Footing calculator
Concrete footing
calculator
Three footing shapes, one calculator. Size round piers poured in cardboard form tubes, bell-bottom footings that flare at the base, or a continuous strip footing under a wall — each in cubic yards, bags and whole form tubes.
Enter measurements ↓Optional
What will it cost?
Enter what you actually pay per cubic yard and the quantities above are priced up. The field starts at a typical US price so you have something to work from — change it to your own.
Typical US range $125–$195 per cubic yard, checked 2026-08-29. Prices move — use your own supplier's figure.
More in this series
Put these figures to work
Sizing one part of a pour is easier when the slab itself is settled. Start with the main estimate, then come back for the detail.
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Based on your result
What you'll need
Quantities update after every calculation. Tubes are counted whole, because a pier is never spliced across two of them.
- Ready-mix concrete
- Bags to fill every footing
- Calculate above
- Form tubes
- Cardboard tubes cut to your depth
- —
- Rebar or dowels
- Vertical cage tying the footing to what sits on it
- —
- Post bases and bolts
- Anchors set while the concrete is wet
- As needed
The maths
How this concrete footing calculator sizes a pour
A pier is a cylinder and a strip footing is a trench, both straightforward. A bell footing is the one that catches people out: the flared base is a truncated cone, not a wider cylinder and not an average of the two ends.
bell = (bell height ÷ 3) × (A_top + A_base + √(A_top × A_base))
strip = length ft × (width in ÷ 12) × (depth in ÷ 12)
tubes = ⌈ count ÷ ⌊ tube length ÷ depth ⌋ ⌉
Sonotube and other cardboard form tubes
Form tubes come in 4, 8 and 12 foot lengths and in diameters from 6 to 24 inches. Cut them to depth with a handsaw, brace the top square before pouring so the shaft does not drift, and strip the cardboard above grade once the concrete has set.
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01
Pick the shape
A deck sits on piers, a heavier load or soft ground gets a bell, and a wall gets a continuous strip footing.
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02
A bell is a cone, not a cylinder
The flare is a truncated cone. Averaging the two diameters undercounts it by about 4%, and averaging the two areas overcounts by about 7%.
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03
Depth is set by frost, not by load
A footing that stops above the frost line will be lifted by the ground every winter, no matter how wide it is.
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04
Tubes are cut, not divided
A 12-foot tube gives three 48-inch piers, not 3.6, so the count rounds down before the tube count rounds up.
Good to know
Footing questions, answered
Below the frost line, which is 12 inches in the warm South and past 48 inches in the northern states. Depth is set by frost heave rather than by the weight of the deck, and a shallow footing will be pushed up every winter.
Eight to twelve inches carries most residential deck posts. Wider footings spread the load over soft ground; the span tables in your local code give the figure that matters.
The flared base spreads the load over more soil without widening the whole shaft, so you dig and pour less concrete than a full-width pier. It also resists uplift, which matters for decks and fences in wind.
Divide the tube length by the pier depth and round down. A 12-foot tube gives three 48-inch piers with a foot left over — and that offcut is not long enough to be a fourth.
Piers usually take two to four vertical bars tied into a small cage, or at minimum a dowel to connect the footing to what sits on it. Strip footings normally run two continuous bars near the bottom.
In firm, self-supporting soil, yes, and it is common for strip footings. A form tube gives a clean round shaft, a known volume and a smooth face above grade, which is why piers usually get one.
About 0.79 cubic feet per foot of depth, so a 48-inch pier is roughly 3.1 cubic feet — six 80 lb bags. Six of them is close to 0.7 of a cubic yard, which is where ready-mix starts to make sense.