Measure the centreline, not the outside
A rectangular perimeter footing is four straight runs that meet at four corners, and the corners belong to two runs at once. If you take the outside dimensions of a 40 by 30 building and multiply the 140 foot perimeter by a 16 inch wide, 8 inch thick footing, you get 124.4 cubic feet. Do the same job as an area subtraction, outer rectangle minus inner rectangle, and you get 119.7. The five cubic foot difference is the four corner squares, counted twice.
The fix is to work on the centreline of the footing. For a rectangle that is twice the length plus twice the width, minus four footing widths. This calculator does that automatically in the perimeter mode. If your building is not a rectangle, run a tape along the middle of the trench, walk it once, and put the result into the linear feet mode instead. Do not add the corners in twice on the way round.
Trench dug in dirt takes more than the drawing says
A footing formed in lumber takes almost exactly its calculated volume, because the sides are flat and the bottom is where you put it. A footing poured against the walls of a trench takes more, and how much more depends entirely on the soil. Clean, tight clay holds a vertical cut and gives you close to the calculated number. Sandy or gravelly soil sloughs into a rough V and swallows an extra tenth of the volume without looking any different from the top. Frozen ground that thaws while the trench is open is worse again.
| Condition | Allowance worth carrying |
|---|---|
| Fully formed, screeded bottom | 10% |
| Machine trench in firm clay | 10-15% |
| Trench in sand, gravel or fill | 20% |
| Hand-dug, irregular bottom | 20% and measure the deep spots separately |
The other thing worth doing before the truck arrives is walking the trench with a tape and checking the depth every ten feet. A footing that is nominally 8 inches thick but averages 10 because the bottom wandered is 25 percent more concrete than the plan, and you will find that out with the drum half empty.
What the bar count in this calculator is and is not
The continuous bar section takes the number of bars and the lap length from you and turns them into linear feet, stick count and weight. It does not decide how many bars a footing should have, what size they should be, or whether it needs transverse steel, dowels into the wall above, or corner bars. Those come from a drawing. The 40 diameter lap in the default is a figure that turns up commonly in residential drawings as a design target, and it is there so the arithmetic runs, not as a specification for your job.
What the calculator does show, and what people underestimate, is how quickly splices add steel. A 134 foot perimeter with two continuous bars is not 268 feet of rebar. At 20 foot sticks it is eight sticks per bar because the lap eats into each one, so sixteen sticks and about 214 pounds of steel to carry. If you need a bigger bar than the drawing shows, that weight climbs faster than the diameter does: a number 6 is more than double the pounds per foot of a number 4.
Pads poured with the strip
Isolated pads under columns, posts or a beam pocket are usually poured at the same time as the perimeter, and they are easy to forget in the order because they are not on the tape measure you walked. A 24 by 24 by 12 inch pad is 4 cubic feet, which sounds trivial until you have six of them and they add a whole cubic yard to the ticket. Put the count and the dimensions in and they go into the total. If the pads are different sizes, run the biggest ones here and add the odd ones on the concrete slab calculator.
One last practical point. Footings and the wall or slab above them are often poured on separate days, which means a construction joint and usually dowels. If you are pouring a slab on top of this footing, the slab volume is a separate calculation and it is worth doing both before you call the plant, because one truck for the whole job is cheaper than two half-loads. If the wall above is block rather than poured, the concrete block wall calculator picks up from the top of the footing.
Questions people ask
How deep does a footing have to go?
Nobody can answer that from a web page, and this calculator deliberately does not try. Footing depth is driven by the frost line where you are, by the soil bearing capacity under the building, and by the load coming down from above. Frost depth alone varies from a few inches in the southern United States to over five feet in northern states, and it is a local figure that your building department publishes. Ask them for the frost depth and the required bearing, or work from a drawing that an engineer stamped. What this page does is turn the width, thickness and length you were given into concrete.
Why does the calculator ask for footing depth and not trench depth?
Because they are different numbers and mixing them up is the most expensive mistake on this page. The trench might be 42 inches deep to get below frost, but the concrete in it might only be 8 inches thick, with the rest of the height taken up by a block or poured wall built on top. Enter the thickness of the concrete pour. If you really are filling the trench solid to the top, then trench depth and footing thickness are the same and you can enter it once.
Can I pour a footing in two loads?
You can, but a horizontal cold joint in the middle of a footing is not the same as a planned construction joint. If the first load stiffens before the second arrives, you get a plane with no bond across it. On a small job people manage it by having the second truck already scheduled thirty minutes behind the first, or by pouring to a planned vertical bulkhead at a corner rather than stopping wherever the concrete ran out. What you should not do is discover the shortfall halfway along a run. Order the quarter yard extra.
Do I need forms if the trench walls are clean?
Trench pours are common and legitimate on residential work in soils that hold a cut, and plenty of building departments accept them. The trade-offs are that you use more concrete, you cannot check the width and level as easily, and you have no way to vibrate against a form face. Forms cost lumber and a day, and give you a footing that is the size you meant. This calculator gives the form board run and stake count either way so you can price the choice rather than assume it.
Does the waste allowance cover the rebar taking up space?
Several times over. Two number 4 bars through a 16 by 8 inch footing displace about a third of one percent of the volume, far below the resolution of anything you can measure in a trench. The allowance is there for the shape of the hole, not the steel in it. What does move the number is a footing bottom that is deeper than intended in places, a keyway formed into the top, or a step down in the footing where the grade drops, and the last of those is worth calculating as a separate block and adding on.