Last updated: August 21, 2026
Concrete Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The Concrete Calculator computes the volume of concrete needed for rectangular slabs and round (circular) pours using standard geometric formulas (V = L×W×T for rectangular; V = π×(d/2)²×T for round). It converts results to cubic yards, cubic feet, and cubic meters, calculates total concrete weight, number of premix bags needed with a configurable waste factor, and mix component volumes (cement, sand, aggregate) from the specified mix ratio. Inputs include slab shape, dimensions, dimension unit, concrete density, bag weight, waste percentage, and mix ratio.
To calculate concrete for a rectangular slab, multiply length × width × thickness in feet, then divide by 27 to get cubic yards. For a round slab, use π × (diameter/2)² × thickness, then divide by 27. Add 5–10% for waste, and divide total weight by bag weight to find the number of bags needed.
Key Takeaways
- Rectangular slab volume: V = L × W × T; divide by 27 to convert ft³ to yd³.
- Round slab volume: V = π × (d/2)² × T — select the 'Round Slab' option and enter the diameter.
- Always add 5–10% waste to your estimate so you never run short mid-pour.
- A 60 lb premix bag yields roughly 0.45 ft³; expect approximately 60 bags per cubic yard.
- Driveways require 5–6 inch thickness (0.42–0.5 ft); pedestrian patios need at least 4 inches (0.33 ft).
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
Rectangular: V = L × W × T | Round: V = π × (d/2)² × T
Where:
- V=Concrete Volume(ft³)
- L=Length(ft)
- W=Width(ft)
- T=Thickness(ft)
- d=Diameter(ft)
Worked Examples
Backyard Patio Slab (10 ft × 12 ft × 4 in)
A homeowner pours a rectangular backyard patio 10 feet wide and 12 feet long with a standard 4-inch thickness, using 60 lb premix bags with 5% waste.
- 1Convert inches to feet: length = 120 / 12 = 10 ft, width = 144 / 12 = 12 ft, thickness = 4 / 12 = 0.333 ft
- 2Volume = L × W × T = 10 × 12 × 0.333 = 40.0 ft³
- 3Convert to cubic yards: 40.0 / 27 = 1.481 yd³
- 4Total weight: 40.0 × 150 = 6,000 lb
- 5Weight with 5% waste: 6,000 × 1.05 = 6,300 lb
- 6Bags needed: ⌈6,300 / 60⌉ = ⌈105.0⌉ = 105 bags
Concrete Driveway (20 ft × 12 ft × 6 in)
A contractor pours a residential driveway 20 feet long and 12 feet wide with a 6-inch structural thickness, using 80 lb bags and 10% waste factor.
- 1Dimensions already in feet: L = 20 ft, W = 12 ft, T = 0.5 ft (6 inches)
- 2Volume = 20 × 12 × 0.5 = 120.0 ft³
- 3Convert to cubic yards: 120.0 / 27 = 4.444 yd³
- 4Total weight: 120.0 × 150 = 18,000 lb
- 5Weight with 10% waste: 18,000 × 1.10 = 19,800 lb
- 6Bags needed: ⌈19,800 / 80⌉ = ⌈247.5⌉ = 248 bags
Round Garden Slab (8 ft diameter × 4 in thick)
A landscaper pours a circular concrete patio 8 feet in diameter and 4 inches thick as a round garden feature, using 60 lb bags with 5% waste.
- 1Radius = 8 / 2 = 4 ft
- 2Area = π × 4² = π × 16 = 50.265 ft²
- 3Volume = 50.265 × 0.333 = 16.738 ft³
- 4Convert to cubic yards: 16.738 / 27 = 0.620 yd³
- 5Total weight: 16.738 × 150 = 2,510.7 lb
- 6Weight with 5% waste: 2,510.7 × 1.05 = 2,636.2 lb
- 7Bags needed: ⌈2,636.2 / 60⌉ = ⌈43.94⌉ = 44 bags
Introduction
The Concrete Calculator determines the exact volume of concrete needed for rectangular slabs, driveways, footings, and round patios. Whether you are estimating premix bags for a backyard project or ordering ready-mix concrete for a full driveway, accurate volume calculation prevents costly shortages and minimises material waste. This calculator applies the standard geometric formulas — L×W×T for rectangular pours and π×(d/2)²×T for round slabs — converts results to cubic yards, cubic feet, and cubic meters, and calculates the number of premix bags and mix component breakdown by cement:sand:aggregate ratio.
How to Use the Concrete Calculator
Select the slab shape (rectangular or round), enter your dimensions in your preferred unit system, and specify the thickness. For rectangular slabs enter length and width; for round slabs enter the diameter. Choose the dimension unit (feet, inches, meters, centimetres, or yards) — the calculator converts all values internally. Set the concrete density (150 lb/ft³ for normal-weight concrete), bag weight (40, 60, or 80 lb), and a waste percentage of 5–10%. The calculator returns volume in cubic yards, cubic feet, and cubic meters, along with total weight, number of premix bags, and optional mix component volumes. For column and post work, also see our Concrete Column Calculator.
Concrete Volume Formulas Explained
Concrete volume is calculated directly from geometry. For a rectangular slab or footing: V = L × W × T, where L is length, W is width, and T is thickness — all in the same unit. For a round (circular) slab: V = π × (d/2)² × T, where d is the diameter. Results in cubic feet are divided by 27 to obtain cubic yards (the standard ordering unit in the United States). To convert to cubic meters, divide cubic feet by 35.3147. These formulas follow the concrete volume estimation approach in ACI 301-16 Specifications for Structural Concrete. For slab-on-grade structural design, consult ACI 360R-10 Guide to Design of Slabs-on-Ground, available at https://www.concrete.org.
Recommended Slab Thickness by Application
Choosing the correct thickness is critical for structural integrity and longevity. Pedestrian patios and walkways: 4 inches (0.33 ft) — sufficient for foot traffic; use a 1:2:4 mix at 15 MPa. Driveways and parking areas: 5–6 inches (0.42–0.5 ft) — must resist vehicle loads; a 1:1.5:3 mix at 20 MPa is recommended. Residential floor slabs: 4 inches minimum on well-compacted fill with a vapour barrier. Structural footings: 6–12 inches depending on bearing load — consult your structural engineer. Concrete sidewalks: 4 inches per ACI 330R-08. For driveway cost estimation, use our Concrete Driveway Cost Calculator.
Concrete Mix Ratios and Compressive Strength
The concrete mix ratio (cement:sand:coarse aggregate) governs both workability and compressive strength. A 1:3:6 mix yields approximately 10 MPa (1450 psi) and is suitable for light-duty fill and non-structural pads. A 1:2:4 mix produces 15 MPa (2175 psi) — the standard for residential patios, paths, and general-purpose slabs. A 1:1.5:3 mix achieves 20 MPa (2900 psi), required for driveways carrying vehicle loads and commercial floor slabs. The Portland Cement Association recommends selecting mix proportions based on the required water-cement ratio, aggregate size, and exposure conditions per ASTM C94/C94M. For full mix design methodology, see PCA EB001.16 at https://www.cement.org.
Waste Factor and Ordering Quantities
Always add a waste factor of 5–10% to your concrete estimate. Pour losses arise from uneven subgrade compaction, formwork leakage, spillage at the chute, and mixing inefficiencies. For small residential pours with accurate formwork, 5% is adequate. For larger pours, irregular shapes, or hand-mixing, budget 8–10%. The number of premix bags is calculated as ⌈(total weight with waste) / bag weight⌉ using ceiling rounding so you never run short mid-pour. For ready-mix truck deliveries, the American Concrete Institute recommends ordering a minimum 8% overage per ACI 304R Guide for Measuring, Mixing, Transporting, and Placing Concrete, available at https://www.concrete.org/tools/concretedesigntools.aspx.
Unit Conversions and International Use
This calculator supports dimensions in feet, inches, meters, centimetres, and yards. All values are converted internally to feet for the geometric computation and then output in cubic yards (US standard), cubic feet, and cubic meters (SI). Key concrete conversions: 1 cubic yard = 27 cubic feet = 0.7646 cubic meters. 1 cubic meter = 35.315 cubic feet = 1.308 cubic yards. Normal-weight concrete density is 150 lb/ft³ (2400 kg/m³). Lightweight concrete ranges from 90–115 lb/ft³. For Canadian and European projects where m³ is the ordering unit, multiply cubic yards by 0.7646. The ISO standard EN 206 governs concrete specification in Europe, available from the British Standards Institution.
Ready-Mix vs. Premix Bags: When to Use Each
Premix bags (40, 60, or 80 lb) are ideal for pours under 1 cubic yard (27 ft³). They are sold at hardware stores, require no specialised equipment, and allow small-batch mixing. A standard 60 lb bag yields approximately 0.45 ft³ of cured concrete, meaning roughly 45 bags per cubic yard (27 / 0.6 = 45). Ready-mix concrete (truck delivery) is cost-effective for pours exceeding 1–2 cubic yards. It is batched to precise specifications, arrives ready to pour, and reduces labour. Most ready-mix suppliers require a minimum order of 1 yard. For reference: ASTM C94/C94M covers ready-mixed concrete standards in the US, while British Standard BS 8500 covers equivalent UK specifications. Our Cubic Yard Calculator can help with volume conversions when ordering ready-mix.
Quick Reference Card
Concrete Quick Reference
Quick reference • Concrete Calculator
Rectangular: V = L×W×T (ft³) ÷ 27 = yd³ | Round: V = π(d/2)²×T (ft³) ÷ 27 = yd³Valid range: Dimensions: 1 in – 500 ft | Thickness: 2 in – 24 in | Waste: 5–15%
Common Values
⚠ Watch Out
- •Divide by 27 — not 26 or 28 — to convert cubic feet to cubic yards.
- •Thickness must be in the same unit as length and width; 4 inches = 0.333 ft.
- •Concrete density varies — use 145 lb/ft³ for air-entrained concrete or 105 lb/ft³ for lightweight.
- •Do not pour below 40°F (4°C) without cold-weather protection; concrete may freeze before it sets.
- •Subgrade compaction is essential — pour on at least 4 inches of well-compacted base gravel.
Pro Tips
- →Order 8–10% extra on first-time pours to cover unexpected voids and subgrade dips.
- →For driveways, use a 1:1.5:3 mix (20 MPa) rather than the standard 1:2:4 to handle vehicle loads.
- →Wet the subgrade before pouring to prevent the ground from drawing moisture out of fresh concrete.
- →Pour in a single continuous operation where possible to avoid cold joints that reduce structural integrity.
- →Use a straightedge and bull float to level the surface immediately after screeding.
FAQs
How do I calculate how much concrete I need for a slab?
Multiply the length × width × thickness (all in the same unit). For example, a 10 ft × 12 ft slab at 4 inches (0.333 ft) thick = 10 × 12 × 0.333 = 40 ft³. Divide by 27 to get cubic yards (40 / 27 = 1.48 yd³). Add 5–10% for waste. This calculator performs all steps automatically in any unit system.
How many bags of concrete do I need for a 10×10 slab at 4 inches thick?
A 10 ft × 10 ft slab at 4 inches (0.333 ft) thick = 33.33 ft³ = 1.235 yd³. At 150 lb/ft³, that is 5,000 lb of concrete. With 5% waste (5,250 lb) and 60 lb bags: ⌈5,250 / 60⌉ = 88 bags. For 80 lb bags: ⌈5,250 / 80⌉ = 66 bags.
How thick should a concrete driveway be?
Residential driveways should be at least 4 inches (100 mm) thick for passenger cars, and 5–6 inches (125–150 mm) for heavier vehicles or when frost heave is a concern. ACI 330R-08 recommends 4 inches minimum for residential use and 5 inches for frequent heavy vehicle access. Always place concrete on a stable, compacted base of at least 4 inches of compacted gravel.
What is the difference between cubic yards and cubic feet in concrete?
1 cubic yard = 27 cubic feet. Ready-mix concrete is ordered in cubic yards in the US; premix bags list yield in cubic feet. To convert: divide cubic feet by 27 to get cubic yards. A 60 lb premix bag yields roughly 0.45 ft³, so approximately 60 bags fill 1 cubic yard.
How do I calculate concrete for a round slab?
Use the formula V = π × (d/2)² × T, where d is the diameter and T is the thickness. For an 8-foot diameter slab at 4 inches (0.333 ft): V = π × 4² × 0.333 = 16.76 ft³ = 0.621 yd³. This calculator handles round slabs automatically — just select 'Round Slab' and enter the diameter and thickness.
How many bags of 60 lb concrete make 1 cubic yard?
A standard 60 lb bag of premix concrete yields approximately 0.45 cubic feet of cured concrete. Since 1 cubic yard = 27 cubic feet, you need approximately 27 / 0.45 = 60 bags per cubic yard. The calculator uses weight-based math (density × volume / bag weight) which gives a very similar result and accounts for the actual density you select.
What waste percentage should I add to my concrete estimate?
Add 5% for straightforward rectangular pours with accurate formwork. Use 8–10% for irregular shapes, hand mixing, steep access, or cold weather pours where extra spills are likely. It is always better to have a small surplus than to run out mid-pour — concrete that sets while you wait for another delivery can create cold joints that weaken the slab.
Can I use this calculator for footings and foundations?
Yes. A rectangular footing is calculated identically to a slab: V = L × W × T. Enter the footing's length, width, and depth (thickness). For stepped or tapered footings, calculate each section separately and sum the volumes. For continuous strip footings, enter the total run length as the length, footing width as width, and footing depth as thickness.