wider buys forgiveness when plumbing the post, but volume rises with the square of the diameter — a 300 mm hole holds 44% more than a 250 mm one at the same depth.
• Depth: one-third of the above-ground height, minimum 600 mm. A 1.8 m fence wants about 600 mm in the ground. This is a minimum, not a target — go deeper for gate posts, corners, ends, soft or sandy soil, and anywhere exposed to wind load.

Cold climates override both: the hole must reach below the local frost line or the post will heave. Frost depth is set by local code and ranges from nothing to well over a metre.

Where the estimate goes wrong

• The gravel base. Most timber posts sit on 75-100 mm of compacted gravel for drainage. Concrete fills only what is above it, so measure depth from the top of the gravel.
• Augers wander. A powered auger rarely cuts a clean cylinder, and hand-dug holes are worse. Sides cave, depths run over, and roots force you wide. Post holes are the one job where the 10% buffer is genuinely a minimum — on a run of more than ten posts, 15% is a fairer allowance.
• Gate and corner posts are bigger. They carry real load and are usually set larger and deeper than line posts. Work them out separately rather than averaging them into the run.
• The crown at the top. Finishing the concrete slightly proud and sloped away from the post sheds water, which is good practice — and adds a little volume per hole.

Bags or ready-mix

Fence posts are a bagged-concrete job almost every time: the concrete goes into scattered holes over a long working session, which suits mixing as you go far better than a truck waiting on site. But a long run adds up. Forty 4×4 posts at two bags each is eighty 80 lb bags — around 1.8 yd³, and well past the point where a trailer mixer or a ready-mix delivery with a couple of helpers saves a day of work.

Use it on the jobsite

The Concrete Calculator app for iOS has this same calculator plus saved projects, history, four more calculator types and full offline support. Get it on the App Store.

Setting something other than a fence? The post hole calculator is the same tool with general-purpose defaults, and the Sonotube calculator covers deck footings and pier columns poured in a round form.