Footing Calculator - Concrete Volume, Bags & Rebar Planning
Footing concrete is volume: length times width times depth for strip footings, count times pad volume for spread footings, or pi radius squared times depth for piers.
Percent added for cuts, waste, settlement, or field loss.
Bag count is shown separately; cost uses ready-mix volume.
Change any value and the results, formula, and diagram update immediately. Use the same unit system throughout one estimate.
Footing Volume Quick Reference
Concrete needed for common residential footing layouts in cubic yards. Real footing dimensions come from the engineer or local code; this is the back-of-envelope check.
| Footing type | Typical dimensions | Concrete per 40 ft of wall | Bags (80 lb) per 40 ft |
|---|---|---|---|
| Strip - 16 in W x 8 in D | Residential single storey | 1.32 yd³ | 60 |
| Strip - 20 in W x 10 in D | Residential double storey | 2.06 yd³ | 93 |
| Strip - 24 in W x 12 in D | Heavy load / soft soil | 2.96 yd³ | 134 |
| Pad - 2 x 2 x 1 ft (4 pads) | Deck or shed posts | 0.59 yd³ | 27 |
| Pier - 12 in dia x 4 ft (4 holes) | Deck posts to frost depth | 0.47 yd³ | 21 |
| Pier - 12 in dia x 6 ft (4 holes) | Cold-climate deck | 0.70 yd³ | 32 |
Footings excavated by hand or with a small backhoe often over-dig - add 15% waste instead of the usual 10% to cover low spots and trench width variation.
Footing Types - Pick The Right One
Strip footing
A continuous concrete trench under a load-bearing wall. The most common residential foundation. Width is usually 2x the wall thickness. The calculator treats this as length x width x depth.
Spread footing (pad)
An isolated rectangular pad under a column, post, or pier. Sized so soil bearing pressure stays below the soil's allowable load. Typical: 2x2x1 ft for a deck post; 3x3x1 ft for a heavier column. Quantity field handles multiple identical pads.
Round pier
A circular hole filled with concrete - the classic deck-post, fence-post, or porch-pier footing. Sonotube formwork shapes the visible part. Width is diameter; depth must reach below the frost line.
Bell footing
A pier with a flared bottom (bell or pier-and-bell). More bearing area at depth, common for elevated decks on poor soil. The calculator handles the cylinder; estimate the bell separately as a cone.
Mat / raft foundation
A single slab footing supporting the entire building. Used on soft soils, swimming pools, water tanks. Estimate as a slab (length x width x thickness) - see the Slab Calculator.
Stepped footing
A strip footing that steps down on a sloped site. Break it into level segments; the calculator sums their volumes. Steps must equal or exceed footing thickness.
Frost Depth - Why Footings Go Deeper Than You Think
Footings must extend below the frost line, where freeze-thaw cycles cannot heave them. Heaving lifts walls, cracks slabs, and tilts decks. Frost depth varies by climate:
| Region | Typical frost depth | Note |
|---|---|---|
| US Gulf Coast / Florida / California | 0-12 in | Code minimum 12 in |
| US Mid-Atlantic / Carolinas | 12-24 in | Most rural counties |
| US Midwest / Northeast | 36-48 in | Chicago 42 in, Boston 48 in |
| US Upper Midwest / Northern Plains | 48-60 in | Minneapolis 60 in, Fargo 60+ in |
| Canada southern provinces | 48-60 in | Toronto 48 in, Calgary 60 in |
| Canada northern provinces | 60-96 in | Edmonton 72 in |
| UK / Northern Europe | 450-750 mm (18-30 in) | Build into BS / Eurocode minimum |
| India | n/a (no freezing in plains) | Depth driven by soil bearing, typically 1-1.5 m residential |
| Australia / Mexico / Philippines | n/a | Depth driven by soil and termite control |
Soil reports trump the table. Expansive clay (Texas, parts of India, southern California) needs deeper or wider footings regardless of frost.
Sizing - Width, Depth, and Bearing
Footing size is driven by load and soil. Code rules of thumb:
- IRC residential rules: footing width 2x wall thickness (16 in for an 8 in wall, 24 in for a 12 in wall). Footing thickness equal to wall thickness, minimum 6 in.
- Soil bearing: typical residential design assumes 1,500 psf (72 kPa). Sandy or clay soils may be 2,000-3,000 psf. Soft clay drops to 1,000 psf or less - widen the footing.
- Pier depth: Frost depth plus 6 in. A 4 ft deck pier in Minnesota is actually 5.5 ft to clear the frost line plus an inch of bearing.
- Pier diameter: 12 in standard for deck and porch posts; 16-24 in for heavier loads. Engineer for anything carrying more than ~5,000 lb point load.
Rebar In Footings
Plain (unreinforced) concrete footings are allowed by code only for lightly-loaded one and two-storey residential work. Anywhere with active seismic, frost heave, or expansive soil, footings carry rebar.
- Strip footing: 2 to 3 longitudinal #4 bars near the bottom, plus #4 ties every 12-24 in. Larger footings carry 4-5 bars.
- Pad / spread footing: a mat of #4 or #5 rebar at 6-12 in OC each way, near the bottom (steel works in tension, soil pushes up).
- Pier: 2-4 vertical bars (#4 or #5) with ties every 12 in. Continuous into the column above.
- Dowels into wall above: bend or hook rebar so it laps 40 bar diameters into the wall pour.
Use the Rebar Calculator for piece counts and lengths.
Cost Estimates Around The World
2026 ready-mix and labour prices for residential footing work. Excavation and disposal are separate line items (commonly $50-150 per cubic yard in the US).
| Region | Ready-mix concrete | Installed footing (concrete + rebar + form + labour) |
|---|---|---|
| United States (USD) | $140-180 / yd³ | $8-20 / linear ft of strip footing |
| Canada (CAD) | C$200-260 / m³ | C$30-70 / linear m |
| United Kingdom (GBP) | £110-140 / m³ | £40-90 / linear m |
| Eurozone (EUR) | €120-160 / m³ | €45-100 / linear m |
| Australia (AUD) | A$220-300 / m³ | A$70-150 / linear m |
| India (INR) | ₹5,500-7,500 / m³ | ₹300-700 / running ft (PCC + RCC footing) |
| Mexico (MXN) | MX$2,800-3,800 / m³ | MX$600-1,400 / m |
| Philippines (PHP) | PHP 5,200-7,000 / m³ | PHP 1,400-3,500 / m |
Pier footings (single hole) tend to price per pier: $80-200 each in the US for hand-dug 12 in dia at 4 ft deep, including concrete and rebar dowel; up to $400-600 each in Canada for 6 ft deep. India: ₹800-2,500 per pier depending on depth and soil.
Common Mistakes
- Using slab math (length x width x thickness) for round piers instead of pi x radius² x depth - off by 22% for a square slot vs round hole.
- Stopping the pier above frost depth - it heaves and the deck tilts within two seasons.
- Entering pier diameter and treating it as radius (or vice versa) - radius is half the diameter.
- Forgetting the quantity field on multiple identical pads or piers.
- Skipping the bearing-area check for spread footings on soft soil.
- No rebar in footings on expansive or seismic soils - cracks within a year.
- Over-excavating the trench width without adding waste percentage - the truck shows up 0.5 yd short.
- Pouring on uncompacted or frozen subgrade.
Footing Calculator FAQ
How much concrete do I need for a 40 ft strip footing?
At 16 in wide x 8 in deep: 40 x 1.333 x 0.667 = 35.6 cu ft = 1.32 yd³. Add 10% waste = 1.45 yd³. Bagged would be about 60 of the 80 lb bags. At 20 in wide x 10 in deep, the same 40 ft footing needs 2.06 yd³.
How deep should my footing be?
Below the local frost line, always. US Gulf Coast: 12 in minimum. Midwest: 36-48 in. Upper Midwest: 48-60 in. UK: 18-30 in. India: typically 1-1.5 m for residential, driven by soil bearing rather than frost.
How wide should a footing be?
IRC rule of thumb: 2x the wall thickness above. An 8 in wall sits on a 16 in wide footing; a 12 in wall on 24 in. Heavy loads, multi-storey homes, or soft soils need wider footings - confirm with the engineer or local code official.
What is the difference between strip, pad, and pier footings?
Strip footings are continuous trenches under a wall. Pad (spread) footings are isolated rectangular pads under a column or post. Piers are round vertical concrete posts from frost depth to grade, sometimes flared at the bottom (bell pier). Each has a different volume formula.
Do I need rebar in a footing?
Plain concrete footings are allowed for low-load one and two-storey residential work in non-seismic areas. Anywhere with seismic activity, expansive soil, or higher loads, footings carry 2-4 horizontal rebars plus ties. Pier footings almost always carry vertical rebar to connect to the column above.
How many bags of concrete for a deck pier?
A 12 in diameter Sonotube 4 ft deep is pi x 0.5² x 4 = 3.14 cu ft. At 0.6 cu ft per 80 lb bag, that is 5.2 bags - call it 6. Four piers need 24 bags. Cold-climate piers at 5-6 ft deep need 7-9 bags each.
Should I use bagged concrete or ready-mix for footings?
Under about 1.5 yd³ total: bagged is fine. Above that: ready-mix beats bag-mixing on cost and labour. Strip footings on a whole house easily run 4-8 yards - call the plant.
How much waste should I add?
15% for hand-dug or over-excavated trenches and rough subgrade. 10% for machine-cut trenches with rigid forms. 20% for piers in collapsing soil where you over-drill to make sure the hole stays open.
Can I pour footings in cold weather?
Above 40°F (5°C) ambient with curing blankets: yes. Below that, use an accelerator and hot-water mix, then insulate. Below 25°F (-4°C): not without a tent and heater. Frozen-ground pours fail - the soil thaws and the footing settles.
What does footing installation cost?
US: $8-20 per linear foot of strip footing installed (concrete + rebar + form + labour). Pier footings: $80-200 each. UK: £40-90 per m of strip. India: ₹300-700 per running foot. Excavation, disposal, and access constraints can double these numbers.
Related Construction Calculators
For other concrete shapes (columns, tubes, curbs, stairs), use the Concrete Calculator. For flat slab pours, see the Slab Calculator. For reinforcement bar counts, use the Rebar Calculator. For cement from raw ingredients, see the Cement Calculator. More tools live on the Construction Calculators hub.
Sources
- ACI 318 - Building Code Requirements for Structural Concrete
- International Residential Code (IRC) Chapter 4 - Foundations
- Portland Cement Association concrete resources
- FEMA P-549 frost depth and foundation guidance
- Bureau of Indian Standards - IS 1904 design of foundations
- NIST Handbook 44 unit conversion tables
This calculator is for planning and ordering conversations. Local code, project drawings, engineered design, and manufacturer instructions control the final work.