Concrete Calculator: Cubic Yards, Bags, Weight & Cost

How to Calculate Concrete: The Complete Guide

To figure out how much concrete you need, multiply the length × width × thickness of the pour in feet, then divide by 27 to turn cubic feet into cubic yards. Ready-mix concrete is sold by the cubic yard, and bagged concrete is sold by weight (40, 60 or 80 lb), so the calculator above shows both. Always add 5–10% extra for uneven ground, spills and form bulge.

The concrete formula

Cubic yards = Length (ft) × Width (ft) × Thickness (in ÷ 12) ÷ 27

• Convert thickness from inches to feet by dividing by 12 (4 in = 0.333 ft, 6 in = 0.5 ft).
• One cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft).
• For round columns and Sonotubes use π × radius² × height instead of length × width.

Worked examples

1. 10 × 10 ft patio, 4 in thick: 10 × 10 × 0.333 = 33.3 ft³ ÷ 27 = 1.23 cubic yards. With 10% extra, order about 1.36 yd³, or about 61 bags of 80 lb concrete.
2. 20 ft strip footing, 16 in wide and 8 in deep: 20 × 1.333 × 0.667 = 17.8 ft³ ÷ 27 = 0.66 cubic yards.
3. 12 in round column, 4 ft tall: 3.1416 × 0.5² × 4 = 3.14 ft³ = 0.12 cubic yards, about six 80 lb bags.
4. 24 × 24 ft garage slab, 6 in thick: 24 × 24 × 0.5 = 288 ft³ ÷ 27 = 10.7 cubic yards, a little more than one full ready-mix truck.

How Many Bags of Concrete Do I Need?

A 60 lb bag of premixed concrete makes about 0.45 cubic feet and an 80 lb bag about 0.60 cubic feet. That means one cubic yard takes roughly 60 bags of 60 lb or 45 bags of 80 lb. The table shows common slab sizes (no waste added):

Above about one cubic yard, ready-mix delivered by truck is usually cheaper and faster than mixing dozens of bags by hand, and it gives a more consistent slab.

How Thick Should a Concrete Slab Be?

Local building codes and soil conditions always come first. A well-compacted gravel base of 4–6 inches under the slab matters as much as the thickness itself.

How Much Does Concrete Weigh?

Normal-weight concrete weighs about 150 lb per cubic foot, which is about 4,050 lb (just over 2 tons) per cubic yard. A 4 in slab weighs roughly 50 lb per square foot and a 6 in slab about 75 lb per square foot. Use the concrete weight calculator for a full chart by cubic foot, yard and meter.

Ready-Mix vs. Bagged Concrete

• Bagged concrete is best for small jobs under about 1 cubic yard: fence posts, steps, small pads and repairs.
• Ready-mix is best for slabs, driveways and footings over 1 cubic yard. A standard mixer truck carries up to about 10 cubic yards; ordering less than a full load often adds a short-load fee.
• Ready-mix is ordered by strength (PSI) and slump. 3,000 PSI suits most residential flatwork; driveways and garages usually get 3,500–4,000 PSI.

Concrete for Posts, Columns and Sonotubes

Round holes and tube forms use the cylinder formula: π × (diameter ÷ 2)² × depth, with all measurements in feet. The table assumes an empty hole; a 4 × 4 post takes up a little of the volume, so the numbers already include a small safety margin.

For deck footings and piers, the Sonotube calculator lists concrete per foot for every tube size from 6 to 36 inches, and the fence post calculator subtracts the post for you.

Concrete Steps, Curbs and Walls

Steps are calculated one riser at a time: each step is a box as wide as the stair, as deep as the run and as tall as all the risers below it. For example, four 7 in risers with 11 in treads and a 48 in width need about 0.8 cubic yards before the landing. Curbs combine the curb and gutter cross-sections, and block walls are calculated from the wall area and the size of the block cells. Use the dedicated steps, curb and block wall calculators for those shapes.

Concrete Mix Ratio for Mixing Your Own

If you mix from scratch instead of using premixed bags, a 1 : 2 : 3 ratio (cement : sand : gravel by volume) with about half a part water gives roughly 3,000 PSI concrete, which suits most slabs, steps and footings. Use 1 : 2 : 4 for mass footings and 1 : 1.5 : 3 when you need extra strength. One cubic yard of 1 : 2 : 3 concrete needs about seven 94 lb bags of Portland cement. The mix calculator works out cement, sand, gravel and water for any volume.

How Long Does Concrete Take to Cure?

• 24–48 hours: firm enough to walk on and remove forms.
• 7 days: about 70% of its design strength; light vehicles can usually drive on a driveway.
• 28 days: full design strength; safe for heavy loads.

Keep fresh concrete moist for the first week (wet curing, curing compound or plastic sheeting) and protect it from frost. Concrete that dries too fast loses strength and cracks.

Common Mistakes That Waste Concrete

• Forgetting to convert inches to feet for thickness, which makes the result 12 times too big.
• Dividing by 9 instead of 27 (9 converts square feet to square yards, not cubic feet to cubic yards).
• Ordering the exact volume. Every pour needs a little extra; running short costs more than a few leftover bags.
• Adding extra water to make concrete easier to place. It noticeably reduces strength and increases cracking.

Tips to Order the Right Amount

• Measure in several places. Ground is rarely level; use the average depth, not the shallowest spot.
• Add 5–10% waste for flat slabs and up to 15% for footings dug in soft soil.
• Split odd shapes into rectangles, circles and triangles, calculate each and add them together.
• Round up to the next quarter yard when ordering ready-mix. Running short mid-pour leaves a cold joint.
• Check access for the truck. If it cannot reach the forms, budget for a pump or wheelbarrows.