Over-excavation, spillage and uneven subgrade. 5-10% for formed work, 10-15% on rough ground.
Yields are the figures printed on the bag. Speciality mixes differ — check yours.
Optional. Leave at zero to skip the cost estimate.
Material quantity estimate for planning and purchasing. Not structural engineering advice, not a bid, and not a code determination.
How much concrete for a 40 ft footing?
A 40 ft run of footing 16 inches wide and 8 inches thick needs 1.32 cubic yards of concrete (35.6 cubic feet). Order about 1.5 yards with waste.
Footing width comes from the load above: the IRC minimum is 12 inches for one storey of light frame, 15 inches for two, and 23 inches for three, on soil with 1,500 psf bearing capacity.
Use 10-15% waste rather than the usual 5-10%. A machine-dug trench in anything but firm clean soil over-excavates, and two extra inches of depth across 40 feet of 16 inch footing quietly swallows another 0.33 cubic yards.
How wide and deep does a footing need to be?
Footing width comes from the load above and the soil below. The IRC gives minimum widths for conventional light-frame construction on soil with 1,500 psf bearing capacity:
| Structure | Footing width | Footing thickness |
|---|---|---|
| 1 storey, light frame | 12 in | 6 in |
| 2 storey, light frame | 15 in | 6 in |
| 3 storey, light frame | 23 in | 8 in |
| 1 storey, brick veneer | 15 in | 6 in |
| 2 storey, brick veneer | 21 in | 8 in |
Frost depth is not optional
A footing has to bear below the frost line or it will heave. Depth is set locally and varies enormously — roughly 0 in along the Gulf Coast, 12-24 in through the mid-Atlantic, 42-48 in across the northern Midwest and New England, and up to 100 in in parts of northern Minnesota. Your building department publishes the number for your jurisdiction and it is the one that counts.
Two more rules worth knowing: the footing should project past the wall by at least its own thickness on each side, and a footing is normally twice the width of the wall it carries.
Ordering footing concrete
Footings are unforgiving of running short, because a cold joint mid-run is a weak point you cannot inspect later. Over-excavation into soft soil also quietly eats volume — a trench dug 2 inches deeper than planned over 40 feet of 16 inch footing swallows an extra 0.33 cubic yards.
Use 10-15% waste on machine-dug trenches in anything but firm, clean soil.
Common questions
How much concrete for a 40 ft footing?
A 40 ft run at 16 in wide and 8 in thick is 35.6 cubic feet, or 1.32 cubic yards. With 10% waste, order about 1.5 cubic yards.
How deep should footings be?
Below your local frost line, and bearing on undisturbed soil. Minimum thickness is 6 inches for one or two storeys of light frame, 8 inches above that. Depth to the bottom of the footing is a code number from your building department, not a rule of thumb.
Can I pour footings and the wall at the same time?
Monolithic footing-and-wall pours are done, but they need forming that most DIY jobs will not support and reinforcement detailing that varies by design. Conventional practice is to pour the footing, set keyway or dowels, then form the wall.