Concrete Footing Calculator
Concrete for footings & foundation trenches
Pouring strip footings, a foundation trench or deck piers? Enter the run in feet with the width and depth in inches and this concrete footing calculator returns cubic yards, bags and the rebar to order with it. Footings are structural and a short load means a cold joint, so the numbers below include waste rather than leaving it to you.
Sizing a strip footing
A foundation concrete calculator has one job on a continuous footing: turn a trench into a delivery.
A continuous footing is a long box: length × width × depth. The only trap is units — the run is in feet while the width and depth are quoted in inches, so a 16-inch-wide footing is 1.33 ft, not 16. This page takes inches directly for both to keep that conversion out of your hands.
Two conventions set the size, and your local code overrides both. Width is commonly twice the thickness of the wall it carries — 16 inches under an 8-inch wall. Depth is commonly at least equal to the width, and never less than 8 inches for anything structural. Soil bearing capacity and the load above can push both larger, which is what an engineer or a plans check is for.
Concrete per 10 feet of footing
Footings are ordered by the run, so the useful unit is yards per ten feet. Multiply by your total length in tens and you have the order.
| Footing width | 8 in deep | 12 in deep | 16 in deep | 24 in deep |
|---|---|---|---|---|
| 12 in | 0.27 yd³ | 0.41 yd³ | 0.54 yd³ | 0.81 yd³ |
| 16 in | 0.36 yd³ | 0.54 yd³ | 0.72 yd³ | 1.09 yd³ |
| 20 in | 0.45 yd³ | 0.68 yd³ | 0.91 yd³ | 1.36 yd³ |
| 24 in | 0.54 yd³ | 0.81 yd³ | 1.09 yd³ | 1.63 yd³ |
Cubic yards per 10 linear feet, including 10% waste.
In bags, the same ten feet of 16 × 12 in footing is 25 bags of 80 lb mix; at 24 in wide and 24 in deep it is 74. That escalation is the whole argument for ready-mix on anything continuous.
Frost depth sets the bottom of the trench
A footing has to bear below the frost line, or the ground under it will heave in winter and drop in spring, and the wall above will crack. Code minimums range from around 12 inches in mild climates to 48 inches or more where winters are hard, and the figure that matters is the one in your local code, not a regional average.
Two rules travel with it. The trench bottom must be flat, firm and undisturbed — footings bear on native soil, never on loose backfill, and if you over-dig you fill with compacted gravel rather than more concrete. And a footing on a slope is stepped, in level runs with vertical risers, rather than poured on a grade.
Rebar in a footing
Most residential footings carry two continuous runs of rebar along their length, kept about 3 inches clear of the soil so the steel stays fully encased. Enter the number of runs and the calculator returns the total length to buy, with 10% added for lap splices at every joint.
On 60 feet of footing that is 132 feet of bar for two runs. Steel is ordered in fixed lengths, so round up to whole sticks and keep the offcuts for the pier cages.
Pier and pad footings for decks and porches
Point loads sit on their own footings. These are ordered per pier, and the numbers get large faster than people expect once the frost depth is real.
| Pier diameter | 30 in deep | 36 in deep | 42 in deep | 48 in deep |
|---|---|---|---|---|
| 12 in | 2.2 ft³ / 4 bags | 2.6 ft³ / 5 bags | 3 ft³ / 6 bags | 3.5 ft³ / 6 bags |
| 16 in | 3.8 ft³ / 7 bags | 4.6 ft³ / 8 bags | 5.4 ft³ / 9 bags | 6.1 ft³ / 11 bags |
| 18 in | 4.9 ft³ / 9 bags | 5.8 ft³ / 10 bags | 6.8 ft³ / 12 bags | 7.8 ft³ / 13 bags |
| 20 in | 6 ft³ / 10 bags | 7.2 ft³ / 12 bags | 8.4 ft³ / 14 bags | 9.6 ft³ / 16 bags |
| 24 in | 8.6 ft³ / 15 bags | 10.4 ft³ / 18 bags | 12.1 ft³ / 21 bags | 13.8 ft³ / 24 bags |
Per pier, 80 lb bags, including 10% waste. For narrow fence-post holes see the concrete calculator.
An 18-inch pier at 36 inches deep is 0.22 yd³, so roughly four of them fill a cubic yard. Nine or ten piers is about where ready-mix starts to beat mixing bags on site, and that is before counting the labour.
Worked example: footings for a 30 × 30 ft addition
A 30 by 30 foot addition has a 120-foot perimeter. At 20 inches wide and 12 inches deep — a common size under a 10-inch wall on average soil — the calculator returns 8.15 cubic yards, or 220 cubic feet, with 10% waste already included. Two runs of rebar come to 264 feet.
In bags that pour would be 367 bags of 80 lb mix, which is roughly nine pallets and a very long weekend. Ready-mix is not close to optional at that size, and 8.15 yards is comfortably above most suppliers' short-load threshold, so you avoid the small-load surcharge as well.
Footing mistakes that are expensive to fix
- Ordering exactly what the arithmetic says. Trench walls slump and the bottom is never perfectly flat, so real footings absorb more than their nominal volume. The 10% here is a floor, not a luxury.
- Running out mid-pour. A continuous footing that sets in two sessions has a cold joint through a structural element. Order the extra yard.
- Pouring on disturbed soil. Backfill compresses. If you dig too deep, bring it back up with compacted gravel and not with loose spoil.
- Rebar laid on the dirt. Steel touching soil rusts and spalls the concrete off. Support it on chairs or dobies with cover all round.
- Ignoring the frost line because the winter was mild. Heave is cumulative and shows up in the wall above, years after the pour, where it is expensive to reach.
- Guessing the width from the wall alone. Twice the wall thickness is a starting point. Soft or clay soils need more bearing area, and that is a code and engineering question.
An estimate for ordering material. Footing size is set by your local building code, the soil and the load — always confirm before you dig.
How this concrete footing calculator works out its numbers
Volume is length × width × depth for a strip footing, or πr² × depth for a round pier, multiplied by the number of footings and then by (1 + waste). Cubic yards are cubic feet ÷ 27. Bag counts divide the volume by the bag yield — 0.375 ft³ for a 50 lb bag and 0.6 ft³ for an 80 lb bag — and always round up.
- Width and depth are entered in inches and converted internally. This is the single most common arithmetic error on footings: a 16-inch width is 1.33 ft, and entering 16 gives a figure twelve times too large.
- Waste defaults to 10% and is included in every figure on this page, tables included. Trench walls slump and over-dig is normal, so footings absorb more than their nominal volume.
- Rebar adds 10% for lap splices. Codes generally specify laps as a multiple of bar diameter, which varies with bar size and concrete strength, so 10% is a practical ordering allowance rather than a code figure. Round up to whole lengths at the merchant.
- Concrete is taken at 150 lb per cubic foot, the standard figure for normal-weight mix. It drives the weight output only, not the volume.
- Nothing here is a structural design. Footing width, depth, frost cover and reinforcement are set by your local building code, the soil bearing capacity and the load above. The rules of thumb quoted are starting points for estimating material, not a substitute for a plans check.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Concrete Footing Calculator: frequently asked questions
How deep should a concrete footing be?
Below the local frost line, which ranges from about 12 inches in mild climates to 48 inches or more where winters are severe, and at least as deep as the footing is wide. The binding figure is in your local building code.
How wide should a footing be?
A common rule of thumb is twice the thickness of the wall it supports — 16 inches under an 8-inch wall. The real requirement depends on the load above and the soil's bearing capacity, so check the code or have it engineered.
How much concrete do I need for footings?
Multiply length by width by depth in consistent units. Ten feet of a 16 by 12 inch footing is 0.54 cubic yards with waste included; a 120-foot perimeter at 20 by 12 inches is 8.15 cubic yards.
Bags or ready-mix for footing concrete?
Small pier and post footings can be bagged. Continuous footings are almost always ready-mix — ten feet of a 24 by 24 inch footing alone is 74 bags of 80 lb mix, and the risk of a cold joint from running short is the bigger issue.
How much rebar goes in a footing?
Two continuous runs along the length is the usual residential minimum, kept about 3 inches clear of the soil. On 60 feet of footing that is 132 feet of bar once 10% is added for lap splices.
How many deck piers can I get from a cubic yard?
About four 18-inch piers at 36 inches deep, since each takes 0.22 cubic yards with waste. Around nine or ten piers is where ready-mix starts to make more sense than mixing bags.
Can I pour a footing on a slope?
Not as a sloping pour. Footings are stepped — level runs joined by vertical risers — so that every section bears flat on undisturbed soil at or below the frost depth.
What if I dig the trench too deep?
Fill back to grade with compacted gravel, not with loose spoil and not with extra concrete. A footing must bear on firm undisturbed ground or properly compacted fill.
How much extra concrete should I order for footings?
At least 10%, which this calculator includes by default. Trench walls slump and bottoms are uneven, so footings routinely take more than their nominal volume, and running short mid-pour is far more costly than a leftover part-yard.