Quick Answer: To calculate concrete, multiply Length (ft) x Width (ft) x Thickness (in) / 12 / 27 to get cubic yards. Concrete weighs 3,915 lbs/cu yd (~1.96 tons). Bags needed: divide total cubic feet by yield per bag (0.6 cu ft for 80-lb, 0.45 for 60-lb, 0.375 for 50-lb, 0.3 for 40-lb). Add 10% for waste. A 10 x 10 ft slab at 4 inches thick needs 1.23 cubic yards, about 56 bags of 80-lb mix.
Calculate how much concrete you need for any project — slabs, footings, columns, walls, and steps. Enter your dimensions to get the exact amount in cubic yards, number of bags, total weight, and estimated cost.
How to Calculate Concrete
The basic formula for calculating concrete volume is:
- Measure the length and width of the area in feet.
- Measure the thickness or depth in inches.
- Multiply length × width × thickness (converted to feet) to get cubic feet.
- Divide by 27 to convert cubic feet to cubic yards.
- Add 10% extra for waste, spillage, and uneven sub-grade.
- Divide total cubic feet by the yield per bag to get the number of bags needed.
Concrete Coverage by Thickness
How much area one cubic yard of concrete covers at different thicknesses:
| Thickness | Coverage per Cubic Yard |
|---|---|
| 2 inches | 162 sq ft |
| 3 inches | 108 sq ft |
| 4 inches | 81 sq ft |
| 5 inches | 65 sq ft |
| 6 inches | 54 sq ft |
| 8 inches | 40.5 sq ft |
| 12 inches | 27 sq ft |
Concrete Bag Size Guide
Quick reference for premixed concrete bags:
| Bag Weight | Yield (cu ft) | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 bags |
| 50 lb | 0.375 cu ft | 72 bags |
| 60 lb | 0.45 cu ft | 60 bags |
| 80 lb | 0.60 cu ft | 45 bags |
Types of Concrete Projects
- Slabs: Patios, garage floors, sidewalks, and shed pads. Typically 4 inches thick for foot traffic or 6 inches for vehicles.
- Footings: Foundation footings for walls, fences, and decks. Width and depth depend on the load they support and local frost depth.
- Columns: Round (Sonotubes) or square columns for deck posts, pergolas, and fence posts. Common diameters are 8, 10, or 12 inches.
- Walls: Retaining walls, foundation walls, and raised beds. Thickness typically ranges from 6 to 12 inches.
- Steps: Front porch steps, patio stairs, and entranceways. Standard rise is 7 inches with an 11-inch run.
Tips for Working with Concrete
- Order 10% extra: Spillage, uneven sub-grade, and form variations mean you will always use more concrete than the theoretical volume.
- Bags vs ready-mix: For projects under 1 cubic yard, bagged concrete is practical. Over 2 cubic yards, order a ready-mix truck to save time and labor.
- Prep the sub-grade: Compact the soil and add a 4-inch gravel base under slabs for drainage and to prevent cracking.
- Use rebar or mesh: Reinforce slabs thicker than 4 inches with wire mesh or rebar to prevent cracking and increase strength.
- Cure properly: Keep concrete moist for at least 7 days. Cover with plastic sheeting or use a curing compound to prevent premature drying and cracking.
- Temperature matters: Do not pour concrete when temperatures are below 40°F or above 90°F. Cold slows curing; heat causes rapid drying and cracking.
Concrete Cost Comparison: Bags vs. Ready-Mix
The most common question when planning a concrete project is whether to buy bags or order a ready-mix truck. Here is how the costs compare:
| Method | Cost per Cu Yd | Best For | Notes |
|---|---|---|---|
| 80-lb bags | $200-290 | <1 cu yd | DIY-friendly, no minimum order |
| 60-lb bags | $230-330 | <0.5 cu yd | Easier to handle, same mix |
| Ready-mix truck | $110-150 | >2 cu yd | 1 cu yd minimum, short-load fee under 3-4 cu yd |
| Short-load truck | $150-200 | 1-3 cu yd | Adds $40-80 fee for small orders |
Rule of thumb: If your project needs more than 90 bags (about 2 cubic yards), a ready-mix truck saves money, time, and your back. For projects between 1 and 2 cubic yards, it depends on whether you value labor savings over material cost.
Concrete Strength Grades
Not all concrete is the same strength. The PSI (pounds per square inch) rating tells you how much weight the cured concrete can support:
- 2,500 PSI: General-purpose residential use. Suitable for patios, sidewalks, and non-structural slabs. Most bagged concrete mixes are 3,500-4,000 PSI.
- 3,000 PSI: Standard residential foundations, footings, and retaining walls. The minimum strength specified in most residential building codes.
- 3,500-4,000 PSI: Driveways, garage floors, and structural slabs. This is what most 80-lb bags of premix deliver.
- 4,500+ PSI: Commercial applications, heavy-traffic industrial floors, and bridge decks. Only available through ready-mix suppliers with specific mix designs.
Formula Summary
Concrete Slab Volume Formula: To calculate the cubic yardage of concrete needed, multiply the Length (ft) x Width (ft) x Thickness (in), divide by 12 to convert thickness to feet, then divide by 27 to convert cubic feet to cubic yards.
V(cu yd) = (L(ft) × W(ft) × T(in)) / (12 × 27)
Round Column Volume: V(cu ft) = π × (d / 2)² × h × N where d = diameter in feet, h = height in feet, N = number of columns.
Bags Formula: Bags = ⌈V(cu ft) / Y(bag)⌉ where Y(bag) is the yield per bag in cubic feet.
Weight Formula: W(lbs) = V(cu yd) × 3915
| Bag Size | Yield (cu ft) | Bags per Cu Yd |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Standard Assumptions: Concrete density is 3,915 lbs per cubic yard (~145 lbs/cu ft). Waste factor of 10% accounts for spillage, uneven sub-grade, and form variations. 1 cubic yard = 27 cubic feet. Supports slab, footing, round column, square column, wall, and steps project types.
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Related Calculators
- Concrete Block Calculator — estimate CMU blocks for walls and foundations
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- Stair Calculator — calculate rise, run, and stringer dimensions
- Gravel Calculator — calculate gravel for driveways, paths, and sub-base
- How Much Concrete Do I Need? — beginner-friendly guide to figuring out your concrete order
Estimates are for planning purposes only. Consult a qualified contractor for critical projects. Actual material requirements may vary based on site conditions, waste, and installation methods.
Pricing last verified: February 2026. Material prices vary by region, season, and supplier. Prices shown are national averages for planning purposes.