Strip footings vs. pad footings
Strip (continuous) footings run under a wall, such as a garage, an addition, a block wall or a foundation. You enter the total length of footing, its width and its depth.
Pad (spread) footings are square or rectangular blocks under individual posts or columns, like a deck, pergola or carport. Enter the length of one pad as "Footing length" and set "Number of footings". For a 24 × 24 in pad, enter 2 ft for the length and 24 in for the width.
For round footings poured in tube forms, use the column calculator. For a footing that combines a tube with a wider base, add the two results together.
Measuring a footing run without double-counting corners
On a rectangular foundation, the easy mistake is adding the outside lengths of all four sides. Each corner then gets counted twice. Measure along the centerline of the footing instead. For a 24 × 30 ft garage with a 16 in footing, the centerline perimeter is 2 × (24 + 30) − 4 × (16 ÷ 12) = 102.7 ft, not 108 ft.
Trenches dug in soil are rarely perfectly square. Measure the actual width and depth in several places after digging, and use the average if they vary.
Formula
Volume = length × (width ÷ 12) × (depth ÷ 12) × number of footings, with width and depth in inches and the result in cubic feet. Divide by 27 for cubic yards. With the defaults (40 ft run, 16 in wide, 8 in deep) that's 40 × 1.333 × 0.667 = 35.6 ft³, and 1.45 yd³ (39.1 ft³) with 10% waste.
How big should a footing be?
Footing size depends on the load, the soil's bearing capacity and local code, so the calculator defaults are not design values. For context, the International Residential Code (Section R403.1.1) sets a minimum width of 12 in and a minimum thickness of 6 in for concrete footings, with larger sizes in its tables depending on soil, the number of stories and snow load. Footings also have to sit below the local frost depth, which your building department publishes.
| Footing | Width × depth | Concrete per 10 ft run (no waste) |
|---|---|---|
| IRC minimum | 12 × 6 in | 0.19 yd³ (5 ft³) |
| Example | 16 × 8 in | 0.33 yd³ (8.9 ft³) |
| Example | 20 × 10 in | 0.51 yd³ (13.9 ft³) |
| Example | 24 × 12 in | 0.74 yd³ (20 ft³) |
| Footing | Width × depth | Concrete per 3 m run (no waste) |
|---|---|---|
| About the IRC minimum | 30 × 15 cm | 0.135 m³ (135 L) |
| Example | 40 × 20 cm | 0.24 m³ (240 L) |
| Example | 50 × 25 cm | 0.375 m³ (375 L) |
| Example | 60 × 30 cm | 0.54 m³ (540 L) |
Deck and pergola pads are usually set by the post load and the soil. Prescriptive guides like the American Wood Council's DCA 6 give footing sizes for decks built to its rules.
Worked examples
Garage footing, 102.7 ft centerline, 16 × 8 inGarage footing, 31.3 m centerline, 40.6 × 20.3 cm
Footing length 102.7 ft · Footing width 16 in · Footing depth 8 in · Number of footings 1 · Waste / overage 10 %Footing length 31.3 m · Footing width 40.6 cm · Footing depth 20.3 cm · Number of footings 1 · Waste / overage 10 %
- Concrete volume: 3.72 yd³ (100.4 ft³)Concrete volume: 2.84 m³
- 80 lb bags: 16840 kg bags: 152
Pergola: four 24 × 24 in pads, 12 in deepPergola: four 61 × 61 cm pads, 30.5 cm deep
Footing length 2 ft · Footing width 24 in · Footing depth 12 in · Number of footings 4 · Waste / overage 10 %Footing length 0.61 m · Footing width 61 cm · Footing depth 30.5 cm · Number of footings 4 · Waste / overage 10 %
- Concrete volume: 0.65 yd³ (17.6 ft³)Concrete volume: 0.498 m³ (498 L)
- 80 lb bags: 3040 kg bags: 27
- 60 lb bags: 4025 kg bags: 43
This is small enough for bags. Check that the bottom of each pad is below frost depth.
Garden block wall, 30 ft long, 20 × 10 in footingGarden block wall, 9.14 m long, 50.8 × 25.4 cm footing
Footing length 30 ft · Footing width 20 in · Footing depth 10 in · Number of footings 1 · Waste / overage 10 %Footing length 9.14 m · Footing width 50.8 cm · Footing depth 25.4 cm · Number of footings 1 · Waste / overage 10 %
- Concrete volume: 1.7 yd³ (45.8 ft³)Concrete volume: 1.3 m³
- 80 lb bags: 7740 kg bags: 70
Tips from the trench
- Keep trench sides clean and vertical where the soil allows. Sloughed soil in the bottom reduces bearing and mixes into the concrete.
- Pour footings on undisturbed or properly compacted soil. Never pour on loose fill, frozen ground or standing water.
- Place rebar on supports (chairs or bricks) so it sits with the required cover. ACI 318 calls for 3 in of cover where concrete is cast against earth. The rebar calculator handles bar counts.
- Get your inspection before the pour. Many jurisdictions inspect footing trenches and rebar before concrete goes in.
Trench pour or formed footing?
Where the soil holds a clean vertical face, many footings are poured straight into the trench (an "earth-formed" footing), and the trench dimensions become the footing dimensions. In sandy or crumbly soil, or where the footing sits partly above grade, builders set wood forms. With a formed footing, measure the inside of the forms. With a trench pour, measure the trench itself in several places, because a trench dug 2 in wider than planned along 100 ft of footing adds noticeable volume.
Common mistakes
- Double-counting corners by adding outside lengths. Use the centerline.
- Underestimating an over-dug trench. A 16 in footing that ends up 18 in wide along 100 ft needs 11.1 ft³ more concrete at 8 in deep.
- Pouring before inspection. Footing trenches and rebar are often inspected before concrete goes in.
- Mixing bags for a big footing. Long footing runs quickly reach ready-mix volumes. One continuous pour avoids cold joints.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 40 ft run, 16 in wide, 8 in deep, no waste: volume | 1.32 yd³ (35.6 ft³) | Hand calculation | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 12.2 m run, 40.6 cm wide, 20.3 cm deep, no waste: volume | 1.01 m³ | Hand calculation | Match |
Frequently asked questions
What is the minimum footing size for a house?
The IRC minimum is 12 in wide and 6 in thick, but the required size often goes up with soil type, the number of stories and snow load. Your local code and plans govern.
How deep should footings be?
Below the local frost line, which ranges from a few inches in warm climates to several feet in cold ones. Ask your building department.
How many bags of concrete for a deck footing?
A 24 × 24 × 12 in pad is 4 ft³, about 7 bags of 80 lb mix before waste.
Should I subtract rebar volume?
No. Rebar takes up well under 1% of the volume.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- International Residential Code, Section R403.1 (footings)
- Quikrete Concrete Mix data sheet (bag yields, minimum thickness)
- American Wood Council DCA 6 deck guide
- ACI 318-19 Table 20.6.1.3.1 (concrete cover)
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.