Concrete Calculator
Calculate the amount of concrete needed for slabs, footings, walls, and columns. Get cubic yards, bags needed, and cost estimates.
Project Dimensions
Project Type:
Units:
Common Depths:
Concrete Needed
1.23 cubic yards
(1.36 yd³ with 10% waste factor)
Pre-Mixed Bags Needed:
56
80 lb bags
75
60 lb bags
Approximate Weight:
5,000 lbs (2,268 kg)
Concrete Coverage Guide
4" Thick Slab
1 cubic yard covers ~81 sq ft
Use for: Sidewalks, patios
6" Thick Slab
1 cubic yard covers ~54 sq ft
Use for: Driveways, garage floors
What Is Concrete Volume Calculation?
Concrete volume calculation determines the amount of concrete needed for slabs, footings, walls, columns, and other structures. Accurate estimation prevents costly over-ordering or project delays from running short. Concrete is typically ordered by the cubic yard in the US and cubic meter elsewhere.
| Project Type | Typical Thickness | Common Use | Concrete Type |
|---|---|---|---|
| Sidewalk | 4 inches | Pedestrian traffic | 3000 PSI |
| Driveway | 4-6 inches | Vehicle traffic | 4000 PSI |
| Patio slab | 4 inches | Light use | 3000 PSI |
| Garage floor | 4-6 inches | Vehicle/storage | 4000 PSI |
| Foundation wall | 8-10 inches | Structural support | 4000-5000 PSI |
| Footings | 8-12 inches | Load distribution | 3500-4000 PSI |
Basic Concrete Volume Formula
Where:
- Length= Length of pour area (ft or m)
- Width= Width of pour area (ft or m)
- Depth= Thickness of concrete (ft or m)
Concrete Volume Formulas for Different Shapes
Different project shapes require different formulas to calculate volume accurately.
| Shape | Formula | Example Application |
|---|---|---|
| Rectangular slab | V = L × W × D | Patios, driveways, floors |
| Cylinder/Column | V = π × r² × h | Round columns, sono tubes |
| Trapezoid | V = ((a + b) / 2) × h × D | Tapered walls, steps |
| Triangle | V = (0.5 × b × h) × D | Ramps, odd corners |
| Steps | V = n × (riser × tread × width) | Exterior stairs |
Column and Cylinder Formula
Where:
- π= Pi (3.14159)
- r= Radius of column
- h= Height of column
Unit Conversions for Concrete
Converting between cubic feet, cubic yards, and cubic meters is essential for ordering concrete.
| From | To | Multiply By | Example |
|---|---|---|---|
| Cubic feet | Cubic yards | ÷ 27 | 54 ft³ = 2 yd³ |
| Cubic yards | Cubic feet | × 27 | 2 yd³ = 54 ft³ |
| Cubic feet | Cubic meters | × 0.0283 | 100 ft³ = 2.83 m³ |
| Cubic meters | Cubic feet | × 35.31 | 1 m³ = 35.31 ft³ |
| Cubic meters | Cubic yards | × 1.308 | 1 m³ = 1.31 yd³ |
| Inches to feet (depth) | Feet | ÷ 12 | 4" = 0.333 ft |
Quick reference: 1 cubic yard = 27 cubic feet = 0.765 cubic meters = 46,656 cubic inches
Waste Factors and Over-ordering
Always order more concrete than the calculated minimum to account for waste, spillage, and form irregularities.
| Project Type | Recommended Waste Factor | Reason |
|---|---|---|
| Simple slabs | 5-7% | Minor spillage, grade variations |
| Complex shapes | 10-12% | More cuts, irregular forms |
| Steps and landings | 10-15% | Multiple small pours |
| Footings in soil | 10-15% | Soil absorption, irregular holes |
| Pump delivery | Add 1/4 yard | Line priming |
| Remote site | 10%+ | Cannot easily order more |
Note: Running short is more expensive than over-ordering. Most suppliers have minimum order quantities (often 1 yard), and short loads may incur fees.
Concrete Mix Types and Strength
Concrete strength is measured in PSI (pounds per square inch) and should match project requirements.
| Strength (PSI) | Mix Ratio | Typical Applications | 28-Day Cure |
|---|---|---|---|
| 2500 PSI | 1:3:5 (cement:sand:gravel) | Non-structural, fence posts | Standard cure |
| 3000 PSI | 1:2.5:4 | Sidewalks, patios, light traffic | Standard cure |
| 4000 PSI | 1:2:3 | Driveways, garage floors | Standard cure |
| 5000 PSI | 1:1.5:2.5 | Commercial, heavy traffic | May need additives |
| 6000+ PSI | Special mix | Industrial, high-rise | Special cure methods |
Concrete weight: Standard concrete weighs approximately 145-150 lbs/ft³ (2,320-2,400 kg/m³). Lightweight concrete: 90-120 lbs/ft³.
Ready-Mix vs Bagged Concrete
Choose between ready-mix delivery and bagged concrete based on project size and logistics.
| Factor | Ready-Mix Truck | Bagged Concrete |
|---|---|---|
| Best for | 1+ cubic yards | Under 1/2 cubic yard |
| Coverage per unit | 1 yd³ = 27 ft³ | 60 lb bag ≈ 0.45 ft³ |
| Cost per cubic yard | $125-200 | $300-400 (in bags) |
| Labor | Less mixing | Manual mixing required |
| Minimum order | Often 1 yard + fees | No minimum |
| Working time | 90 mins to place | Flexible timing |
| Quality | Consistent mix | Varies with mixing |
Rule of thumb: For projects over 1/2 cubic yard, ready-mix is usually more economical and provides better quality.
Calculating Bags of Concrete Needed
For small projects using bagged concrete, calculate the number of bags required.
| Bag Size | Coverage (approx) | Bags per Cubic Yard | Weight |
|---|---|---|---|
| 40 lb bag | 0.30 ft³ | 90 bags | 40 lbs (18 kg) |
| 50 lb bag | 0.375 ft³ | 72 bags | 50 lbs (23 kg) |
| 60 lb bag | 0.45 ft³ | 60 bags | 60 lbs (27 kg) |
| 80 lb bag | 0.60 ft³ | 45 bags | 80 lbs (36 kg) |
Bags Needed Formula
Where:
- Bags= Number of bags to purchase
- Volume= Total cubic feet needed
- Coverage= Cubic feet per bag
Worked Examples
Calculate Concrete for a Driveway
Problem:
A driveway is 20 feet long, 10 feet wide, and needs to be 4 inches thick. How many cubic yards of concrete are needed?
Solution Steps:
- 1Convert thickness to feet: 4 inches ÷ 12 = 0.333 feet
- 2Calculate volume: 20 × 10 × 0.333 = 66.6 cubic feet
- 3Convert to cubic yards: 66.6 ÷ 27 = 2.47 cubic yards
- 4Add 10% waste: 2.47 × 1.10 = 2.72 cubic yards
- 5Round up: Order 3 cubic yards
Result:
Order 3 cubic yards of 4000 PSI concrete for this driveway. This provides adequate material with waste allowance.
Calculate Concrete for Round Columns
Problem:
You need to pour 4 round columns (sono tubes) that are 12 inches in diameter and 4 feet tall. How much concrete is needed?
Solution Steps:
- 1Convert diameter to feet: 12 inches = 1 foot, radius = 0.5 feet
- 2Calculate volume per column: π × 0.5² × 4 = 3.14 cubic feet
- 3Total for 4 columns: 3.14 × 4 = 12.56 cubic feet
- 4Convert to cubic yards: 12.56 ÷ 27 = 0.47 cubic yards
- 5Add 10% waste: 0.47 × 1.10 = 0.52 cubic yards
Result:
Order 0.6-0.75 cubic yards, or use approximately 32 bags of 80 lb concrete mix (60 bags of 60 lb mix).
Calculate Bags for a Small Pad
Problem:
A 4×4 foot pad, 4 inches thick, using 60 lb bags. How many bags are needed?
Solution Steps:
- 1Calculate volume: 4 × 4 × 0.333 = 5.33 cubic feet
- 260 lb bag covers 0.45 cubic feet
- 3Bags needed: 5.33 ÷ 0.45 = 11.8 bags
- 4Add 10% waste: 11.8 × 1.10 = 13 bags
- 5Round up to nearest bag
Result:
Purchase 13-14 bags of 60 lb concrete mix. This allows for slight overfill and spillage.
Tips & Best Practices
- ✓Convert all measurements to the same unit (feet) before calculating, then convert to cubic yards.
- ✓1 cubic yard = 27 cubic feet—memorize this conversion for quick estimates.
- ✓Always add 5-10% extra for waste, spillage, and grade variations.
- ✓Ready-mix concrete typically needs to be placed within 90 minutes of batching.
- ✓For small jobs under 1/2 cubic yard, bagged concrete is often more practical.
- ✓Check your forms and grade before the truck arrives—you can't send concrete back.
- ✓Consider pump delivery for hard-to-reach areas; add 1/4 yard for line priming.
Frequently Asked Questions
Sources & References
Last updated: 2026-01-22