Concrete Footing Calculator
Work out concrete volume and premix bags for strip or pad footings.
Last reviewed September 19, 2026
Result
Buy 33 bags (80 lb)Estimate
Excavated trenches and holes are rarely perfectly square or level, so the headline includes 10% extra by default.
- Concrete, allowance included
- 0.7243 yd³ (19.5556 ft³)
- Minimum if everything goes perfectly
- 17.7778 ft³ (0.6584 yd³)
- Concrete per footing
- 17.7778 ft³
https://calcgallery.com/concrete/concrete-footing-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Choose strip or pad footing. The choice labels the job; both use rectangular volume.
- Enter the excavated length, width and depth and the number of identical footings.
- Choose the concrete bag size you can buy.
- Read the bag count and the bulk volume.
How it is calculated
A strip or rectangular pad footing is a rectangular prism:
Volume = length × width × depth × quantity
Bags = ceil(volume with allowance ÷ yield per bag)All measurements are converted to metres before multiplication. Bag yield is editable because the number printed by the manufacturer is the one that matters.
Practical limits
This is a material calculator, not a footing design tool. It does not choose width, depth, concrete strength, reinforcement, cover or frost protection. Use the construction drawing or advice appropriate to the site.
Measure after excavation. An extra inch across a long trench is a meaningful amount of concrete. Check several places rather than measuring the neatest section.
Keep loose soil out. Soil knocked into the trench takes up volume but does not replace concrete structurally. Clean the bottom before the pour.
Plan the whole pour. A footing is difficult to stop halfway. Compare the result with ready-mix availability, access and the time needed to mix the stated number of bags.
With your numbers
- One footing = 20 ft × 16 in × 8 in = 17.78 ft³
- 1 footing, plus 10% = 19.56 ft³
- Rounded up to 33 bags
Quick reference
Excavated trenches and holes are rarely perfectly square or level, so the headline includes 10% extra by default.
| Job | Bags to buy |
|---|---|
| 20 ft × 16 in × 8 in strip | 33 |
| 40 ft × 18 in × 8 in strip | 74 |
| 12 ft × 24 in × 12 in strip | 44 |
| Four 3 × 3 × 1 ft pads | 6 |
| 6 m × 400 mm × 200 mm strip | 57 |
| Four 900 × 900 × 300 mm pads | 115 |
Common questions
- How much concrete is in a 20 ft footing 16 in wide and 8 in deep?
- The exact volume is 17.78 ft³. With 10% extra it is 19.56 ft³, or 33 bags when each 80 lb bag yields 0.60 ft³.
- Does this choose the correct footing size?
- No. It prices material for dimensions you already have. Soil bearing, loads, frost depth, reinforcement and local requirements determine the design.
- Can I calculate several pad footings?
- Yes. Enter one pad length, width and depth, then set the quantity. The result multiplies the identical pads before applying the allowance.
- Should I measure the drawing or the excavation?
- Use the actual excavation before ordering if you can. Trenches shed soil, sides bow and bottoms are rarely perfectly level.