A foundation and footing calculator turns the size of a footing, pier, post hole or foundation wall into the concrete, bags and masonry it needs. A foundation is the part of a structure that carries its load down to the ground, so the volumes are simple but the quantities add up fast: a modest strip footing can take more than a cubic yard, and a single pier can take half of one.
The tools below cover the whole foundation job, from a deck post hole to a full basement wall. Every calculator is free, runs in your browser, and shows the formula and sources behind its numbers. If you are still working out the general method, start with the concrete calculators overview; this page goes deeper on footings, piers and walls.
Footing, Column and Pier Calculators
Footings spread a load into the soil, while piers, columns and posts carry it down to a footing or to bearing ground. Pick the tool that matches the shape you are forming.
Foundation Wall Calculators
Foundation and stem walls can be poured, stacked from concrete block, or built from insulated forms that stay in place. Each is estimated differently, so each has its own tool.
Which Foundation Calculator Should You Use?
Most foundation jobs fit one of these shapes. If yours is a combination, such as a footing under a block wall, run each part and add the results.
| If you are building | Use | Why |
|---|---|---|
| A footing under a wall or a pad under a post | Footing calculator | Length times width times depth, with a count for repeated pads |
| A fence or deck post | Post hole calculator | Subtracts the post from the hole and follows the 3-times-width hole rule |
| A pier poured into a cardboard tube | Sonotube calculator | Cylinder volume by tube diameter, with bags per tube |
| A drilled pier with a widened base | Bell pier calculator | Adds the shaft, the flared bell and the toe |
| A structural column or round pier | Column calculator | Round or square section times height |
| A poured, block, ICF or brick wall | The matching wall calculator | Each material is counted differently |
How Footings and Piers Are Estimated
A footing is a rectangular prism, so its volume is length times width times depth. Footings are usually specified in inches and run in feet, so convert width and depth to feet by dividing by 12, multiply, divide the cubic feet by 27 for cubic yards, and add a waste allowance for an uneven trench.
Round pier or tube: cubic feet = pi x radius squared x height, in feet.
As an example, a 40 foot strip footing 16 inches wide and 8 inches deep holds 40 x 1.333 x 0.667 = 35.6 cubic feet, or 1.32 cubic yards. With a 10 percent allowance that is 1.45 cubic yards, roughly 66 eighty-pound bags, which is why most footings are ordered as ready-mix. The concrete footing calculator does this for your own sizes.
Sonotube and column sizes
Round forms are sold by inside diameter. Sonoco lists its round concrete forms from 6 to 60 inches, and the volume per foot grows with the square of the diameter: an 8 inch tube holds about 0.35 cubic feet per foot of height, a 12 inch tube about 0.79 and a 24 inch tube about 3.14. Four 12 inch tubes 4 feet tall hold 12.6 cubic feet; with 10 percent waste that is about 0.51 cubic yards, or 24 eighty-pound bags.
Bell piers hold more than the shaft suggests
A belled pier widens at the bottom to spread its load over more soil, and that flare is a large share of the concrete. A 12 inch shaft 8 feet deep on a 30 inch bell holds about 6.3 cubic feet in the shaft and another 6.3 in the bell and toe, so half the concrete sits in the bottom. Estimating only the shaft would order half of what the pier needs.
Setting posts
For fence and deck posts, QUIKRETE’s setting-posts guidance calls for a hole about three times the width of the post, a depth of about one third of the post’s overall length plus 6 inches of gravel for drainage, and, for structural posts, setting them at least 6 inches below the frost line. The post hole concrete calculator applies that rule and subtracts the post, since a post displaces concrete. For a quick pour, a 50 pound bag of fast-setting concrete yields about 0.375 cubic feet.
Foundation Walls Compared
The same wall can be built four ways, and the unit you count changes with the material.
| Wall type | Estimated by | Key figure |
|---|---|---|
| Poured concrete | Face area times thickness | A 20 by 8 foot wall at 8 inches is 3.95 cubic yards before waste |
| Concrete block | Blocks by area, plus mortar | About 1.125 blocks per square foot for an 8 by 8 by 16 inch block, so 180 for a 20 by 8 foot wall |
| Insulated concrete forms | Blocks by area, plus core concrete | A 16 by 48 inch block covers 5.33 square feet; a 6 inch core holds half a cubic foot per square foot of wall |
| Brick | Bricks by area, plus mortar | 6.75 modular bricks per square foot, from Brick Industry Association Technical Note 10 |
Block and brick figures come from published estimating guides, and ICF dimensions vary by manufacturer, so enter your own block size where the tool allows it. For a stacked block wall, the concrete block calculator also gives courses and blocks per course. For the concrete itself, see the concrete wall calculator, and for the steel inside it, the concrete rebar calculator.
Common Mistakes When Estimating Foundations
1. Mixing up inches and feet
Width, depth and thickness are quoted in inches but the volume formula needs feet. Entering inches where feet are expected inflates the answer twelvefold; entering feet where inches are expected shrinks it. Each tool labels its units, so read them.
2. Forgetting how many footings there are
A deck or porch may need six or eight identical pier footings. Size one, then set the quantity, so a small error per footing is not multiplied unnoticed.
3. Ignoring over-dig
Concrete poured against soil fills whatever space the soil leaves. A ragged or over-excavated trench or hole takes more than the geometry says, so keep a waste allowance of at least 5 to 10 percent.
4. Treating volume as a design
These tools estimate how much concrete a footing, pier or wall needs once you know its size. The size itself, the depth below the frost line and any reinforcement come from your plans, local code or an engineer. A calculator cannot judge your soil.
Frequently Asked Questions
How much concrete do I need for a footing?
Multiply length by width by depth in feet and divide by 27. A 40 foot footing 16 inches wide and 8 inches deep is about 1.32 cubic yards, or 1.45 with 10 percent waste. The footing calculator does the conversion for your own dimensions.
How big should a post hole be?
QUIKRETE’s guidance is a hole about three times the post’s width and roughly one third of the post’s length deep, plus 6 inches of gravel at the bottom. For a post with an actual width of 3.5 inches that is a hole about 10.5 inches across.
How many bags of concrete fill a sonotube?
A 12 inch tube holds about 0.79 cubic feet per foot of height, so a 4 foot tube takes 3.14 cubic feet, or about 5.2 eighty-pound bags at 0.60 cubic feet per bag. Add a little for waste, and use the sonotube calculator for other sizes.
Why does a bell pier take so much concrete?
The flared base spreads the load over more soil, and it holds a large share of the volume. In a 12 inch pier 8 feet deep with a 30 inch bell, the bell and toe hold about as much concrete as the shaft above them.
How deep does a footing need to be?
That depends on the load, the soil and the local frost depth, so it comes from your plans, local building code or an engineer, not from a calculator. QUIKRETE advises setting structural posts at least 6 inches below the frost line. Once you know the depth, these tools turn it into concrete.
How many concrete blocks are in a square foot of wall?
About 1.125 for a standard 8 by 8 by 16 inch block with a 3/8 inch mortar joint. A 20 by 8 foot wall, 160 square feet, needs about 180 blocks before waste.
Do I use the footing calculator or the wall calculator for a foundation?
Both. The footing under the wall and the wall itself are separate pours, so run the footing calculator for the footing and the wall, block or ICF calculator for the wall, then add the totals.
More Concrete Calculators
Know your footing size? Start with the concrete footing calculator, then price the pour with the concrete cost calculator.