Concrete Calculator: Square Feet to Yards

This concrete calculator converts an area measured in square feet and a slab thickness in inches into the volume of concrete needed in cubic yards. You enter the length and width in feet, the thickness in inches, and the calculator returns the cubic yards of concrete required, along with the equivalent cubic feet and cubic meters for reference.

Formula

The basic formula for concrete volume is:

Volume (cubic yards) = (Length (ft) × Width (ft) × Thickness (ft)) / 27

Since thickness is usually given in inches, convert it to feet first: Thickness (ft) = Thickness (in) / 12.

Variables:

• Length – the longer horizontal dimension of the slab, in feet (ft).
• Width – the shorter horizontal dimension, in feet (ft).
• Thickness – the vertical depth of the concrete, in inches (in).
• 27 – the number of cubic feet in one cubic yard (3 ft × 3 ft × 3 ft = 27 ft³).

The result is the volume of concrete needed before adding any waste or overage. Most ready-mix suppliers recommend ordering 5–10% extra to account for uneven subgrade, spillage, or slight over-excavation.

Worked Example

Using the calculator defaults: a slab that is 10 feet long, 10 feet wide, and 4 inches thick.

1. Convert thickness to feet: 4 in ÷ 12 = 0.3333 ft
2. Calculate volume in cubic feet: 10 ft × 10 ft × 0.3333 ft = 33.33 ft³
3. Convert to cubic yards: 33.33 ft³ ÷ 27 = 1.2346 yd³

The calculator rounds to two decimals, giving 1.23 cubic yards. That matches the headline output above. For a real order, you would likely order 1.5 yards (the nearest quarter-yard increment) or about 1.35 yards if you add 10% for waste.

When to Use This Conversion

You need this calculation any time you are ordering ready-mix concrete for a slab, footing, or pad. Common projects include:

• Patio or walkway slabs
• Garage or shed floors
• Footings for decks or walls
• Driveway aprons
• Basketball court or equipment pads

Concrete is sold by the cubic yard in the US and by the cubic meter in most other countries. Knowing the volume in cubic yards lets you call a ready-mix plant and order the right amount. It also helps you compare prices from different suppliers, since they quote per yard.

How to Read the Result

The main number is the exact volume of concrete if the slab were perfectly rectangular and the ground perfectly level. In practice, you will need more because:

• The subgrade is rarely perfectly flat; low spots consume extra concrete.
• Forms may bow outward slightly, increasing thickness.
• Some concrete sticks to the chute, wheelbarrow, or tools.
• You may want a small surplus to avoid a short pour (a cold joint is a structural weakness).

A common rule of thumb is to add 10% to the calculated volume. For a 1.23 yd³ slab, that brings the order to about 1.35 yd³. Ready-mix trucks typically deliver in quarter-yard increments, so you would round up to 1.5 yards.

Common Mistakes and How to Avoid Them

Unit mix-ups cause most errors. Here are the ones to watch:

• Mixing inches and feet: Always convert thickness to feet before multiplying. Forgetting to divide by 12 gives a volume 12 times too large.
• Forgetting the 27: One cubic yard is 27 cubic feet, not 3. If you divide by 3, you get a number three times too small.
• Measuring in different units: If you measure length in feet and width in inches, convert both to the same unit first.
• Ignoring waste: Ordering exactly the calculated amount often leaves you short because of uneven ground and spillage.
• Rounding too early: Round only the final answer, not intermediate steps, to avoid accumulating errors.

If you are working with metric measurements, the equivalent formula is: Volume (m³) = Length (m) × Width (m) × Thickness (m). One cubic yard equals approximately 0.7646 cubic meters.

Limits of the Formula

This calculator assumes a simple rectangular slab with uniform thickness. It does not account for:

• Sloped surfaces (a ramp or sloped driveway needs a different calculation based on average thickness).
• Footings or beams that are wider or deeper than the slab.
• Columns, piers, or other vertical elements.
• Irregular shapes (L-shaped or curved areas). For those, break the area into rectangles and sum the volumes.

For a sloped slab, the average thickness is the average of the shallow end and the deep end. For an L-shaped slab, divide it into two rectangles, calculate each volume, and add them together. The formula is exact for any shape that can be decomposed into rectangular prisms.

Ordering Concrete: Practical Tips

Once you have the cubic yards, you need to communicate with the ready-mix supplier. Key points:

• Tell them the volume (with the 10% overage) and the concrete strength (e.g., 3000 or 4000 psi).
• Ask if they charge for a short load (often under 3–4 yards).
• Check if the truck can reach the pour site; if not, you may need a pump or wheelbarrow.
• Have a plan for placing and finishing the concrete before the truck arrives – concrete starts setting within 30–60 minutes.

For small projects (under 1 yard), bagged concrete mix is often more practical than ordering a truck. A 60-lb bag yields about 0.45 cubic feet, and an 80-lb bag yields about 0.6 cubic feet. Divide your cubic feet by that number to get the bag count.

Frequently Asked Questions

How many square feet does a yard of concrete cover at 4 inches thick?

One cubic yard is 27 cubic feet. At 4 inches thick (1/3 foot), it covers 27 ÷ (1/3) = 81 square feet. This is a common quick reference: 1 yard covers about 80 sq ft at 4 inches.

What if my slab is 6 inches thick?

At 6 inches (0.5 ft), one yard covers 27 ÷ 0.5 = 54 square feet. For a 10×10 slab (100 sq ft) at 6 inches, you need 100 × 0.5 = 50 cubic feet, which is 50 ÷ 27 = 1.85 yards.

How do I calculate concrete for a round column?

Use the cylinder volume formula: π × r² × h, where r is the radius and h is the height, both in feet. Convert the result to cubic yards by dividing by 27. For example, a column 1 foot in diameter and 4 feet tall has a radius of 0.5 ft, so volume = π × 0.5² × 4 = 3.14 ft³, or 0.12 yd³.

Should I order extra concrete?

Yes, most professionals add 5–10% to the calculated volume. This covers uneven subgrade, spillage, and slight errors in measurement. For a small slab, 10% is safer; for large pours, 5% may be enough.

How many 80-lb bags do I need for 1 cubic yard?

An 80-lb bag of concrete mix yields about 0.6 cubic feet. One cubic yard is 27 cubic feet, so you need 27 ÷ 0.6 = 45 bags. For a 60-lb bag (0.45 ft³), you need 60 bags.

Can I use this calculator for footings?

Yes, if the footing is rectangular. Multiply the length, width, and depth (all in feet) and divide by 27. For stepped footings, calculate each step separately and add the volumes.

Related

• Board Foot to Square Foot Calculator
• Board Foot Calculator: How to Measure Lumber
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in l | Length | ft | 10 | 0
in w | Width | ft | 10 | 0
in t | Thickness | in | 4 | 0
out yards | Concrete needed | yd³ | (l * w * (t / 12)) / 27 | 2
out ft3 | Volume | ft³ | l * w * (t / 12) | 2
out m3 | Volume | m³ | (l * w * (t / 12)) * 0.0283168 | 2