Concrete Footing Calculator
Work out exactly how much concrete a strip or pad footing needs — bags, cubic yards, rebar guidance and cost.
Your footing
Rule of thumb: 2× the wall width above it.
Check local frost-line depth.
Full perimeter or wall length the footing runs under.
= 6 footings (3 × 2 grid)
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Per linear ft breakdown
0.1 × 80 lb bags per linear ft
Mixing time
30 m
One person, 5 bags
Rebar & formwork planner
The short answer
A typical 16 inch wide, 8 inch deep strip footing needs about 0.9 cubic feet of concrete per running foot — roughly one 80 lb bag every 7 to 8 feet. Work it out precisely by multiplying width × depth × length in feet, adding 5–10% waste, and dividing by your bag’s yield. Pad footings work the same way, per footing, then multiplied by how many you’re pouring.
Bags per linear foot, strip footings
Figures below assume standard concrete mix, no waste added.
| Footing size | Concrete per ft | 80 lb bags / 10 ft | 60 lb bags / 10 ft | 40 lb bags / 10 ft |
|---|---|---|---|---|
| 12″ wide × 6″ deep | 0.50 cu ft | 9 | 12 | 17 |
| 16″ wide × 8″ deep | 0.89 cu ft | 15 | 20 | 30 |
| 18″ wide × 10″ deep | 1.25 cu ft | 21 | 28 | 42 |
| 24″ wide × 12″ deep | 2.00 cu ft | 34 | 45 | 67 |
For pad footings, run the calculator per footing — a 24″ × 24″ × 12″ pad needs roughly 4.0 cu ft, or about 7 × 80 lb bags, before waste.
How deep and how wide should a footing be?
| Structure | Typical footing width | Typical depth |
|---|---|---|
| Garden wall / fence wall | 10–12″ | 6–8″ |
| Single-storey wall | 16″ | 8″ |
| Two-storey wall | 18–20″ | 10″ |
| Deck or porch post pad | 18–24″ square | 10–12″ |
| Garage / shed column pad | 24–30″ square | 12″ |
Width is generally about twice the width of the wall it carries. Depth is driven less by the load and more by frost line — footings above the frost line in cold climates can heave and crack as the ground freezes and thaws, so always check your local code minimum before digging.
How to calculate concrete for a footing
A footing is a simple rectangular volume — no material to subtract, unlike a post hole.
- Convert to feet. Width and depth are usually easiest to measure in inches — divide by 12.
- Find the volume. Multiply width × depth × length (or width × depth × length × number of footings for pads).
- Add waste. 5–10% covers spillage, uneven subgrade and over-dug trenches.
- Divide by bag yield and round up. 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb — round up per footing so no single pour comes up short.
Worked example
A strip footing 16″ wide, 8″ deep, running 40 ft, using 80 lb bags:
- Width in feet16 ÷ 12 = 1.33 ft
- Depth in feet8 ÷ 12 = 0.67 ft
- Volume per foot1.33 × 0.67 = 0.89 cu ft
- Total volume (× 40 ft)35.6 cu ft
- Plus 10% waste39.2 cu ft
- Divide by bag yield39.2 ÷ 0.60 = 65.3
- Round up66 × 80 lb bags total
That’s roughly 5,280 lb to move and around 5 hours of mixing for one person — a strong argument for ready-mix or a helper on runs this long.
Strip vs pad footings
| Strip footing | Pad footing | |
|---|---|---|
| Shape | Continuous trench under a wall | Single block under one column or post |
| Typical use | Foundation walls, garden walls | Deck posts, porch columns, sheds |
| Rebar | 2+ continuous horizontal bars | Grid or cage sized to the load |
| Formwork | Long straight forms or trench walls | Short forms, often a single box |
| Pour style | Usually one continuous pour | Poured individually or together |
A structure often uses both — a strip footing under the perimeter walls and pad footings under interior posts or columns carrying point loads.
Pouring a footing: a quick checklist
- Dig to undisturbed soilFootings need to bear on firm, undisturbed ground — not loose backfill.
- Check the depth against local codeFrost line and load requirements vary by area; confirm before pouring.
- Add a gravel base if needed2–4 inches of compacted gravel improves drainage under the footing.
- Set forms and check levelStraight, level forms keep the footing’s dimensions accurate along its full length.
- Place rebar with proper coverSupport bars on chairs so they sit inside the concrete, not on the ground.
- Pour, consolidate and cureTamp out air pockets, screed level, and keep the surface damp while it cures.
Common mistakes
- Footing too shallow for the frost lineGround freeze-thaw cycles can heave a footing that doesn’t reach below frost depth, cracking the structure above it.
- Skipping rebarConcrete is strong in compression but weak in tension — rebar carries the tension loads that plain concrete can’t.
- Pouring on loose or disturbed soilA footing needs firm bearing soil beneath it, not backfill or loose spoil from digging.
- Rounding the total instead of per footingRound up bags per footing first, then add them up, so no individual pour runs short.
- Underestimating trench overdigReal trenches are rarely perfectly straight — the waste allowance exists for exactly this.
- Loading the footing too soonBuilding on a footing before it has enough strength risks settling or cracking.
What this calculator assumes
Adjust your result if your project differs from these.
- Bag yields of 0.60, 0.45, 0.375 and 0.30 cu ft for 80, 60, 50 and 40 lb bags
- Footings treated as simple rectangular volumes (width × depth × length)
- Standard concrete mix, not fast-setting or high-strength products
- Water of roughly 3 quarts per 80 lb bag
- Mixing at about 6 minutes per 80 lb bag, per person
- Rebar guidance is a general rule of thumb, not a stamped engineering design
- Pad footing count from a column grid rounds to whole footings, evenly spaced
- Prices are whatever you enter; they are not live market rates
Common questions
How much concrete do I need for a footing?
Multiply the footing’s width, depth and length in feet to get cubic feet, add 5 to 10 percent waste, then divide by your bag’s yield. A typical 16 in wide, 8 in deep strip footing needs about 0.9 cubic feet per running foot.
How deep should a concrete footing be?
Most residential footings are 8 to 12 inches deep, but local code usually requires footings to sit below the frost line, which can be much deeper in cold climates.
How wide should a footing be?
A common rule is twice the width of the wall it supports, so an 8 inch wall typically sits on a 16 inch wide footing. Pad footings under posts or columns are sized to the load, often 18 to 30 inches square.
Do I need rebar in a footing?
Most codes require at least two continuous horizontal bars in a strip footing, with more for wider footings or heavier loads. Pad footings typically use a rebar grid or cage sized to the load.
What is the difference between a strip footing and a pad footing?
A strip footing is a continuous trench of concrete that runs under a wall. A pad footing, also called an isolated or spread footing, is a single block of concrete under one column or post.
Should footings go below the frost line?
In climates that freeze, yes. Footings above the frost line can heave as the ground freezes and thaws, cracking the structure above. Local building codes set the minimum frost depth for your area.
Can I pour footings without forms?
If the trench walls are firm and hold their shape, concrete can be poured directly against the soil. Loose or sandy soil usually needs wood forms to keep the footing’s dimensions accurate.
How long does a footing need to cure before building on it?
Footings typically reach enough strength to build on in 2 to 7 days, though 24 to 48 hours is common for light loads. Full strength takes about 28 days.
How much does a footing cost per linear foot?
Material cost per linear foot depends on the footing’s width and depth, plus your local concrete price. Multiply the concrete volume per foot by your price per cubic yard or per bag to estimate it.
How many bags of concrete for a small shed footing?
A typical run of pad footings for a small shed — four 24 in square, 12 in deep pads — needs roughly 12 to 16 × 80 lb bags total, depending on waste allowance.
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Concrete Footing Estimate
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Project
| Footing type | — |
| Footing size | — |
| Length / count | — |
| Waste allowance | — |
Per unit
| Concrete volume | — |
| 80 lb bags | — |
Total shopping list
| 80 lb bags | — |
| 60 lb bags | — |
| 40 lb bags | — |
| Water needed | — |
| Dry weight to move | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes rectangular footing volumes and standard concrete mix with yields of 0.60, 0.45, 0.375 and 0.30 cu ft for 80, 60, 50 and 40 lb bags. Confirm quantities and rebar with your engineer or supplier before buying.