What a footing takeoff has to account for
A footing spreads the load of a wall or a column over enough soil that the ground beneath it does not settle. Its volume is trivial geometry — a long rectangular prism for a strip footing, a short one for a pad — but the takeoff goes wrong for two reasons that have nothing to do with arithmetic: the run is measured on the wrong line, and the trench holds more concrete than the drawing says.
Start with the run. For a rectangular building the footing follows the perimeter, so the run is 2 × (length + width). Measure those dimensions along the centreline of the footing, not to the outside of the sheathing and not to the inside of the foundation wall. On a 40 × 30 building the difference between measuring to the outside of an 8 inch wall and to the footing centreline is a few inches per side — small — but the same mistake made on interior bearing walls and returns compounds quickly. Interior footings under load-bearing partitions, garage returns, chimney bases and pilasters all add run that a simple perimeter never picks up.
Then there are the pads. Isolated spread footings under posts, columns and piers carry point loads and are sized independently of the strip. Each one is a small volume — a 24 × 24 × 12 inch pad is exactly 4 cubic feet — but eight of them add 32 cubic feet, or 1.19 cubic yards, which is enough to matter when you are rounding an order to the quarter yard.
This calculator keeps the strip and the pads separate in the output for exactly that reason: when the total does not match your expectation, you can see immediately which half of the takeoff is wrong.
The formula, and the number worth memorising
The strip footing is width × thickness × length. Because width and thickness are given in inches and the run in feet, you divide by 144 to convert the two inch dimensions and then by 27 to reach cubic yards. Each pad is width × length × thickness with all three in inches, so it divides by 1,728 to reach cubic feet.
The quantity to carry in your head is the cross-sectional area, which this calculator reports as concrete per lineal foot. A 16 × 8 inch footing is (16 ÷ 12) × (8 ÷ 12) = 0.889 ft² of cross-section, so every lineal foot of it takes 0.889 cubic feet of concrete. Multiply by the run and divide by 27 and you are done. The convenient special case is the 24 × 12 inch footing: exactly 2 ft² of section, so 2 cubic feet per lineal foot and 100 lineal feet is 200 cubic feet, or 7.41 cubic yards.
Per lineal foot is also the figure to use when the run changes on site. If the excavator finds rock and the foundation shifts 6 feet, you do not redo the takeoff — you multiply 6 by the cross-section and adjust. That is why it is an output here rather than an intermediate step.
One thing the formula deliberately does not do is subtract anything for the foundation wall or the stem that will sit on top. The footing is poured first and full width; the wall is a separate pour with its own takeoff. Nor does it add the trench over-dig, which is what the waste allowance is for.
Worked example: a 40 ft × 30 ft crawlspace foundation with three pier pads
You are pouring footings for a 40 ft × 30 ft crawlspace foundation. The strip footing is 16 inches wide and 8 inches thick on the centreline of the perimeter, there are three interior pier pads at 24 × 24 × 12 inches under a girder, and you want a 10% allowance because the footings are earth-formed in a trench.
- Footing run. 2 × (40 + 30) = 140 lineal feet.
- Cross-section. (16 ÷ 12) × (8 ÷ 12) = 1.3333 × 0.6667 = 0.8889 ft², so 0.889 cubic feet per lineal foot.
- Strip volume. 0.8889 × 140 = 124.44 ft³.
- One pad. 24 × 24 × 12 ÷ 1,728 = 6,912 ÷ 1,728 = 4.0 ft³. Check it the other way: 2 ft × 2 ft × 1 ft = 4 ft³.
- Three pads. 3 × 4.0 = 12.0 ft³. Count only pads that sit clear of the strip; anything falling inside the trench line is already in the strip volume.
- Total cubic feet. 124.44 + 12.0 = 136.44 ft³.
- Cubic yards. 136.44 ÷ 27 = 5.053 yd³, or 3.864 m³.
- Add 10% waste. 5.053 × 1.10 = 5.559 yd³.
- Round up to the next quarter yard. 5.559 ÷ 0.25 = 22.2, so 23 quarter-yards = 5.75 yd³ to order.
Now test how sensitive that is. Widen the footing from 16 inches to 20 inches because the soil report comes back at a lower bearing value: the cross-section rises to (20 ÷ 12) × (8 ÷ 12) = 1.111 ft², the strip becomes 155.6 ft³, and the total climbs to 6.21 cubic yards neat — 23% more concrete for four inches of width. Footing width is the single most expensive dimension on a foundation, which is why the code tabulates it rather than leaving it to judgement.
Checking the size against the code before you check the volume
A volume is only useful if the footing is the right size, so run the dimensions past the residential minimums first. The 2021 International Residential Code governs one- and two-family dwellings in most of the United States, and Section R403 is where footings live:
- Thickness. Section R403.1.1 sets a minimum footing thickness of 6 inches. Eight inches is the common built thickness because it matches the wall forms and gives room for a keyway.
- Width. Table R403.1(1) tabulates minimum widths against the soil's load-bearing value and the number of storeys, with light-frame construction on weaker soils driving the widest footings. Do not guess this from the wall thickness — a 16 inch footing under an 8 inch wall is a convention, not a code requirement, and the table can demand considerably more.
- Depth below grade. Section R403.1.4 requires footings to extend below the frost line for the jurisdiction, and in any case at least 12 inches below the undisturbed ground surface. That depth changes the excavation, not the concrete volume, so it does not appear in this calculator — but it is what makes a footing trench expensive.
- Projection. A plain concrete footing is normally proportioned so that the concrete projecting beyond the face of the wall does not exceed the footing thickness. When it does, the footing is in bending and needs reinforcement designed to ACI 318 rather than a code table.
Soil bearing capacity is the input behind all of this. If your soil report or local presumptive value is lower than the code default, the footing gets wider, and every inch of width costs concrete in direct proportion. Size the footing first with the soil bearing footing size calculator, then bring the dimensions here for the takeoff.
On the volume itself, expect the trench to take more than the drawing. Earth-formed footings slough, the excavator bucket is wider than the design width, and the bottom is rarely flat. Ten per cent is a reasonable allowance in firm soil with a clean trench; in loose or wet ground the real overrun is larger, and formed footings are the only way to control it.
Strip footing concrete per lineal foot
| Footing size (W × D) | Cross-section | ft³ per lineal ft | yd³ per 100 lineal ft | yd³ per 140 ft perimeter |
|---|---|---|---|---|
| 12 in × 6 in | 0.500 ft² | 0.500 | 1.85 | 2.59 |
| 12 in × 8 in | 0.667 ft² | 0.667 | 2.47 | 3.46 |
| 16 in × 8 in | 0.889 ft² | 0.889 | 3.29 | 4.61 |
| 16 in × 10 in | 1.111 ft² | 1.111 | 4.12 | 5.76 |
| 20 in × 8 in | 1.111 ft² | 1.111 | 4.12 | 5.76 |
| 20 in × 10 in | 1.389 ft² | 1.389 | 5.14 | 7.20 |
| 24 in × 12 in | 2.000 ft² | 2.000 | 7.41 | 10.37 |
| 30 in × 12 in | 2.500 ft² | 2.500 | 9.26 | 12.96 |
| 36 in × 12 in | 3.000 ft² | 3.000 | 11.11 | 15.56 |
Neat volumes with no waste allowance. The 140 ft column corresponds to a 40 ft × 30 ft building perimeter.
Where footing takeoffs go wrong
- Measuring the perimeter to the wall face rather than the footing centreline. The footing is wider than the wall and its centreline sits where the wall centreline sits, not at the outside of the sheathing.
- Leaving out interior bearing walls. A centre bearing wall on a 40 ft building adds 40 lineal feet — nearly 30% more strip on a simple rectangle.
- Forgetting garage returns, porches and chimney bases. These are short runs and small pads, and there are usually several of them.
- Counting the stem wall as part of the footing. The wall above is a separate pour with its own forms and its own volume; adding it here double-counts nothing but hides the real quantity of each.
- Using a zero waste allowance on an earth-formed trench. The bucket is wider than the design and the sides slough. Ten per cent is a floor, not a ceiling.
- Applying a wall-thickness rule instead of the code table. "Twice the wall width" is a habit. IRC Table R403.1(1) is the requirement, and it depends on the soil.
- Ignoring stepped footings on sloping ground. Each step adds a vertical section of concrete that a flat-run takeoff misses entirely.
This is a quantity tool, not a foundation design
The calculator tells you how much concrete fills a footing of a stated size. It does not tell you whether that size is adequate. Footing width follows from the load above and the soil beneath it, and both are site-specific. The presumptive load-bearing values in IRC Table R401.4.1 are a starting point only where a geotechnical report is not required; where the code official requires one, the report governs. Where the projection beyond the wall exceeds the footing thickness, or where the footing spans soft spots, the design belongs to a licensed engineer working to ACI 318. Get the size right first, then use this page to buy the concrete.
How this fits with the rest of the foundation takeoff
Footings are the first of four quantities on a foundation, and they are the only one this page covers.
The excavation. The trench is deeper and wider than the footing, and the spoil has to go somewhere. Take it off with the trench excavation volume calculator, which also gives you the swell volume you will be hauling away.
The stem wall or foundation wall. Poured walls are a slab-shaped volume turned on edge: thickness × height × run. Run them through the slab calculator using the wall height as the length and the wall thickness as the thickness.
The reinforcement. Continuous footings normally carry longitudinal bars with laps at the corners, and pads carry a mat. Bar counts come from the rebar spacing calculator and the weight to order from the rebar weight calculator.
The slab. If the floor is a slab on grade, it is a separate pour unless the design is monolithic. For a monolithic slab with a thickened edge, take the slab off at its constant thickness and treat the turndown as a strip footing here, then add the two.
For deck and pier work the geometry changes: piers are cylinders rather than prisms, so use the Sonotube and column calculator for the volume and the deck footing size calculator to size the bearing area first. And on small jobs — a few pads, a short return — check whether the total is worth a delivery at all by running it through the bag calculator.