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Footing and Pier Calculator

Concrete for cylindrical footings, piers and post holes.

Your measurementsEvery assumption editable01

The tube or auger diameter, not the post.

Below grade. Frost depth governs this wherever the ground freezes.

Concrete requiredCalculated live02

Result

28× 80 lb bags

16.49 cu ft across 6 footings

Per footing12 in × 42 in deep
2.7489 cu ft
Number of footings
6
Total volume0.611 cu yd
16.493 cu ft
Bags per footing
4.58
Bags to buyat 0.6 cu ft per bag
28 × 80 lb

Volume scales with the square of the diameter. Going from a 10 in to a 12 in tube is only two inches wider but takes forty-four per cent more concrete, which is why footing size drives cost far more than depth does.

Depth is normally set by frost line rather than by load. A footing that does not reach below the frost depth will lift and settle with the seasons regardless of how much concrete is in it.

A flared or belled base holds more than a straight cylinder, and this calculates a straight one. If you are belling the bottom, add the extra volume separately.

Enter the hole diameter, depth and how many. Volume scales with the square of the diameter, so a couple of inches wider costs far more concrete than a few inches deeper — which is the opposite of most people's intuition.

Why use this tool?What it does differently03

Why use this tool?

Per footing and total

Both figures, so you can see whether a bag or two per hole is realistic.

Bags and yards

Bag count for small jobs, cubic yards once ready-mix starts to make sense.

Diameter dominates

Going from a 10 in to a 12 in tube takes 44% more concrete for two inches of width.

Frost depth flagged

Depth is normally set by frost line rather than by load.

How this worksThe method04

How this footing and pier calculator works

Volume is π times the radius squared times the depth, in cubic inches, divided by 1,728 for cubic feet. Multiplying by the number of footings gives the total, and dividing by bag yield gives a bag count.

Because the radius is squared, diameter drives cost far more than depth does. A 12 in tube holds forty-four per cent more than a 10 in one at the same depth, while adding six inches of depth to the 10 in tube adds only about fourteen per cent.

This calculates a straight cylinder. A flared or belled base — common where bearing capacity matters — holds meaningfully more, and that extra volume has to be added separately.

How to use itStep by step05

How to use it

  1. Step 1: Measure the hole

    Tube or auger diameter, not the post that goes in it.

  2. Step 2: Set the depth

    Below grade. Frost line governs this wherever the ground freezes.

  3. Step 3: Count the footings

    All of them, so you order in one trip.

  4. Step 4: Add for a bell

    If the base is flared, that extra volume is not included here.

Example usageWorked figures06

Example usage

A deck on six piers
12 in tubes at 42 in deep, six of them: 2.7489 cu ft each, 16.493 cu ft total — 0.61 cubic yards, or 28 eighty-pound bags.
Why diameter matters
Dropping to 10 in tubes on the same job takes the total from 16.49 to 11.45 cu ft — nine fewer bags for two inches of diameter.
Deeper instead
Keeping 12 in tubes but going to 48 in deep raises it to 18.85 cu ft. Depth costs far less than width.
Frequently asked questionsCommon questions07

Frequently asked questions

How much concrete for a post hole?

A 12 inch diameter hole 42 inches deep takes about 2.75 cubic feet, or roughly 5 eighty-pound bags per hole.

How deep should footings be?

Below the frost line wherever the ground freezes — that depth varies by region and is set by local code. A footing above frost depth will lift and settle seasonally however much concrete is in it.

Does diameter or depth cost more?

Diameter, substantially. Volume scales with the square of the radius, so going two inches wider adds far more concrete than going several inches deeper.

How many bags per footing?

For a 12 by 42 inch hole, about 4.6 eighty-pound bags — so five per hole in practice, since you cannot buy part of a bag.

Does this include a belled base?

No. This is a straight cylinder. A flared or belled bottom holds noticeably more and needs calculating separately.

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