Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 40 ft; Width 16 in = 1.3333 ft; Depth 8 in = 0.6667 ft
- Volume = 40 × 1.3333 × 0.6667 × 1 = 35.5556 ft³
- Net volume = 35.5556 ft³ ÷ 27 = 1.3169 yd³ (× 0.0283168 = 1.0068 m³)
- Order quantity = 1.3169 yd³ × (1 + 10%) = 1.4486 yd³ = 39.1111 ft³
- 80 lb bags = 39.1111 ft³ ÷ 0.6 ft³ per bag = 65.19 → 66 bags (always round up)
- Weight = 39.1111 ft³ × 150 lb/ft³ = 5,867 lb
How to measure
For a strip footing, measure the centerline length of the trench and use the width and depth of the excavated trench or the forms. For a rectangular building add the four sides, but count each corner only once. Subtract nothing for the foundation wall that will sit on top, because the footing is below it.
For spread (pad) footings under posts or piers, enter the side length of the pad as both the length and the width, the depth as its thickness, and put the number of pads in the quantity box.
The footing formula
A footing is a rectangular box, so the formula is the same as a slab: length × width × depth, all in feet, divided by 27 for cubic yards. What changes is that two of the three dimensions are small and given in inches. Convert them exactly: 16 in is 1.3333 ft and 8 in is 0.6667 ft.
Because footings are poured into trenches, the sides are rarely perfectly square. Trench walls cave, the bottom is uneven and the width may be wider than the form. Ten percent extra is a sensible default, and for a trench poured without forms, 15% is not unusual.
Formula: Volume (ft³) = L × (W ÷ 12) × (D ÷ 12) × quantity.
Worked example: a 120 ft strip footing
A 30 × 30 ft building has 120 ft of perimeter. With a footing 16 in wide and 8 in deep:
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 120 ft; Width 16 in = 1.3333 ft; Depth 8 in = 0.6667 ft
- Volume = 120 × 1.3333 × 0.6667 × 1 = 106.6667 ft³
- Net volume = 106.6667 ft³ ÷ 27 = 3.9506 yd³ (× 0.0283168 = 3.0205 m³)
- Order quantity = 3.9506 yd³ × (1 + 10%) = 4.3457 yd³ = 117.3333 ft³
- 80 lb bags = 117.3333 ft³ ÷ 0.6 ft³ per bag = 195.56 → 196 bags (always round up)
- Weight = 117.3333 ft³ × 150 lb/ft³ = 17,600 lb
| Quantity | Result |
|---|---|
| Net volume | 3.951 yd³ (106.67 ft³, 3.020 m³) |
| Order with 10% waste | 4.35 yd³ (3.32 m³) |
| 80 lb bags | 196 |
| 60 lb bags | 261 |
| Weight at 150 lb/ft³ | 17,600 lb |
That is about 4.35 yd³, a full truck order rather than bags. The footing alone weighs around 8.8 tons.
Worked example: six pad footings
Six 24 × 24 in pads, each 12 in thick, under deck posts:
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 24 in = 2 ft; Width 24 in = 2 ft; Depth 12 in = 1 ft
- Volume = 2 × 2 × 1 × 6 = 24 ft³
- Net volume = 24 ft³ ÷ 27 = 0.8889 yd³ (× 0.0283168 = 0.6796 m³)
- Order quantity = 0.8889 yd³ × (1 + 10%) = 0.9778 yd³ = 26.4 ft³
- 80 lb bags = 26.4 ft³ ÷ 0.6 ft³ per bag = 44 → 44 bags (always round up)
- Weight = 26.4 ft³ × 150 lb/ft³ = 3,960 lb
Total 0.98 yd³ with waste, or 44 bags of 80 lb mix. This is the size of job where bagged concrete is still sensible: one wheelbarrow of mix per pad. Set the posts or piers before the concrete skins over.
How big should a footing be?
Footing size is set by the load, the soil bearing capacity and your local building code, not by this calculator. Many residential strip footings are somewhere between 12 and 24 in wide and 8 to 12 in deep, but that range is only context. Frost depth sets how deep the bottom of the footing must be, and that can be 4 ft or more in cold climates.
Take dimensions from your plans or the building inspector. If the footing size is not specified, ask the inspector or an engineer rather than copying a number from the internet.
Common footing mistakes
- Using the wall thickness instead of the footing width. Footings are normally wider than the wall above them.
- Counting corners twice. Where two trench legs meet, the overlap is one square of footing, not two.
- Forgetting the steps in a stepped footing on a sloping lot.
- Not allowing for steel. Rebar takes up little volume, so you do not subtract it, but you should price it with the rebar calculator.
- Ordering for the planned depth when the trench was dug deeper to reach good soil. Measure the finished trench.
Next steps
Footings usually come before the walls or slab. Add the wall calculator for the stem wall, the slab calculator for the floor, and carry the totals into the Pour List for one combined order. If the footing is poured by truck, read how to order ready-mix concrete first.
Frequently asked questions
How do I calculate concrete for a footing?
Multiply the length of the trench by its width and depth, all in feet, then divide by 27 for cubic yards. For a 40 ft run, 16 in wide and 8 in deep: 40 × 1.3333 × 0.6667 = 35.56 ft³, or 1.32 yd³.
How much concrete is in a linear foot of footing?
A footing 16 in wide and 8 in deep holds 0.889 ft³ per linear foot, which is 0.033 yd³. A footing 24 in wide and 12 in deep holds 2 ft³ per linear foot.
Do I subtract the rebar volume?
No. Rebar takes up well under one percent of the footing volume, and the waste allowance is larger than that.
Should I order extra for a trench footing poured without forms?
Yes. Earth-formed trenches are wider and uneven, so 10 to 15% extra is common. Ask your supplier what they recommend for your soil.
How many bags do I need for a deck footing?
A 24 × 24 in pad that is 12 in thick holds 4 ft³, about 7 bags of 80 lb mix with no waste. A 12 in round pier that is 36 in deep is much less, about 5 bags with waste.
Can I use the slab calculator for a footing?
Yes, the geometry is identical. This page just defaults to footing-sized dimensions and mixed units.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.