Concrete Calculator

Multiply length × width × thickness, then divide by 27 to convert cubic feet to cubic yards. The formula assumes everything is in feet — so a 4-inch thick slab uses thickness = 0.333 ft (4 ÷ 12). For a 10 × 12 × 4″ slab: 10 × 12 × 0.333 = 40 cubic feet, then 40 ÷ 27 = 1.48 cubic yards. The calculator above handles the unit conversion automatically and adds your waste factor.

A slab is just a rectangular volume: length × width × thickness ÷ 27 = cubic yards. The catch is the thickness — it's almost always given in inches but the formula needs feet. For a typical 4-inch slab: thickness = 4 ÷ 12 = 0.333 ft. For a 6-inch slab: 0.5 ft. Add 5–10% waste because concrete is wet — you can't recover overage from a partial truck. Use the slab mode in the calculator above for instant results.

One 80-lb bag yields about 0.022 cubic yards (0.6 cubic feet) of mixed concrete. So one cubic yard takes about 45 bags of 80-lb concrete. For 60-lb bags it's about 60 bags per yard, and for 40-lb bags it's about 90 bags per yard. Bags get heavy fast — anything over 1 cubic yard is usually cheaper and faster as ready-mix delivery rather than mixing by hand.

They measure the same thing — volume of mixed concrete — but for different buyers. Cubic yards is how ready-mix is sold and delivered by truck (typically minimum 1 yard, $130–$180 per yard depending on region). Bags is how DIY concrete is sold at hardware stores (40, 60, or 80-lb dry mix that you add water to). Below ~1 cubic yard, bags usually make sense; above that, ready-mix is cheaper and faster.

Standard residential thicknesses depend on use. Sidewalks and patios: 4 inches. Driveways for cars: 4 inches (5 inches for heavier vehicles or trucks). Garage floors: 4–6 inches. Shed and small outbuilding floors: 4 inches. Commercial floors and shop slabs: 6 inches or more, often with reinforcement. Always check local building codes — some jurisdictions require minimum thicknesses for certain applications.

Below about ½ cubic yard, mixing your own bags is usually cheaper. Above 1 cubic yard, ready-mix delivery wins on both cost and time. The breakeven is near ¾ to 1 yard depending on your region's bag and delivery prices. Ready-mix has a minimum order (typically 1 yard) and a "short load" fee for orders under 4–5 yards. For a 10×12×4″ slab (1.48 yards), ready-mix is almost always the better choice.

Footings are usually rectangular trenches: length × width × depth ÷ 27 = cubic yards. A 30-foot footing trench at 16″ wide × 12″ deep = 30 × 1.333 × 1 ÷ 27 = 1.48 cubic yards. Columns and round piers use π × radius² × height ÷ 27. A 4-foot Sonotube at 12″ diameter = 3.14 × 0.5² × 4 ÷ 27 = 0.12 cubic yards. The calculator above handles both shapes — pick "Footing" or "Column" mode.

5–10% is the standard waste allowance. Concrete is wet and unforgiving — you can't bring a partial truck back, and an extra trip costs hundreds in delivery fees. For simple rectangular slabs, 5% covers normal spillage and form variation. For complex shapes, multiple pours, or sloped pours, use 10–15%. Always round UP to the nearest ¼ cubic yard when ordering ready-mix — suppliers won't deliver fractions smaller than that.

The calculator accepts inputs in feet and inches, but every result shows the metric equivalent in parentheses (cubic meters alongside cubic yards). To convert your measurements: 1 meter = 3.281 feet, and 1 cubic meter = 1.308 cubic yards. For a 3m × 4m × 0.1m slab, that's roughly 9.84 ft × 13.12 ft × 4 inches in US units. Bag counts and yardage are universal once volume is calculated.

The math is exact for the geometry it models — rectangular slabs, footings, columns, and stairs all use straightforward volume formulas. Real-world accuracy depends on input precision and waste factor selection. Measure dimensions carefully, account for any irregular sections separately, and order with a 5–10% buffer. For ready-mix orders, confirm with your supplier — they often have minimum delivery quantities and can advise on PSI strength for your application.