Post Hole Concrete Calculator

How much concrete one post hole takes

A post hole is a cylinder with a post stuck through the middle of it, so the concrete you need is the hole volume minus the buried part of the post:

• Hole volume = π × (diameter ÷ 2)² × depth
• Post volume = post width × post depth × the same depth
• Concrete = hole volume − post volume

The calculator also subtracts the gravel base, because the concrete sits on top of the gravel rather than filling the whole hole. With a 36-inch hole and a 6-inch gravel bed, the concrete depth is 30 inches.

Concrete is sold by weight but mixed and placed by volume, which is where most people go wrong. Bag yields are printed on the bag and are consistent across brands: 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb. Those are the numbers this calculator uses, and they match the yield tables published by Quikrete's concrete calculator.

Worked example: 12 fence posts, 9-inch holes, 3 feet deep

A common backyard fence job: 12 posts, 9-inch diameter holes, 36 inches deep, 4x4 posts, a 6-inch gravel base in each hole.

• Concrete depth: 36 − 6 = 30 inches
• Hole volume: π × 4.5² × 30 = 1,909 cubic inches = 1.104 cu ft
• 4x4 post (3.5 in square): 3.5 × 3.5 × 30 = 368 cubic inches = 0.213 cu ft
• Concrete per hole: 1.104 − 0.213 = 0.892 cu ft
• All 12 holes: 0.892 × 12 = 10.70 cu ft, or 0.396 cubic yards
• 80 lb bags: 10.70 ÷ 0.60 = 17.8, so buy 18 bags
• 60 lb bags: 10.70 ÷ 0.45 = 23.8, so buy 24 bags

At about $6.50 for an 80 lb bag that is roughly $117 in concrete. The same volume in 60 lb bags at about $5.25 is $126, so the bigger bag wins on price and on the number of trips. Total weight is around 1,552 lb, three-quarters of a ton to move, mix and place by hand.

Two variations worth running yourself: skip the post subtraction and the same 12 holes need 13.25 cubic feet, which is 23 bags of 80 lb, and dig 12-inch holes instead of 9-inch and the total jumps to 21.0 cubic feet, 35 bags. Hole diameter is the most expensive number on this page.

Bags per hole at a glance

Concrete per hole for a 4x4 post with a 6-inch gravel base, and the bags to buy. Always round up, a half-filled hole is a wobbly post:

Read the table left to right and the pattern is clear: going from an 8-inch hole to a 12-inch hole at the same depth more than quadruples the concrete, because volume grows with the square of the diameter. Depth is the cheaper way to buy holding power. A 9-inch hole 36 inches deep uses less concrete than a 12-inch hole 24 inches deep and resists frost heave far better.

How deep and how wide should a post hole be?

Two rules cover almost every fence and deck post in the US:

• Diameter: three times the width of the post. A 4x4 post is 3.5 inches actual, so a 10-inch hole is ideal and 9 inches is fine in firm soil. Loose or sandy ground wants 12 inches.
• Depth: one-third of the post above ground, two-thirds below. A 6-foot fence means a 3-foot hole. Add depth if your frost line is deeper, in the northern US, 42 inches is standard for a 6-foot fence.

Frost is the reason depth matters more than diameter. Water in the soil expands when it freezes and lifts anything in its path, and a shallow footing rises with it. The International Code Council publishes the residential code provisions that local inspectors use, including the requirement that footings extend below the frost line, and most building departments will tell you your local depth for free.

Shape matters too. A hole that is wider at the bottom than at the top, a bell, resists lifting better than a straight cylinder of the same volume. If you are digging by hand, it is worth the extra five minutes with a post-hole digger to flare the base. And do not backfill with soil instead of concrete on a load-bearing post: soil settles, and the post moves with it.

Bag yields and price per cubic foot

Bag size is a price question, not a strength question, every one of these mixes reaches the same compressive strength when mixed correctly. What changes is how much you pay per cubic foot of concrete:

On our 12-post example the totals work out to $117 with 18 bags of 80 lb, $126 with 24 bags of 60 lb, $131 with 29 bags of 50 lb and $135 with 36 bags of 40 lb. The gap is about 15%, and it grows with the size of the job.

Where a small bag does win is small jobs. If you need 0.4 cubic feet for a single mailbox post, a 40 lb bag is the right purchase because you will not be left with half a bag of open concrete turning to stone in the garage. Bags absorb moisture from the air and start to set within a few weeks of opening, so buy the size that matches the job rather than the best price per cubic foot.

Pouring and curing without wasting bags

Mixing and placing is where a well-calculated job turns into a second trip to the store. A few habits keep you on budget:

• Mix only what you can place in 30 minutes. A wheelbarrow holds about 2 cubic feet, which is roughly three 80 lb bags. Mix, place and tamp that batch, then start the next one.
• Measure the water. A 60 lb bag wants about 2.5 quarts and an 80 lb bag about 3 quarts. Extra water makes the mix easier to pour and up to 30% weaker.
• Tamp as you go. Poke the mix with a stick or a spare bar to release air pockets, or the concrete bridges and leaves voids around the post.
• Crown the top. Slope the concrete slightly away from the post so rain runs off instead of pooling at the base of the wood.
• Watch the weather. Concrete needs to stay above about 40°F while it cures; check the National Weather Service forecast before you start a pour in shoulder season.
• Let it cure. 24 hours before any load, 48 to 72 hours before a gate, and do not water the lawn against the post for the first day.

One more piece of arithmetic worth doing before you buy: every bag you open has to be carried, mixed and placed, and 18 bags of 80 lb concrete is 1,440 lb of material. If the job is bigger than a dozen posts, compare the bag total against a short-load delivery from a ready-mix plant, under about 1 cubic yard most plants will not deliver, but above 2 cubic yards the truck is usually cheaper and much easier on your back.