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Footing Concrete Calculator | Foundation & Post Footings

Quickly determine the concrete volume needed for rectangular footings in building foundations, fence posts, and retaining walls.

Enter the length of the footing

ft

Enter the width of the footing

ft

Enter the depth of the footing

ft

Add extra percentage for spillage, uneven ground, etc.

%
, ,

Enable to estimate the number of pre-mix concrete bags needed

Optional: Enter the cost per unit volume for a total cost estimate

$
Calculate Concrete Volume
Enter the dimensions for your concrete project to calculate the volume needed.

Documentation Contents

Understanding Concrete Calculations

Concrete is one of the most versatile and widely used construction materials, with applications ranging from simple garden paths to massive structural foundations.

This calculator estimates the volume of concrete needed for common construction shapes including slabs (rectangular and circular), footings, columns, and stairs. Accurate concrete estimation is crucial for project planning, budgeting, and ensuring you have enough material on site for your pour.

Using this calculator will help you:

  • Determine the precise amount of concrete needed
  • Account for waste and spillage with a customizable percentage
  • Calculate pre-mix bag quantities for smaller projects
  • Estimate costs based on volume or bag pricing
  • Calculate reinforcement requirements for your project

Whether you're a DIY homeowner planning a small patio or a contractor estimating material needs for a commercial project, accurate concrete volume calculation is the foundation of successful project planning.

How to Use This Calculator

Basic Steps

  1. Select the shape of your concrete project from the dropdown (Slab, Circular Slab, Rectangular Footing, Square Column, Circular Column, or Stairs).
  2. Choose your unit system (Imperial or Metric) based on your measurement preference.
  3. Enter the dimensions for your selected shape in the appropriate fields.
  4. Adjust waste percentage if needed (default is 10%).
  5. Optionally enable reinforcement calculation to estimate rebar, wire mesh, or fiber reinforcement needs for your project.
  6. Optionally enable bag calculation for smaller projects to estimate the number of pre-mix concrete bags needed.
  7. Click Calculate to get your results.

Input Field Descriptions

Rectangular Slab

  • Length and Width: The horizontal dimensions of your slab in feet (imperial) or meters (metric).
  • Thickness: The vertical depth of your slab in inches (imperial) or centimeters (metric).

Circular Slab

  • Diameter: The width of your circular slab in feet (imperial) or meters (metric).
  • Thickness: The vertical depth of your slab in inches (imperial) or centimeters (metric).

Thickened Edge Slab

  • Slab Length/Width: The horizontal dimensions of your main slab in feet (imperial) or meters (metric).
  • Slab Thickness: The vertical depth of your main slab in inches (imperial) or centimeters (metric).
  • Edge Width: The width of the thickened edge in inches (imperial) or centimeters (metric).
  • Edge Depth: The total depth of the edge in inches (imperial) or centimeters (metric). Must be greater than the slab thickness.
  • Edge Position: Select where the thickened edges will be located (all sides, length sides only, etc.).

Rectangular Footing

  • Length, Width, Depth: The dimensions of your footing in feet (imperial) or meters (metric).

Columns

  • Side/Diameter: For square columns, the side length; for circular columns, the diameter.
  • Height: The vertical dimension of your column.

Stairs

  • Number of Steps: Total number of stair steps in your project.
  • Riser Height: The vertical height of each step in inches (imperial) or centimeters (metric).
  • Tread Depth: The horizontal depth of each step in inches (imperial) or centimeters (metric).
  • Width: The horizontal width of the entire staircase in feet (imperial) or meters (metric).

Reinforcement Options

  • Include Reinforcement: Enable to calculate reinforcement materials.
  • Reinforcement Type: Choose between rebar grid, wire mesh, or fiber reinforcement.
  • Rebar Spacing: For rebar grid, specify the spacing between bars (in inches or centimeters).
  • Wire Gauge: For wire mesh, select the gauge/type of wire mesh.
  • Fiber Dosage: For fiber reinforcement, specify the dosage rate (lbs per cubic yard or kg per cubic meter).

Interpreting Your Results

The calculator provides several key pieces of information to help with your project planning:

  • Base Volume: The exact concrete volume without accounting for waste. Provided in both cubic yards and cubic meters.
  • Volume with Waste: The recommended concrete volume including your specified waste percentage. This is the value you should use when ordering concrete to ensure you have enough.
  • Number of Pre-mix Bags: If you enable bag calculation, this shows how many bags of the selected size you'll need to complete your project.
  • Estimated Cost: Based on either the cost per cubic yard/meter or cost per bag, depending on your inputs.
  • Reinforcement Estimates: If reinforcement is enabled, you'll see specific material requirements based on your selected type:
    • Rebar Grid: Total linear footage of rebar needed for your grid pattern.
    • Wire Mesh: Square footage of mesh and number of standard sheets required.
    • Fiber Reinforcement: Amount of fiber additive needed (in pounds or kilograms).

For ready-mix concrete, most suppliers sell by the cubic yard (imperial) or cubic meter (metric). For small projects, pre-mix concrete bags are a convenient alternative.

Calculation Formulas

This calculator uses standard geometric formulas to calculate the volume of concrete needed:

Rectangular Slab

Volume=Length×Width×ThicknessVolume = Length \times Width \times Thickness

Circular Slab

Volume=π×(Diameter2)2×ThicknessVolume = \pi \times \left(\frac{Diameter}{2}\right)^2 \times Thickness

Thickened Edge Slab

Volumemain=Length×Width×ThicknessVolume_{main} = Length \times Width \times Thickness
Volumeedge=EdgeLength×EdgeWidth×(EdgeDepth−Thickness)Volume_{edge} = EdgeLength \times EdgeWidth \times (EdgeDepth - Thickness)
Total Volume=Volumemain+VolumeedgeTotal\,Volume = Volume_{main} + Volume_{edge}

Rectangular Footing

Volume=Length×Width×DepthVolume = Length \times Width \times Depth

Square Column

Volume=Side2×HeightVolume = Side^2 \times Height

Circular Column

Volume=π×(Diameter2)2×HeightVolume = \pi \times \left(\frac{Diameter}{2}\right)^2 \times Height

Stairs

Volume=0.5×Steps×Riser×Tread×WidthVolume = 0.5 \times Steps \times Riser \times Tread \times Width

The waste calculation simply adds your specified percentage to the base volume:

Volumewith waste=Volume×(1+Waste%100)Volume_{with\,waste} = Volume \times (1 + \frac{Waste\%}{100})

For bag calculations, the calculator divides the volume (with waste) by the yield of each bag size:

Bags=⌈Volumewith wasteBag Yield⌉Bags = \lceil\frac{Volume_{with\,waste}}{Bag\,Yield}\rceil

Note: The ceiling function (⌈ ⌉) rounds up to the nearest whole bag.

Reinforcement Calculations

For reinforcement materials, the calculator uses area and spacing-based calculations:

Rebar Grid

For rectangular areas:

Rowsin one direction=⌈LengthSpacing⌉Rows_{in\,one\,direction} = \lceil\frac{Length}{Spacing}\rceil
Rowsin other direction=⌈WidthSpacing⌉Rows_{in\,other\,direction} = \lceil\frac{Width}{Spacing}\rceil
Total Length=(Rowsin one direction×Width)+(Rowsin other direction×Length)Total\,Length = (Rows_{in\,one\,direction} \times Width) + (Rows_{in\,other\,direction} \times Length)

For circular areas (simplified approximation):

Rows=⌈DiameterSpacing⌉Rows = \lceil\frac{Diameter}{Spacing}\rceil
Total Length=Rows×Diameter×1.2Total\,Length = Rows \times Diameter \times 1.2

Note: The 1.2 factor accounts for the additional length needed in a circular pattern.

Wire Mesh

Sheets=⌈AreaSheet Size⌉Sheets = \lceil\frac{Area}{Sheet\,Size}\rceil

Note: Standard sheet size is typically 5' × 10' (50 sq ft).

Fiber Reinforcement

Fiber Amount=Volume×Dosage RateFiber\,Amount = Volume \times Dosage\,Rate

Real-World Applications & Examples

Common Uses

  • Rectangular Slab: Patios, driveways, shed foundations, sidewalks, concrete floors
  • Circular Slab: Fire pit surrounds, circular patios, round garden features, hot tub bases, column bases
  • Thickened Edge Slab: Garage floors, shed foundations, load-bearing structures, monolithic slab foundations
  • Rectangular Footing: Building foundations, retaining walls, fence posts
  • Square Column: Support columns, pillars, posts
  • Circular Column: Deck supports, pier foundations, sonotubes
  • Stairs: Outdoor steps, basement stairs, garden terracing

Example Scenarios

Garden Patio Slab

For a 10' × 10' patio with 4" thickness, you would need approximately 1.23 cubic yards of concrete (with 10% waste).

Circular Fire Pit Base

For a 6' diameter circular slab with 4" thickness, you would need approximately 0.35 cubic yards of concrete (with 10% waste).

Fence Post Footings

For 10 footings at 1' × 1' × 2' each, you would need approximately 0.74 cubic yards of concrete.

Concrete Steps

For a staircase with 5 steps (7" rise, 11" tread, 3' wide), you would need approximately 0.35 cubic yards of concrete.

Important Considerations & Disclaimers

This calculator provides estimates for material volume only. It is not a substitute for professional engineering advice.

Limitations

  • This calculator assumes regular, simple shapes. Complex or irregular shapes may require breaking down into simpler components or consulting a professional.
  • Site conditions (uneven subgrade, slopes, etc.) can significantly affect actual concrete needed.
  • The stair calculation is a simplification and may not account for all variables in complex staircase designs.
  • Reinforcement calculations are approximations intended for planning purposes, not structural design.

Safety & Compliance

  • Always consult local building codes for requirements specific to your project.
  • For structural applications, consult a qualified engineer for proper concrete mix design and reinforcement specifications.
  • Wear appropriate Personal Protective Equipment (PPE) when working with concrete, including gloves, eye protection, and respiratory protection.
  • Concrete is caustic and can cause chemical burns. Avoid skin contact and wash thoroughly if contact occurs.

Concrete Basics

Components of Concrete

Concrete is a composite material composed of:

  • Cement: The binding agent that hardens when mixed with water
  • Fine Aggregates: Sand that fills spaces between coarse aggregates
  • Coarse Aggregates: Crushed stone, gravel, or recycled concrete
  • Water: Initiates the chemical reaction with cement (hydration)
  • Admixtures: Optional chemicals to adjust concrete properties (workability, setting time, strength)

These ingredients are mixed in specific proportions to achieve the desired strength and workability.

Types of Concrete

By Mixing Method

  • Ready-Mix: Professionally mixed at a plant and delivered by truck
  • Pre-Mix Bags: Dry ingredients pre-mixed in bags, just add water
  • Site-Mixed: Ingredients mixed on-site using a mixer or by hand

By Strength

  • 2,500-3,000 PSI: General purpose (sidewalks, patios)
  • 3,500-4,000 PSI: Structural (foundations, driveways)
  • 4,500+ PSI: High-strength (commercial, heavy load applications)

Specialty Concrete

  • Fiber-Reinforced: Contains fibers for crack resistance
  • High-Early-Strength: Achieves strength more quickly
  • Self-Consolidating: Highly flowable, requires minimal vibration

Subgrade Preparation

Proper subgrade preparation is critical to the long-term performance of any concrete project. A well-prepared subgrade provides uniform support, helps prevent settlement, and improves drainage.

Basic Steps

  1. Remove Topsoil: Strip away all organic material, including grass, roots, and topsoil.
  2. Establish Grade: Determine proper elevation and slope for drainage (typically 1/4" per foot away from structures).
  3. Add Base Material: Add 4-6" of granular base material (crushed stone, gravel, or road base).
  4. Compact: Use a plate compactor or roller to achieve 95% compaction.
  5. Check Level: Verify proper grade and elevation before pouring concrete.

Common Subgrade Problems

  • Soft Spots: Areas that feel spongy under foot or equipment weight.
  • Poor Drainage: Water pooling that could cause future settlement.
  • Tree Roots: Can lift and crack concrete over time.
  • Expansive Soils: Clay soils that expand when wet and contract when dry.
  • Underground Utilities: Always call for utility location before excavating.

Pro Tip: For improved drainage in frost-prone areas, consider installing a layer of geotextile fabric beneath the granular base.

Reinforcement Guide

Reinforcement strengthens concrete by compensating for its low tensile strength. Properly reinforced concrete can resist cracking, handle greater loads, and have improved longevity.

Rebar Grid

Steel reinforcing bars (rebar) arranged in a grid pattern.

Applications

  • Structural slabs and footings
  • Driveways and heavy-load areas
  • Retaining walls

Installation Tips

  • Position rebar at the middle or upper third of the slab thickness
  • Use rebar chairs to maintain proper height
  • Overlap bars by at least 40 times the bar diameter
  • Typical spacing: 12-18" for residential work
  • Common sizes: #3 (3/8") or #4 (1/2") for residential projects

Wire Mesh

Welded wire fabric/mesh consisting of steel wires in a grid pattern.

Applications

  • Thin slabs (patios, sidewalks)
  • Non-structural concrete
  • Crack control in slabs

Installation Tips

  • Raise mesh to middle of the slab using chairs or lifting during pour
  • Overlap sheets by at least one full square
  • Cut with wire cutters around obstacles
  • Common types: 6×6-W1.4×W1.4 (lighter duty) or 6×6-W2.9×W2.9 (heavier duty)

Fiber Reinforcement

Synthetic or steel fibers mixed directly into concrete.

Applications

  • Crack resistance in slabs
  • Areas with complex shapes
  • Thin overlays
  • Supplementary reinforcement

Installation Tips

  • Add fibers according to manufacturer recommendations
  • Typical dosage: 1-3 lbs per cubic yard (0.6-1.8 kg per cubic meter)
  • Ensure thorough mixing for proper distribution
  • May leave some fibers visible at surface

Important: While this calculator provides reinforcement material estimates, structural design of reinforced concrete should be performed by a qualified engineer.

Weather Considerations

Weather conditions significantly affect concrete placement, curing, and finished quality. Planning your pour according to weather forecasts can help avoid problems.

Hot Weather (Above 85°F/29°C)

Challenges

  • Accelerated setting time
  • Increased water demand
  • Plastic shrinkage cracking
  • Difficulty finishing

Recommendations

  • Pour early in the morning
  • Use cooled mixing water or ice
  • Dampen subgrade before pouring
  • Consider retarding admixtures
  • Provide sun/wind protection during placement
  • Cure immediately and thoroughly
  • Use evaporation retardant during finishing

Cold Weather (Below 50°F/10°C)

Challenges

  • Delayed setting and curing
  • Freezing damage if below 32°F/0°C
  • Reduced early strength
  • Cold joints between batches

Recommendations

  • Don't pour on frozen ground
  • Use heated mixing water
  • Consider accelerating admixtures
  • Insulate formwork and new concrete
  • Use concrete blankets or tarps for protection
  • Maintain temperature above 50°F/10°C for at least 3 days
  • Don't allow concrete to freeze for at least 7 days

Rainy Weather

Challenges

  • Excess water in the mix
  • Surface erosion
  • Weakened surface strength
  • Uneven color and texture

Recommendations

  • Check weather forecasts before scheduling
  • Have plastic sheeting ready for protection
  • Delay finishing until bleeding water disappears
  • Avoid adding water to rainwater-affected concrete
  • If rain begins before setting, protect with plastic and avoid creating depressions

Sustainability Considerations

Concrete production accounts for approximately 8% of global CO₂ emissions. Making environmentally conscious choices can significantly reduce the environmental impact of your concrete projects.

Sustainable Concrete Options

  • Supplementary Cementitious Materials (SCMs): Fly ash, slag cement, and silica fume can replace a portion of Portland cement, reducing CO₂ emissions.
  • Recycled Aggregates: Using crushed recycled concrete as aggregate reduces landfill waste and natural resource extraction.
  • Carbon-Cured Concrete: Some manufacturers now offer concrete that absorbs CO₂ during curing.
  • Alternative Cements: Geopolymer and calcium sulfoaluminate cements have a lower carbon footprint than traditional Portland cement.
  • Local Materials: Sourcing materials locally reduces transportation emissions.

Best Practices

  • Precise Estimation: Calculating volume accurately minimizes waste.
  • Proper Planning: Well-planned projects require fewer repairs and replacements.
  • Pervious Concrete: Consider permeable options for improved stormwater management.
  • Recycling Excess: Arrange for unused concrete to be recycled rather than landfilled.
  • Washout Management: Use designated concrete washout stations to prevent water contamination.
  • Long-Term Durability: Properly designed, reinforced, and cured concrete lasts longer, reducing lifetime environmental impact.

When requesting concrete from suppliers, ask about their sustainability options, such as mixes with reduced cement content or recycled materials. Many suppliers now offer "green concrete" alternatives with Environmental Product Declarations (EPDs) documenting their reduced environmental impact.

Frequently Asked Questions

What units can I use in this calculator?

You can choose between Imperial (feet/inches) and Metric (meters/centimeters) units. For Imperial, slab thickness is in inches while other dimensions are in feet. For Metric, slab thickness and stair dimensions (riser/tread) are in centimeters, while other dimensions are in meters.

What waste percentage should I use?

For most projects, a 10% waste factor is standard. For complex projects with many corners or slopes, consider 15%. For simple projects with experienced workers, 5% might be sufficient. This accounts for spillage, uneven subgrades, and slight variations in form dimensions.

Should I use bags or ready-mix concrete?

For projects under 1/2 cubic yard (about 20 bags), pre-mixed bags are often practical. For larger projects, ready-mix concrete is more cost-effective and ensures consistent quality. Ready-mix typically has minimum order quantities (often 1 cubic yard) and may charge extra for small loads.

How do I calculate concrete for an L-shaped or irregular slab?

Break the shape into regular rectangles or circles, calculate each volume separately, then add them together. For an L-shape, divide it into two rectangles, calculate each, and sum the results.

What reinforcement should I use for my project?

For most residential slabs 4" thick or more, either wire mesh (6×6-W1.4×W1.4) or #3 (3/8") rebar in a 16-18" grid is adequate. For driveways or heavier loads, #4 (1/2") rebar in a 12-16" grid is recommended. Fiber reinforcement is suitable for crack resistance in thinner non-structural applications. Consult local building codes or a structural engineer for specific requirements.

How long should concrete cure before use?

Concrete continues to cure and gain strength for weeks. As a general rule:

  • 24-48 hours: Light foot traffic
  • 7 days: Regular foot traffic and light furniture
  • 28 days: Full strength for vehicle traffic or heavy loads

Proper curing (keeping concrete moist) for the first 7 days is essential for optimal strength.

Does this calculator account for forms and stakes?

No, this calculator focuses on concrete volume. You'll need to purchase forming materials (lumber, stakes, screws) separately based on your project dimensions.

How deep should my concrete be?

Standard thickness recommendations:

  • Sidewalks: 3-4 inches
  • Patios: 4 inches
  • Driveways: 4-6 inches (6 inches recommended for climates with freezing temperatures)
  • Footings: Depends on soil conditions and load, but typically 8-12 inches for residential

Local building codes may specify minimum requirements based on your climate and application.

Related shape calculators

Slab
ThickenedEdgeSlab
CircularSlab
RectangularFooting
Columns
Stairs

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