Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Area to cover 100 ft²; Thickness 4 in = 0.3333 ft
- Volume = 100 ft² × 0.3333 ft = 33.3333 ft³
- Net volume = 33.3333 ft³ ÷ 27 = 1.2346 yd³ (× 0.0283168 = 0.9439 m³)
- Order quantity = 1.2346 yd³ × (1 + 10%) = 1.358 yd³ = 36.6667 ft³
- 80 lb bags = 36.6667 ft³ ÷ 0.6 ft³ per bag = 61.11 → 62 bags (always round up)
- Weight = 36.6667 ft³ × 150 lb/ft³ = 5,500 lb
How to measure
Area is length × width for a rectangle. For other shapes: a triangle is base × height ÷ 2, a circle is π × radius², and any complex outline can be split into rectangles and triangles. Add the areas, then enter the total.
The unit selector accepts square feet, square yards and square meters. Thickness works in inches, centimeters or millimeters.
The area-to-yards formula
Cubic yards = area in square feet × thickness in feet ÷ 27. Because thickness is usually given in inches, it is easier to use the other way: yards = area × inches ÷ 324. The 324 comes from 12 inches per foot times 27 cubic feet per yard. For a quick check, 1 yd³ covers 81 ft² at 4 in.
Shortcut: yd³ = area (ft²) × thickness (in) ÷ 324.
Worked example: 400 ft² at 4 in
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Area to cover 400 ft²; Thickness 4 in = 0.3333 ft
- Volume = 400 ft² × 0.3333 ft = 133.3333 ft³
- Net volume = 133.3333 ft³ ÷ 27 = 4.9383 yd³ (× 0.0283168 = 3.7756 m³)
- Order quantity = 4.9383 yd³ × (1 + 10%) = 5.4321 yd³ = 146.6667 ft³
- 80 lb bags = 146.6667 ft³ ÷ 0.6 ft³ per bag = 244.44 → 245 bags (always round up)
- Weight = 146.6667 ft³ × 150 lb/ft³ = 22,000 lb
| Quantity | Result |
|---|---|
| Net volume | 4.938 yd³ (133.33 ft³, 3.776 m³) |
| Order with 10% waste | 5.43 yd³ (4.15 m³) |
| 80 lb bags | 245 |
| 60 lb bags | 326 |
| Weight at 150 lb/ft³ | 22,000 lb |
Using the shortcut: 400 × 4 ÷ 324 = 4.94 yd³ before waste, and 5.43 yd³ with 10% waste.
How far does a yard go?
| Thickness | Square feet | Square meters |
|---|---|---|
| 2 in | 162.0 | 15.1 |
| 3 in | 108.0 | 10.0 |
| 4 in | 81.0 | 7.5 |
| 5 in | 64.8 | 6.0 |
| 6 in | 54.0 | 5.0 |
| 8 in | 40.5 | 3.8 |
The 4 in row (81 ft²) is the figure behind most rules of thumb. Notice how fast the coverage falls: a 6 in slab covers only two thirds of what a 4 in slab does.
Ordering from area
Ready-mix plants sell by the cubic yard and most take orders in quarter-yard steps. Round your result up to the next quarter, and make sure the truck’s total includes the base, because a base that is 1 in low adds 1 in of concrete across the whole area. If the number is small, bags are a better fit: see the bag calculator.
Mistakes with area-based estimates
- Using the area of the whole yard instead of the poured area.
- Mixing square yards and square feet. A square yard is 9 ft², not 3.
- Using the thickness of the nominal slab when the base is uneven.
- Counting a sloped surface by its plan view and forgetting extra thickness.
- Forgetting that decorative edges or thickened borders add concrete.
Mental math with the coverage numbers
Memorize the coverage: one yard covers 81 ft² at 4 in, 65 ft² at 5 in and 54 ft² at 6 in. A quick check of a 250 ft² patio at 4 in: 250 ÷ 81 = 3.1 yd³ net; the calculator gives 3.09 yd³ net and 3.40 yd³ with 10% waste. Divide the area by the coverage and add waste, and you have an answer without a calculator.
Start from an area when the shape is irregular, for example a curved patio measured from a plan or a driveway with a flare. Measure the area in square feet (or square meters), then enter the thickness. If the area comes from a drawing, check the scale and add up the pieces.
Frequently asked questions
How many cubic yards in 100 square feet at 4 inches?
100 ft² × 4 in ÷ 324 = 1.23 yd³.
How do I convert square feet to cubic yards?
Multiply the area by the thickness in feet, then divide by 27. Or multiply by the thickness in inches and divide by 324.
How many square feet does a yard of concrete cover?
At 4 in, 81 ft². At 6 in, 54 ft². At 3 in, 108 ft².
How much concrete for 1,000 square feet?
At 4 in thick that is 12.3 yd³ before waste, and about 13.6 yd³ with 10% waste.
How do I measure an irregular area?
Divide it into rectangles and triangles, calculate each area and add them.
Does the calculator include waste?
Yes. The default is 10% and you can change it from 0 to 25% or type your own value.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.