Concrete Job Calculator: Mix & Cost Estimator

This concrete job calculator works out how much concrete you need for a slab, footing, column, or wall, and estimates the number of bags or cubic yards to order. Enter the dimensions (length, width, depth) and optionally the price per bag or per cubic yard, and you get the volume in cubic feet and cubic yards, plus the estimated cost and bag count. The calculator below uses the standard volume formula (length × width × depth) and converts to the units your supplier uses.

Formula

The core formula is the volume of a rectangular prism:

Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft)

Because depth is usually given in inches, convert it first: Depth (ft) = Depth (in) ÷ 12

Then convert to cubic yards: Cubic yards = Cubic feet ÷ 27

For bagged concrete, the number of bags depends on the bag size. A 60-lb bag yields about 0.45 cubic feet (or 0.017 cubic yards). So Bags = (Cubic feet ÷ 0.45) rounded up. The calculator uses a default of 60 bags per cubic yard (which matches 0.45 cu ft per bag) but you can adjust that if you use 40-lb or 80-lb bags.

Worked Example

Using the defaults from the calculator: a slab that is 10 ft long, 8 ft wide, and 4 in deep.

1. Convert depth: 4 in ÷ 12 = 0.333 ft
2. Volume in cubic feet: 10 × 8 × 0.333 = 26.64 cu ft
3. Convert to cubic yards: 26.64 ÷ 27 = 0.9867 cu yd (round to 0.99)
4. Bags needed: 26.64 cu ft × (60 bags / 27 cu ft) = 59.2, round up to 60 bags
5. Cost: 60 bags × $5.50 = $330.00

The calculator shows the same: 26.64 cu ft, 0.99 cu yd, 60 bags, $330.00.

When to Use This Calculator

Use this for any rectangular pour: patios, sidewalks, footings, shed bases, or small slabs. It works for any shape that can be broken into rectangles—just calculate each section separately and add the volumes. For round columns or curved areas, use a different formula (πr²h) and then apply the same conversion.

How to Read the Results

The first number (cubic feet) is the raw volume. The second (cubic yards) is what ready-mix companies use—they deliver by the cubic yard. The bag count assumes you are mixing bagged concrete yourself, which is practical for small jobs (under 1 cubic yard). The cost estimate is based on the bag price you enter; it does not include delivery, reinforcement, or labor.

Common Mistakes and How to Avoid Them

• Mixing units: Always convert depth to feet before multiplying. A common error is using inches directly, which overestimates volume by 12×.
• Rounding too early: Round only at the end. If you round the depth to 0.3 ft instead of 0.333, you lose about 1%—not huge, but for large slabs it adds up.
• Forgetting waste: Concrete shrinks, spills, and sticks to tools. Add 5–10% extra. For a 1-yard job, order 1.1 yards.
• Assuming all bags are the same: A 40-lb bag yields about 0.30 cu ft, an 80-lb bag yields about 0.60 cu ft. Adjust the “bags per yardâ€� accordingly (40-lb: ~90 bags/yard, 80-lb: ~45 bags/yard).

Limits of This Formula

This calculator assumes a flat, uniform thickness. For sloped surfaces, use the average depth. It also ignores the slight volume loss when mixing with water—dry concrete expands a bit when wet, but the difference is negligible for most projects. For large pours (over 1 yard), ready-mix is usually cheaper than bags, but you must order a minimum quantity (often 1–3 yards) and pay for delivery.

Bagged vs. Ready-Mix

Bagged concrete is easier for small jobs, but the per-yard cost is higher. A typical 60-lb bag costs $4–$7, so 60 bags (1 yard) runs $240–$420. Ready-mix concrete costs $100–$200 per yard plus delivery, but you need a truck and a crew to place it. For a 10×8×4-inch slab (0.99 yd), bagged concrete costs about $330 (using $5.50/bag), while ready-mix might be $150 plus delivery—so ready-mix is cheaper if you can handle the logistics.

Frequently Asked Questions

How many 80-lb bags do I need for a 10×8×4-inch slab?

An 80-lb bag yields about 0.60 cu ft. The volume is 26.64 cu ft, so 26.64 ÷ 0.60 = 44.4, round up to 45 bags. In the calculator, change “bags per yard� to 45 (since 27 ÷ 0.60 = 45).

Do I need to add extra concrete for waste?

Yes, add 5–10% to the total volume. For a slab that calculates to 1.0 cubic yard, order 1.05–1.1 cubic yards. This covers spillage, uneven subgrade, and the concrete that stays in the mixer or wheelbarrow.

What depth should I use for a sidewalk?

Residential sidewalks are typically 4 inches thick, which is the default. For driveways or areas with vehicle traffic, use 5–6 inches. Always check local building codes for minimums.

Can I use this for a round column?

No, this calculator is for rectangular shapes. For a round column, use the formula V = π × (radius²) × height. For example, a 12-inch diameter column 4 feet tall: radius = 6 in = 0.5 ft, so V = π × 0.25 × 4 = 3.14 cu ft, which is about 0.12 cubic yards.

How do I convert cubic feet to cubic meters?

1 cubic foot = 0.0283168 cubic meters. Multiply your cubic feet by 0.0283 to get cubic meters. For the example, 26.64 cu ft × 0.0283 = 0.754 m³.

What is the difference between concrete and cement?

Cement is a powder ingredient (usually Portland cement) that binds the mix. Concrete is the final mixture of cement, sand, gravel, and water. When you buy a bag of “concrete mix,� it already contains the cement and aggregates—you just add water.

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in l | Length | ft | 10 | 0
in w | Width | ft | 8 | 0
in d | Depth | in | 4 | 0
in price_bag | Price per 60-lb bag | $ | 5.50 | 0
in bags_per_yard | Bags per cubic yard (60-lb) | bags | 60 | 0
out cubic_ft | Concrete volume | cu ft | l * w * (d / 12) | 2
out cubic_yd | Concrete volume | cu yd | cubic_ft / 27 | 2
out bags | Bags needed | 60-lb bags | ceil(cubic_ft * (bags_per_yard / 27)) | 0
out cost | Estimated cost | $ | bags * price_bag | 2