How much concrete a footing takes
A footing is its width times its depth times its run. The 40 ft of 16 by 8 in footing this page opens with is 40 × 16/12 × 8/12 = 35.556 cubic feet, which is 35.556 ÷ 27 = 1.32 cubic yards. Add the 10% waste in the box and it is 1.45, ordered as 1.5 in quarter-yard steps — though a supplier bills a 2 yard minimum, which the page says next to the figure. In bags it is 66 of 80 lb: 5,280 lb to carry and mix.
A trench holds more than it looks. Thirty feet of ordinary 16 by 8 in
footing is a cubic yard, so a footing run crosses into truck territory at a
length most people would still call small. Width and depth are typed in
inches here — a 20-inch footing, the way the trade says it — and the length in
feet; type 16, 1'4" or 16 in in the width
box and all three mean the same footing.
How wide, how deep, and what this page can tell you
Two numbers decide a footing, and neither of them is the volume. This page counts concrete and then says plainly which of the two it cannot answer.
| Question | What the code says | What this page says |
|---|---|---|
| How wide? | IRC 2021 R403.1.1 — never less than 12" wide or 6" thick. Above that floor, Table R403.1(1) sets the width from the load above and the soil below. | Below 12" by 6" it says the footing does not meet the code. Above it, not checked: the load and the soil are not on this page, and a width that is legal under a one-storey frame wall is not legal under three storeys of brick. |
| How deep below grade? | R403.1.4 — at least 12" below undisturbed ground for an exterior footing, and R403.1.4.1 sends the frost figure to Table R301.2(1), which each jurisdiction fills in itself. | Not checked, on every project, with the clause named. There is no national frost depth to check against: it is 12" in one county and 60" in another, and your building department holds the number. |
That is why both rows of the Standards panel are marked "not checked" when the page opens. A calculator that printed "meets code" over a footing whose load it has never seen would be making the one promise it has no right to make.
Yards per foot of footing
Before waste, at the yield on the 80 lb bag's data sheet:
| Footing | Cubic feet per foot of run | Feet of run per cubic yard | 80 lb bags per 10 ft |
|---|---|---|---|
| 12" × 6" | 0.50 | 54 | 9 |
| 16" × 8" | 0.89 | 30.4 | 15 |
| 20" × 8" | 1.11 | 24.3 | 19 |
| 24" × 10" | 1.67 | 16.2 | 28 |
| 24" × 12" | 2.00 | 13.5 | 34 |
The bag column is rounded up, per 10 ft and before waste. Round the whole job up once instead — not each run, and never to the nearest bag.
Bags or a truck
The opening 40 ft run prices at $370 for the truck against $374.88 in 80 lb bags at the reference prices in the boxes: close enough that your supplier's own figures decide it, which is why both sides are priced from boxes you can type over. What the money does not show is the other difference — 66 bags is a day of mixing, and a trench wants the whole run poured before the first end sets. The short-load fee a supplier charges under their minimum load is in that $370, and so is the 2 yard minimum they bill; both are rows under Assumptions.
A footing, a stem wall and a column in one job
The shape picker here offers the three things poured against a form or a trench wall: a footing, a wall and a round column or form tube. Most foundations are two of them — a footing with a stem wall on top — so add a part and the breakdown gives each its own line and its share of the total, with one order covering both. Pick the column and the buy list adds the form tubes, one row per diameter and height.
Post holes for fence posts, a flight of steps and a slab are each a different question, and the concrete calculator takes all of them in the same project as these three.
Common questions
How many yards of concrete for a 40 foot footing?
At 16 by 8 in, 1.32 cubic yards, or 1.45 with 10% waste. At 12 by 6 in the same 40 ft is 0.74, and at 24 by 12 in it is 2.96. Change the width or the depth box and the three lines under the answer move with it.
What is the minimum size for a concrete footing?
Where the IRC is in force, 12 inches wide and 6 inches thick is the floor — R403.1.1 — and that is a floor, not a recommendation for your house. The width your footing needs comes out of Table R403.1(1) from the load it carries and the bearing value of your soil, and a plan reviewer reads both. This page flags anything under the floor and declines to bless anything over it.
How deep does a footing have to be?
Deep enough to sit on undisturbed soil below the frost line, which is a local number. The IRC asks at least 12 inches below undisturbed ground for an exterior footing and then hands the frost depth to the local table — so the honest answer is the one on the panel: ask your building department before you dig. A footing poured above the frost line will lift, and no amount of concrete fixes that.
Does this count the rebar in the footing?
No. The steel in a footing is sized with the footing, by whoever sized the footing, and this page counts the steel in a slab only. The yards, the bags, the water and the form tubes are here; the bars are a drawing you should be working from.
Can I pour the footing and the wall at once?
You can order them as one load — add both parts and the total is what arrives on the truck. A true monolithic pour, where the footing and the slab edge are one shape, is not one of the shapes here yet, and counting it as two parts slightly overstates the concrete where they meet.
Is the waste added twice?
No. Waste is multiplied in exactly one place in this site, and the result shows the volume before and after it so you can see it happened once. Set the box to 0 and the two lines agree.