Concrete Footing Calculator
Calculate concrete volume and bag count for strip, spread, stepped, or continuous footings. Pick an application preset for typical sizes, or enter your own dimensions.
Results
Estimates only. Verify dimensions on site, set footings below your local frost line and on undisturbed soil, and check building code for required width, depth, and rebar.
How to use this calculator
- 01Pick an Application preset to auto-fill typical dimensions, or leave it on Custom to enter your own. The presets cover the most common footing sizes — light residential strip, 2-story strip, deck pier, fence post, and shed perimeter.
- 02Choose a Footing Type. Strip and Continuous run under walls. Spread is a single rectangular pad under a column or post. Stepped is for sloped sites where the footing changes depth in segments. The math is the same for all four — the type affects the dimension labels and the helpText.
- 03Enter Length in feet, plus Width and Depth in inches. For multiple identical footings (deck piers, fence posts), set Number of Footings — the calculator returns the project total in one shot.
- 04Pick your Bag Size and Waste Factor. 80-lb bags are standard for footings; 10% waste covers the partial-bag overage that cannot be returned. For projects over ~1 cubic yard, ready-mix delivery is usually cheaper than bags.
- 05Read the results. Total Concrete Needed includes waste and is the orderable cubic yardage. Total Bags is computed from total volume directly, so it can be slightly less than per-footing bags multiplied by the count — that is normal and reflects actual minimum demand.
Understanding the math
Every footing is a rectangular volume: length × width × depth. Length is given in feet; width and depth come in inches and need to be converted:
volume ft³ = (lengthFt × widthIn × depthIn) / 144 · yards = ft³ / 27
For multi-footing projects, multiply the per-footing volume by the count to get total cubic feet. Total bag count is computed from total volume directly — not by multiplying per-footing bags by the count. That single ceiling step gives the actual minimum bag demand, which can be slightly lower because per-footing rounding inflates each footing independently.
Worked example: a 30-foot strip footing at 16″ wide × 8″ deep, single run, 80-lb bags, 10% waste. Volume = (30 × 16 × 8) / 144 = 26.67 cubic feet, or 0.99 cubic yards. With 10% waste: 1.09 cubic yards. Bags = 26.67 × 1.10 / 0.6 = 48.89, rounded up to 49 bags. Weight = 26.67 × 150 = 4,000 lbs.
Footing reference chart
Volume and 80-lb bag count for common footing applications, computed at 10% waste. Per-foot rows are for strip footings — multiply by your trench length to get the project total.
| Application | Dimensions | Volume | 80-lb Bags |
|---|---|---|---|
| Light residential strip | per ft × 16″ × 8″ | 0.033 yd³/ft | 2/ft |
| 2-story residential strip | per ft × 24″ × 10″ | 0.062 yd³/ft | 4/ft |
| Deck pier (rectangular) | 24″ × 24″ × 12″ | 0.15 yd³ | 8 |
| Fence post | 12″ × 12″ × 36″ | 0.11 yd³ | 6 |
| Light pole / mailbox | 18″ × 18″ × 36″ | 0.25 yd³ | 13 |
| Shed perimeter strip | per ft × 12″ × 8″ | 0.025 yd³/ft | 2/ft |
| Heavy spread footing | 36″ × 36″ × 18″ | 0.50 yd³ | 25 |
Frequently asked questions
How much concrete do I need for a footing?
A footing is a rectangular volume: length × width × depth. Multiply all three in feet, then divide by 27 to convert cubic feet to cubic yards. For a 30-foot strip footing at 16″ wide × 8″ deep: 30 × 1.333 × 0.667 = 26.7 cubic feet, or 0.99 cubic yards. Add 5–10% waste because concrete is wet and unforgiving — partial trucks cannot be returned. The calculator above handles unit conversion and waste automatically.
How deep should a footing be?
Footings must extend below the frost line for your region — this is a building code requirement, not a recommendation. Typical residential depths: northern US 42-48 inches, mid-latitudes 30-36 inches, southern US 12-24 inches. Footings supporting load-bearing walls also need to reach undisturbed, compacted soil. Always check local codes before pouring — depth varies by climate, soil type, and structural load.
How wide should a footing be?
The 8/16/24 rule: a footing should be at least twice the width of the wall it supports. For an 8-inch foundation wall, the footing is typically 16 inches wide. For poured concrete walls or heavy brick, 24-inch footings are common. Heavy commercial footings reach 36 inches or more. Local building codes specify minimums based on soil bearing capacity — sandy or weak soils require wider footings to spread the load.
What is the difference between a strip footing and a spread footing?
A strip footing (also called a continuous footing) is a long, narrow trench under a wall — it spreads the wall's load along its entire length. A spread footing (or pad footing) is a square or rectangular pad under a single column, post, or pier — it concentrates the load through one isolated point. Strip footings support walls; spread footings support columns. Most residential foundations use strip footings under perimeter walls and spread footings under interior posts.
Do footings need rebar?
Almost always yes. Building codes typically require at least two horizontal #4 or #5 rebar runs along the length of strip footings, and a grid of rebar in spread footings. Rebar prevents the footing from cracking under bending stress as the soil settles unevenly. The exception is very small footings (under 200 square inches of cross-section) in some residential codes. Always verify with local codes — requirements depend on jurisdiction, soil conditions, and structural load.
How do I calculate concrete for a stepped footing?
Stepped footings are used on sloped sites where one continuous depth would mean excessive digging. Calculate each step segment as a separate rectangular footing, then add the volumes together. For example: three steps of 10 ft × 16″ × 8″ at the lowest level, 10 ft × 16″ × 16″ at the middle, and 10 ft × 16″ × 24″ at the top. The calculator above handles single rectangular sections — for stepped footings, calculate each step separately and sum the results.
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Estimates only. Verify dimensions on site, set footings below your local frost line and on undisturbed soil, and check building code for required width, depth, and rebar. TakeoffCalc is not responsible for material over- or under-orders.