Concrete Footing Calculator
Estimate the volume, weight, and cost of concrete needed for strip footings, pad footings, or trench fills. Numbers update as you type.
Concrete Footing Calculator: Strip, Pad & Trench Fill Volume Guide [2026]
A concrete footing calculator gives you exact cubic yards, cubic feet, and bag count for strip footings, pad footings, and trench fills — before you call the ready-mix plant. Enter footing type, dimensions, and number of footings. The calculator returns total volume with waste allowance, weight in pounds (lb) and kilograms (kg), and material cost estimate.
Most footing estimates fail before the calculator opens — wrong footing width, insufficient depth for local frost line, or confusing trench depth with concrete depth. This guide covers how to size footings correctly before entering any numbers, so your calculation matches what inspectors and structural engineers expect.
Need accurate volume for a related project? Use our concrete column calculator for pier and post footings or our concrete cost calculator for full project budgeting.
3 Footing Types the Calculator Supports
The calculator handles 3 footing types. Each uses a different formula and serves a different structural purpose.
Footing Type | Volume Formula | Best Application |
|---|---|---|
Strip footing (continuous) | Length × Width × Depth | Foundation walls, load-bearing walls, retaining walls |
Pad footing (isolated) | Length × Width × Depth × Number of pads | Posts, columns, deck footings, point loads |
Trench fill | Length × Width × Concrete depth in trench | Narrow trenches filled with mass concrete, no forming needed |
Strip footings run continuously under walls and distribute wall loads across a long contact area. Pad footings support individual point loads like posts and columns. Trench fill is a mass concrete method that fills the full trench without forming — faster but uses 30–60% more concrete than a formed footing of the same depth.
How to Size a Footing Before You Calculate Volume
Entering wrong dimensions into the calculator gives an accurate answer for the wrong footing. Get sizing right first.
Footing Width Rule: 2x Wall Thickness
Standard footing width equals 2 times the supported wall thickness. This rule applies to residential foundation walls under IRC 2024:
Wall Thickness | Minimum Footing Width | Minimum Footing Depth | Concrete Volume per LF |
|---|---|---|---|
6 in (15.2 cm) | 12 in (30.5 cm) | 6 in (15.2 cm) | 0.25 ft³ / 0.007 m³ |
8 in (20.3 cm) | 16 in (40.6 cm) | 8 in (20.3 cm) | 0.59 ft³ / 0.017 m³ |
10 in (25.4 cm) | 20 in (50.8 cm) | 10 in (25.4 cm) | 1.16 ft³ / 0.033 m³ |
12 in (30.5 cm) | 24 in (61 cm) | 12 in (30.5 cm) | 1.78 ft³ / 0.050 m³ |
These are minimum dimensions under standard residential code. Poor soil conditions, heavy loads, or engineered designs require larger footings. When in doubt, add 2–4 inches (5.1–10.2 cm) to both width and depth — the concrete cost is minimal compared to the cost of a failed foundation.
Frost Line Depth: The Most Critical Number
The bottom of any footing must sit below the local frost line depth. Footings above the frost line heave every winter as frozen soil expands upward by 0.5–2 inches (1.3–5.1 cm) per freeze cycle. Over 5 years, a heaving footing can shift a structure enough to crack walls, bind doors, and separate floor systems.
Region / State | Frost Line Depth | Notes |
|---|---|---|
Florida, Gulf Coast TX | 0–12 in (0–30.5 cm) | Frost line not a concern in most areas |
Virginia, Tennessee, NC | 12–20 in (30.5–50.8 cm) | Check local code — varies by county |
Ohio, Indiana, Missouri | 24–36 in (61–91.4 cm) | Standard Midwest depth |
Pennsylvania, New York | 36–42 in (91.4–106.7 cm) | Always confirm with local building dept |
Minnesota, Wisconsin, MI | 42–52 in (106.7–132.1 cm) | Deep frost zone — significantly more concrete |
Montana, Wyoming, Maine | 48–60 in (121.9–152.4 cm) | Deepest frost lines in contiguous US |
Colorado (mountain) | 36–48 in (91.4–121.9 cm) | Elevation adds 6–12 in vs valley areas |
Always confirm frost line depth with your local building department before digging. Using the state average instead of local code depth is the most common footing depth mistake on residential projects.
How the Concrete Footing Calculator Works
Strip Footing Calculation
Strip footing volume = total length × width × depth. All dimensions in feet. Divide by 27 for cubic yards.
Example: A house foundation with 120 feet (36.6 m) of continuous strip footing, 16 inches (40.6 cm) wide, 10 inches (25.4 cm) deep:
- Convert to feet: width = 1.33 ft, depth = 0.83 ft
- Volume: 120 × 1.33 × 0.83 = 132.5 ft³ (3.75 m³)
- Cubic yards: 132.5 ÷ 27 = 4.91 yd³ (3.75 m³)
- With 10% waste: order 5.4 yd³ (4.13 m³)
- 80 lb bags if bagged: 5.4 × 45 = 243 bags — use ready-mix at this volume
Pad Footing Calculation
Pad footing volume = length × width × depth × number of pads. Each pad is calculated individually, then multiplied by count.
Example: 8 deck footings, each 18 inches (45.7 cm) × 18 inches (45.7 cm) × 12 inches (30.5 cm) deep:
- Volume per pad: 1.5 × 1.5 × 1.0 = 2.25 ft³ (0.064 m³)
- Total volume: 2.25 × 8 = 18.0 ft³ (0.510 m³)
- Cubic yards: 18.0 ÷ 27 = 0.67 yd³ (0.510 m³)
- With 10% waste: 0.74 yd³ (0.566 m³) — 34 bags of 80 lb mix
Trench Fill Calculation
Trench fill volume = trench length × trench width × depth of concrete fill. The concrete depth is not always the full trench depth. Many trench fill designs fill only the bottom 18–24 inches (45.7–61 cm) with mass concrete, leaving the upper section as backfill.
Example: 60-foot (18.3 m) trench, 18 inches (45.7 cm) wide, 24 inches (61 cm) of concrete fill:
- Volume: 60 × 1.5 × 2.0 = 180 ft³ (5.10 m³)
- Cubic yards: 180 ÷ 27 = 6.67 yd³ (5.10 m³)
Use ready-mix — no practical way to mix 6+ yards by hand (300 bags of 80 lb mix)
Footing Concrete Volume Reference Chart
Use this table for quick estimates. Multiply cubic feet per linear foot by your total footing length for strip footings.
Footing Width | Footing Depth | ft³ per Linear Foot | yd³ per 100 LF | 80 lb Bags per 100 LF |
|---|---|---|---|---|
12 in (30.5 cm) | 6 in (15.2 cm) | 0.50 ft³ | 1.85 yd³ | 84 bags |
16 in (40.6 cm) | 8 in (20.3 cm) | 0.89 ft³ | 3.30 yd³ | 149 bags |
16 in (40.6 cm) | 12 in (30.5 cm) | 1.33 ft³ | 4.93 yd³ | 222 bags |
20 in (50.8 cm) | 10 in (25.4 cm) | 1.39 ft³ | 5.15 yd³ | 232 bags |
24 in (61 cm) | 12 in (30.5 cm) | 2.00 ft³ | 7.41 yd³ | 334 bags |
24 in (61 cm) | 18 in (45.7 cm) | 3.00 ft³ | 11.11 yd³ | 500 bags |
Any footing requiring over 100 bags needs ready-mix delivery. Mixing 100 bags by hand takes a 3-person crew a full day. A ready-mix truck delivers the same volume in 20–30 minutes at $125–$195 per yard — cheaper and stronger than hand-mixed bags for large pours.
Soil Bearing Capacity: Why It Determines Footing Width
Soil bearing capacity is the maximum load per square foot that soil can support without excessive settlement. Poor soil requires wider footings to spread the same load across more contact area. The IRC 2024 residential code assumes 1,500 psf (71.8 kPa) for prescriptive footing sizes — below that, engineering is required.
Soil Type | Bearing Capacity | Footing Width Adjustment | Notes |
|---|---|---|---|
Solid bedrock | 8,000–15,000+ psf (383–718 kPa) | Can use narrower than standard | Engineer must verify — never assume |
Dense gravel / compacted fill | 3,000–6,000 psf (144–287 kPa) | Standard width applies | Most residential sites |
Dense sand | 2,000–3,000 psf (96–144 kPa) | Standard width applies | Common in coastal areas |
Stiff clay / loam | 1,500–2,000 psf (72–96 kPa) | Code minimum — standard width | Watch for seasonal movement |
Soft clay / organic soil | 500–1,000 psf (24–48 kPa) | Widen footing 50–100% | Engineer assessment strongly recommended |
Fill / disturbed soil | Unknown | Do not use prescriptive sizes | Geotechnical report required |
If your soil is soft clay, organic material, or fill, get a soils report before pouring any footing. A $500–$1,500 soils test prevents a $20,000–$80,000 foundation repair later. Prescriptive footing sizes from the IRC code assume stable, undisturbed soil — they are not safe for poor soil conditions.
Monolithic Pour vs Separate Footing and Wall
Most homeowners pour footings and foundation walls as separate operations. A monolithic pour combines both into one continuous concrete placement.
Method | When to Use | Volume Calculation | Advantage |
|---|---|---|---|
Separate footing + wall | Most residential foundations, any wall over 24 in (61 cm) tall | Calculate footing and wall volumes separately, add together | Easier forming, standard practice |
Monolithic T-footing | Slabs on grade, frost-protected shallow foundations | Single L-shaped cross-section: footing + thickened slab edge combined | Fewer pours, no cold joint between footing and wall |
Monolithic slab (thickened edge) | Garages, small additions, shed slabs | Slab volume + edge thickening volume combined | Fastest pour, fewest cold joints |
A cold joint between footing and foundation wall is the most common concrete foundation weakness. Water infiltrates the cold joint, freezes, and expands — widening the crack over 10–20 winters. A monolithic pour eliminates this joint entirely where soil conditions allow.
Rebar Requirements for Footings
All load-bearing footings require horizontal rebar. The rebar handles tensile stress that concrete cannot absorb on its own. Without rebar, footings can crack through the center under normal settlement loads.
Horizontal Rebar Sizing by Footing Width
Footing Width | Rebar Size | Number of Bars | Spacing |
|---|---|---|---|
12 in (30.5 cm) | #4 (1/2 in / 12.7 mm) | 2 bars | 4 in (10.2 cm) from each edge |
16 in (40.6 cm) | #4 (1/2 in / 12.7 mm) | 2 bars | 4–5 in (10.2–12.7 cm) from each edge |
20 in (50.8 cm) | #4 or #5 | 2–3 bars | 4 in from edges, 1 bar centered |
24 in (61 cm) | #5 (5/8 in / 15.9 mm) | 3 bars | 4 in from edges, 1 bar centered |
Over 30 in (76.2 cm) | Engineer specification | Engineer specification | Geotechnical/structural design required |
Rebar must sit on concrete chairs (dobies) 3 inches (7.6 cm) above the trench bottom. Rebar resting directly on soil corrodes from the bottom up — destroying footing integrity within 15–25 years. Plastic or concrete chairs cost $0.10–0.30 each and are the cheapest insurance in any footing project.
Vertical Rebar Dowels: Connecting Footing to Wall
Vertical rebar dowels connect the footing to the foundation wall or column above. Set dowels while the footing concrete is still wet — within 15 minutes of placement. Use a string line and level to align dowels with the planned wall centerline.
- Standard residential dowel: #4 rebar, 24 inches (61 cm) long
- Embedment in footing: 12 inches (30.5 cm) minimum
- Projection above footing: 12 inches (30.5 cm) minimum into wall
- Spacing: 24–48 inches (61–121.9 cm) on center, or per structural engineer
Waste Factor for Footings: How to Set It Correctly
Set waste factor at 5% for formed footings in stable soil and 10–15% for unformed trenches or irregular excavations. Formed footings have precise dimensions — the form prevents over-pour. Unformed trenches (including trench fill) lose concrete to soil absorption, trench wall sloughing, and irregular bottom profiles.
Pour Type | Recommended Waste Factor | Reason |
|---|---|---|
Formed strip footing | 5% | Form controls volume precisely |
Formed pad footing | 5% | Small volume, easy to control |
Unformed trench fill | 10–15% | Soil absorbs and sloughs into concrete |
Irregular excavation | 12–15% | Bottom is uneven, sides crumble |
Rocky or gravelly soil | 8–12% | Voids in subgrade consume extra concrete |
Any footing over 4 yd³ | 5% min | Large pours: always round up to nearest 0.5 yd³ |
Running short on a footing pour creates a cold joint. A cold joint between 2 separate pours is a structural weakness — not just cosmetic. Order slightly more than your calculation shows. Extra concrete costs $150–$300. A foundation repair costs $8,000–40,000.
Footing vs Foundation Wall vs Slab: What to Calculate Separately
Many residential projects combine multiple concrete elements. Calculate each separately, then sum for total order volume.
Element | Calculator to Use | Typical PSI | Notes |
|---|---|---|---|
Strip footing | Footing calculator (this page) | 3,000–3,500 PSI | Use as base for foundation wall |
Foundation wall | Concrete wall calculator | 3,000–3,500 PSI | Calculate separately from footing |
Basement floor slab | Concrete floor calculator | 3,500–4,000 PSI | Thicker than typical patio slab |
Column / pier footing | Concrete column calculator | 3,500–4,000 PSI | Pad footing under column |
Full project cost | Concrete cost calculator | N/A | Add all elements for total budget |
Calculate your foundation wall volume separately using our concrete wall calculator. Then add footing + wall + slab volumes for your full project ready-mix order. Ordering everything in one truck eliminates short-load fees and saves $200–$600 on most residential foundation projects.
Permits and Inspections: What Inspectors Check
Concrete footing inspections happen before pouring — not after. Inspectors verify width, depth, rebar placement, and frost line compliance with the concrete forms and rebar exposed. Once the concrete is poured, the inspection cannot happen.
What a footing inspector checks on-site:
- Footing width and depth match the approved plans
- Footing bottom sits on undisturbed soil — no loose fill or disturbed material
- Rebar size, spacing, and cover (clearance from forms and soil) match specs
- Vertical dowels are placed and aligned
- Footing bottom is below local frost line depth
- No water pooling in the trench — water in the trench weakens the concrete mix
Schedule the footing inspection before you call for concrete delivery. Most inspections require 24–48 hours notice. The concrete truck cannot wait for an inspector — it costs $2–5 per minute after the standard 7-minute grace period.
5 Footing Calculation Mistakes That Cause Problems
- Using home square footage instead of footing length: Footings are linear elements. Measure the total run length of each footing section, not the building footprint area.
- Ignoring frost line depth: A footing 6 inches above the frost line fails within 3–5 winters. Always confirm local frost depth with the building department before setting footing depth.
- Confusing trench depth with concrete depth for trench fill: Many trenches are dug 48 inches (121.9 cm) deep but only filled 24 inches (61 cm) with concrete. Enter the concrete depth — not the total trench depth.
- Not adding overbreak waste for unformed trenches: Soil falls into unformed trenches during digging. Use 10–15% waste for all trench fill and unformed footing pours.
- Ordering bags instead of ready-mix for large footings: A typical house foundation needs 4–10 cubic yards. At 45 bags per yard, that is 180–450 bags — physically impossible for a DIY crew in one day. Use ready-mix for any footing over 1 cubic yard (0.76 m³).
Frequently Asked Questions
How do I calculate concrete for a footing?
Multiply length × width × depth (all in feet), then divide by 27 for cubic yards. For a 40-foot strip footing, 16 inches (40.6 cm) wide, 8 inches (20.3 cm) deep: 40 × 1.33 × 0.67 ÷ 27 = 1.32 yd³ (1.01 m³). Add 10% waste: order 1.45 yd³.
What is the minimum footing size for a house?
For an 8-inch (20.3 cm) concrete foundation wall, the minimum footing is 16 inches (40.6 cm) wide and 8 inches (20.3 cm) deep under IRC 2024 for standard residential soil conditions. Larger loads, poor soil, or multi-story buildings require wider and deeper footings per engineering specifications.
How deep should a footing be below the frost line?
The footing bottom must sit at or below the frost line depth — not 6 inches below it. The frost line is the minimum depth. Going 6 inches deeper than the frost line is common practice for safety margin, but the footing must reach at least to the frost line depth for your specific location.
Do I need rebar in a footing?
Yes — all load-bearing footings require horizontal rebar. Footings without rebar crack through the center under normal soil movement and structural loads. The IRC 2024 code requires a minimum of 2 bars of #4 rebar in residential footings up to 16 inches (40.6 cm) wide.
What is the difference between a footing and a foundation?
A footing is the horizontal concrete base that distributes structural loads to the soil. A foundation is the complete below-grade system that includes the footing plus the foundation wall, piers, or slab that sits on top of the footing. The footing is always the lowest element.
Can I pour a footing in cold weather?
Yes, but only with cold weather concrete practices. Concrete must not freeze within the first 48 hours of pouring. Temperatures below 40°F (4.4°C) require insulating blankets, heated enclosures, or hot water in the mix. Never pour a footing directly on frozen ground — the frozen soil thaws and settles after the pour, creating uneven support.
How much does a concrete footing cost?
A typical residential strip footing costs $15–25 per linear foot installed in 2026, including concrete, rebar, forming, labor, and backfill. A 120-foot (36.6 m) house footing runs $1,800–$3,000 total. Material alone costs $4–8 per linear foot. Use our concrete cost calculator for a full material budget based on your specific footing dimensions.
What PSI concrete is best for footings?
Use 3,000 PSI (20.7 MPa) for standard residential footings and 3,500–4,000 PSI (24.1–27.6 MPa) for footings supporting heavy loads or in freeze-thaw climates. In northern states where footings freeze and thaw repeatedly, 3,500 PSI with air-entrainment significantly extends footing lifespan.