Online Concrete Calculator

Free browser-based concrete estimating tool. Enter dimensions in standard US customary units (feet and inches) to instantly calculate ready-mix cubic yards, bag counts, weight, and materials.

Instant Online Estimator

Adjust dimensions below to compute cubic yards and bags in real time.

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⚡ 1-Click Popular Presets:

Slab Length (Feet)

1 ft 40 ft 80 ft

Slab Width (Feet)

1 ft 40 ft 80 ft

Slab Depth / Thickness (Inches)

Waste & Spillage Allowance 10% Recommended

Ready-Mix To Order (With Waste)

1.50 yd³ ≈ 1.15 m³

Exact net: 1.36 yd³ Volume: 33.3 cu ft

Pre-mixed Concrete Bags Includes 10% waste

80 lb Bags (0.60 cu ft)

62 bags

≈ 90 × 25kg

60 lb Bags (0.45 cu ft)

82 bags

≈ 89 × 25kg

Estimated Cured Weight: 5,500 lbs (2,495 kg)

Surface Area: 100.0 sq ft (9.3 m²)

Ready-mix or bags?

Projects over 2 cu yds are almost always cheaper with ready-mix.

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Quick Reference Table: Concrete Slabs by Dimensions (4" & 6" Thick)

Pre-calculated cubic yards and bag requirements including standard 10% waste allowance.

Includes 10% Waste

Slab Size (Ft)	Depth	Surface Area	Ready-Mix Order (Yds)	80 lb Bags	60 lb Bags
4' × 4'	4"	16 sq ft	0.25 yd³	10 bags	14 bags
4' × 4'	6"	16 sq ft	0.50 yd³	15 bags	20 bags
6' × 6'	4"	36 sq ft	0.50 yd³	23 bags	30 bags
6' × 6'	6"	36 sq ft	0.75 yd³	33 bags	44 bags
8' × 8'	4"	64 sq ft	1.00 yd³	40 bags	53 bags
8' × 8'	6"	64 sq ft	1.50 yd³	59 bags	79 bags
10' × 10'	4"	100 sq ft	1.50 yd³	62 bags	82 bags
10' × 10'	6"	100 sq ft	2.25 yd³	92 bags	123 bags
10' × 12'	4"	120 sq ft	1.75 yd³	74 bags	98 bags
10' × 12'	6"	120 sq ft	2.50 yd³	110 bags	147 bags
12' × 12'	4"	144 sq ft	2.00 yd³	89 bags	118 bags
12' × 12'	6"	144 sq ft	3.00 yd³	132 bags	176 bags
12' × 16'	4"	192 sq ft	2.75 yd³	118 bags	157 bags
12' × 16'	6"	192 sq ft	4.00 yd³	177 bags	235 bags
12' × 20'	4"	240 sq ft	3.50 yd³	147 bags	196 bags
12' × 20'	6"	240 sq ft	5.00 yd³	220 bags	294 bags
14' × 14'	4"	196 sq ft	2.75 yd³	120 bags	160 bags
14' × 14'	6"	196 sq ft	4.00 yd³	180 bags	240 bags
16' × 16'	4"	256 sq ft	3.50 yd³	157 bags	209 bags
16' × 16'	6"	256 sq ft	5.25 yd³	235 bags	313 bags
20' × 20'	4"	400 sq ft	5.50 yd³	245 bags	326 bags
20' × 20'	6"	400 sq ft	8.25 yd³	367 bags	489 bags
24' × 24'	4"	576 sq ft	8.00 yd³	353 bags	470 bags
24' × 24'	6"	576 sq ft	11.75 yd³	528 bags	704 bags

💡 Note: Orders are rounded up to the nearest 0.25 yd³ as standard for ready-mix dispatch. Ready-mix density estimated at 150 lbs/cu ft (4,050 lbs/yd³).

Worked Example: 20' × 20' Two-Car Garage Apron (5 Inches Thick)

Suppose you are planning to pour a backyard slab measuring 20 feet long by 20 feet wide with a standard residential depth of 5 inches. Here is the verified 5-step arithmetic:

Convert thickness from inches to feet: Since dimensions must be in consistent units, divide depth by 12:

 5" ÷ 12 = 0.417 feet

Calculate cubic feet volume: Multiply Length × Width × Depth in feet:

 20' × 20' × 0.417' = 166.67 cubic feet

Convert cubic feet to cubic yards: There are exactly 27 cubic feet in 1 cubic yard. Divide by 27:

 166.67 cu ft ÷ 27 = 6.173 cubic yards (exact net)

Add 10% waste margin: Concrete forms bow slightly, excavation depth is never laser-flat, and spillage occurs. Multiply by 1.10:

 6.173 yd³ × 1.10 = 6.79 cubic yards with waste

Final Purchasing Recommendation:

Ready-Mix Delivery: Order 7.00 yd³ (rounded up to nearest 0.25 yd³ increment).
Pre-mixed 80 lb Bags: Purchase 306 bags (0.60 cu ft yield per bag).
Pre-mixed 60 lb Bags: Purchase 408 bags (0.45 cu ft yield per bag).

Calculation Methodology & Estimating Formulas

Standards-compliant math based on ASTM C94 and American Concrete Institute (ACI 318) guidelines.

1. The Core Volume Formula

Concrete volume is measured in three dimensions and converted to cubic yards (1 cubic yard = 27 cubic feet):

Slab / Rectangle:
Volume (yd³) = [Length (ft) × Width (ft) × (Thickness (in) ÷ 12)] ÷ 27

Cylinder / Pier / Sonotube:
Volume (yd³) = [π × (Diameter (in) ÷ 24)² × Depth (ft)] ÷ 27

2. Why We Add a 10% Waste Margin

Ordering the exact geometric volume almost always results in a short pour. A minimum 10% waste buffer is critical for three physical reasons:

Subgrade Unevenness: Just 1/4 inch of extra grade depth across a 400 sq ft slab requires an additional 0.31 cubic yards.
Form Deflection: Wooden 2×4 or 2×6 forms bulge outward under 150 lb/cu ft liquid concrete pressure.
Equipment Residuals: Concrete sticks to truck chutes, pump hoses, wheelbarrows, and screed boards.

3. Bagged Concrete Yield Constants

Pre-mixed bagged concrete (e.g., Quikrete, Sakrete) is pre-measured with Portland cement, sand, and gravel. Standard yields:

Bag Size	Cubic Feet Yield	Bags Per 1 yd³
80 lb bag	0.60 cu ft	45 bags
60 lb bag	0.45 cu ft	60 bags
50 lb bag	0.375 cu ft	72 bags
40 lb bag	0.30 cu ft	90 bags

4. Structural Weight & Load Assumptions

Cured normal-weight structural concrete is modeled at 150 pounds per cubic foot (ASTM C94 benchmark). This means:

1 cubic yard weighs approximately 4,050 pounds (2.025 US tons).
For sub-base preparation, always lay 4 inches of compacted crushed gravel (dense grade aggregate) under slabs to prevent frost heave and settlement cracking.

Maddy inspecting concrete site conditions

Written & Reviewed by Maddy

✓ ACI & ASTM Verified

Construction Estimating Specialist & Civil Calculation Modeler

Last Reviewed: October 6, 2026 Report a calculation discrepancy →

Governing Industry Standards: This calculator executes mathematical models grounded strictly in ASTM C94 (Standard Specification for Ready-Mixed Concrete), ACI 318-19 (Building Code Requirements for Structural Concrete), and NRMCA volumetric batch tolerances.

Need to verify our references and derivation? Explore our dedicated Engineering Sources & Specifications directory or read our Calculation Formulas guide.

Online Concrete Calculator Questions

Common contractor questions, bag ratios, thickness standards, and curing guidance.

Why use an online concrete calculator instead of manual math? ▼

An online calculator automatically handles fractional depth conversions (converting inches to feet), incorporates standard 10% spillage allowances, and translates cubic yards into exact commercial bag counts (80 lb, 60 lb, 50 lb) while rounding ready-mix orders to official batch plant increments (0.25 yd³).

How accurate is this online concrete calculator? ▼

Our calculation engine strictly implements ASTM C94 and American Concrete Institute (ACI) geometric algorithms. It calculates exact three-dimensional volume to three decimal places before applying recommended site buffers.

How do I calculate concrete for irregular or L-shaped areas? ▼

For L-shaped patios or multi-section pours, divide the area into separate rectangular sections. Calculate the cubic yards for each rectangle individually, and add the quantities together. Remember to add 10% total waste.