Concrete Calculator
Calculate concrete cubic yards for slabs, patios, footings, and columns. Enter your dimensions and thickness in inches to get slab yardage, ready-mix cubic yards, 60 lb and 80 lb bag counts, and a waste-adjusted order quantity.
Estimated Required
Base Estimate
With 10% Waste
How to Use This Calculator
Select the project shape, enter your dimensions, choose a waste factor, and read the result. The base estimate is the theoretical volume. The with-waste figure is what you should actually order — it accounts for uneven subgrade, form imprecision, and spillage.
Formula & Methodology
This calculator estimates concrete volume by converting your project dimensions into cubic yards, then calculating the equivalent number of pre-mixed bags based on published bag yield specifications.
Slab / Footing:
Volume (yd³) = (L ft × W ft × Depth in ÷ 12) ÷ 27
Column:
Volume (yd³) = (π × (d ÷ 2 ÷ 12)² × Height in ÷ 12) ÷ 27
Bags:
Bag count = Volume (yd³) × 1,728 ÷ Bag Yield (cu ft)
Bag yields: 80 lb bag ≈ 0.60 cu ft; 60 lb bag ≈ 0.45 cu ft. Confirm yield on the bag label before purchasing.
Sources & Assumptions
Formula: Standard volumetric calculation for slabs/footings and cylinder formula for columns, both divided by 27 cu ft/yd³.
Bag yield: 80lb bag ≈ 0.60 cu ft; 60lb bag ≈ 0.45 cu ft. Based on published yield specifications from Quikrete® and Sakrete®. Actual yield varies slightly by mix design — verify on the bag label before purchasing.
Waste recommendation: 5–10% is standard; higher for uneven subgrade or irregular form shapes. Running short mid-pour can be costly — err on the high side.
Confirmation: Verify ready-mix cubic yards with your concrete supplier; confirm bag yield on packaging before purchasing.
Last reviewed: May 2026
Recommended Depths & Coverage
Use these common thickness ranges for planning. Actual thickness may vary based on subgrade preparation, reinforcement, load requirements, and local building codes.
| Project Type | Recommended Depth | Approx. Coverage per 1 yd³ | Notes |
|---|---|---|---|
| Sidewalk / walkway | 4 in | ~81 sq ft | Common residential walkway thickness |
| Patio slab | 4 in | ~81 sq ft | Typical light-use slab |
| Residential driveway | 4–5 in | ~65–81 sq ft | Increase thickness for heavier loads |
| Garage / heavy-use slab | 5–6 in | ~54–65 sq ft | Depends on reinforcement and subgrade |
Coverage values assume 1 cubic yard before waste and compaction considerations. Add the selected waste factor for ordering quantities.
Project Examples
Common slab, patio, and footing projects with volume estimates. All figures include 10% overage — the minimum recommended for any concrete pour.
| Project | Dimensions | Thickness | Cubic Yards |
|---|---|---|---|
| 10 × 10 slab | 10 × 10 ft | 4 in | 1.23 yd³ |
| 12 × 20 patio | 12 × 20 ft | 4 in | 2.96 yd³ |
| 20 × 20 driveway | 20 × 20 ft | 4–5 in | 4.94–6.17 yd³ |
| Post footing | 12-in dia, 24 in deep | — | ~0.058 yd³ |
Common Ordering Mistakes
- !Not adding a waste factor: Even a small short pour means an extra hardware store trip or a cold joint in your slab. Add at least 5–10% overage to every concrete order.
- !Choosing bags for a large pour: Mixing more than 1 cubic yard by hand or with a small drum mixer is exhausting and inconsistent. A ready-mix truck is faster, produces more uniform concrete, and is usually cheaper above that threshold.
- !Using the wrong bag size in the estimate: 60 lb bags yield ~0.45 ft³ and 80 lb bags yield ~0.60 ft³. Calculating with 80 lb bags when you buy 60 lb bags leaves you short by about 25%.
- !Not accounting for subgrade variation: Soft spots and low areas in the subgrade consume more concrete than a flat-surface estimate predicts. Compact and level the base before pouring.
- !Starting the pour without all bags on hand: Concrete cures continuously once water is added. Running out mid-pour creates a cold joint — a structural weakness — between the two pours.
Concrete Cubic Yards and Bag Counts
Understanding the relationship between cubic yards and bags helps you decide whether to order ready-mix or buy bags — and how many of each.
1 cubic yard = 27 cubic feet: All concrete volumes convert through this relationship. Slabs use Length (ft) × Width (ft) × Thickness (in) ÷ 12 to get cubic feet, then ÷ 27 for cubic yards. This is the number quoted on ready-mix delivery tickets.
80 lb bags yield ~0.60 cubic feet each: That works out to 45 bags per cubic yard before waste. Add a 10% waste factor and a typical order is 50 bags per yard. Even a modest 10 × 10 slab at 4 inches needs ~62 bags — this is why bag counts climb quickly on anything but the smallest pours.
60 lb bags yield ~0.45 cubic feet each: Roughly 60 bags per cubic yard before waste, or about 66 with 10% overage. For the same 10 × 10 slab you'd need ~82 bags — reinforcing why ready-mix becomes the economical choice above about 1 cubic yard.
Bags vs. ready-mix break-even: For most homeowners the crossover is around 1 cubic yard. Below that, bags are convenient. Above that, a half-yard or full-yard ready-mix delivery saves time and usually money. See the Concrete Bags vs. Ready-Mix guide for a full cost comparison.
Estimating for a standard backyard slab? See the How Much Concrete for a 10×10 Slab? guide for a complete walkthrough with material options and cost ranges.
Frequently Asked Questions
Related Calculators & Guides
Need to estimate a gravel base layer first? Use the Gravel Calculator. Paving the surface with asphalt? Try the Asphalt Calculator.
Estimating for a 10×10 slab specifically? See the How Much Concrete for a 10×10 Slab? guide.
Bags or ready-mix? Concrete Bags vs Ready-Mix: Which Should You Use?
Disclaimer
Results are estimates only. Actual quantities vary with subgrade conditions, form dimensions, and mix design. Order 5–10% extra. Consult a qualified contractor for large pours.