Length × Width × Thickness — Instant Concrete Quantity
Estimate cubic feet, cubic yards, concrete bags, total weight and optional material cost for rectangular slabs, patios, driveways, garage floors, pads, walkways and other flat concrete pours.
Calculate one rectangle, combine multiple sections, estimate bags or convert volume into concrete weight and cost.
Estimate concrete for slabs, patios, driveways, garage floors and rectangular pads.
Split L-shaped, stepped or irregular flatwork into rectangles and combine the concrete volumes.
Convert slab volume into approximate 40 lb, 60 lb or 80 lb concrete bag quantities.
Convert known rectangular concrete volume into approximate weight and optional ready-mix material cost.
Calculate concrete from length, width and thickness in seconds.
Combine non-overlapping rectangles for irregular flatwork.
Estimate 40, 60 or 80 lb bags for smaller pours.
Convert cubic yards into approximate mass and material cost.
The Rectangular Concrete Calculator is designed for concrete shapes with straight sides and constant thickness. Measure the finished length and width in feet, then enter thickness in inches. The calculator converts thickness automatically and returns area, cubic feet and cubic yards.
Use the single-rectangle panel for slabs, patios, driveways, garage floors and pads. For L-shaped or stepped work, divide the footprint into non-overlapping rectangles and use the multiple-section calculator.
Use the finished dimension.
Measure across the slab.
Use inches for slab depth.
Choose a planning percentage.
Check supplier increments.
Rectangular concrete is calculated from three dimensions. When length and width are in feet and thickness is in inches, divide thickness by 12 first.
Use finished concrete dimensions rather than excavation dimensions.
For a 20 ft by 12 ft patio that is 4 inches thick, the footprint is 240 square feet. Four inches equals 0.333 foot.
With a 10% allowance, the planning quantity becomes about 3.26 cubic yards before supplier rounding.
| Slab Size | Thickness | Cubic Feet | Cubic Yards |
|---|---|---|---|
| 10 × 10 ft | 4 in | 33.33 | 1.23 |
| 12 × 12 ft | 4 in | 48.00 | 1.78 |
| 20 × 12 ft | 4 in | 80.00 | 2.96 |
| 20 × 20 ft | 4 in | 133.33 | 4.94 |
| 24 × 24 ft | 5 in | 240.00 | 8.89 |
Volume changes directly with thickness. If length and width remain fixed, a 5-inch slab uses 25% more concrete than a 4-inch slab, while a 6-inch slab uses 50% more.
This calculator estimates quantity only. Use the Concrete Thickness Calculator for thickness-related planning and follow the project requirements for the final design thickness.
L-shaped patios, widened driveways and stepped slab areas can usually be divided into two or more rectangles. Calculate each section separately and add the volumes.
| Project | Method | Related Tool |
|---|---|---|
| Patio | L × W × thickness | Concrete Patio Calculator |
| Driveway | One or more rectangles | Concrete Calculator |
| Garage Floor | Rectangular slab | Concrete Floor Calculator |
| Walkway | Long narrow rectangle | Concrete Walkway Calculator |
Small rectangular pours can be converted from cubic feet to bag quantity. The calculator uses approximate yields of 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag.
Manufacturer yields differ, so verify the actual product label. For larger bagged-concrete work, use the Concrete Mixer Capacity Calculator to plan bags per batch and mixing cycles.
Concrete weight is useful for demolition, lifting, hauling and dead-load planning. The calculator multiplies cubic feet by a selected density.
For more conversions, use the Concrete Volume to Weight calculator.
Enter a local ready-mix price per cubic yard for a basic material estimate. Delivery, short-load charges, reinforcement, pumping, finishing and labor are not included.
For truck and fee planning, use the Concrete Delivery Calculator.
Exact volume assumes perfectly uniform forms and subgrade. Real projects can require slightly more concrete because of grade variation, form movement, over-excavation, thickness differences and material left in equipment.
The calculator includes several selectable allowances so you can compare exact geometry with a practical planning quantity.
For concrete design, placement and construction guidance, consult the American Concrete Institute (ACI) and National Ready Mixed Concrete Association (NRMCA). Verify final dimensions, thickness, reinforcement, supplier increments and project specifications before placing concrete.
Good concrete estimates begin with finished dimensions. Measure length and width at more than one location when forms or existing edges are not perfectly straight. For a new slab, measure inside the forms rather than outside form dimensions. If one side varies, divide the slab into smaller rectangles instead of using a single average dimension that may hide a large difference.
Thickness deserves the same attention. A nominal 4-inch slab can consume more concrete when the subgrade is uneven or when edges are intentionally thickened. If a garage floor has a 4-inch field with a 12-inch thickened edge, calculate those zones separately. This creates a more useful takeoff than applying the thickened dimension to the entire slab.
The calculator is optimized for feet and inches because these are common on U.S. concrete projects, but the same geometry works in metric units. A rectangular slab measured in meters produces cubic meters when length, width and thickness are all expressed in meters.
| Conversion | Equivalent | Use |
|---|---|---|
| 12 inches | 1 foot | Convert slab thickness to feet |
| 27 ft³ | 1 yd³ | Ready-mix ordering |
| 1 meter | 3.28084 ft | Metric length conversion |
| 1 m³ | 1.30795 yd³ | Metric-to-U.S. volume conversion |
| 25.4 mm | 1 inch | Thickness conversion |
When doing a manual metric calculation, multiply length in meters × width in meters × thickness in meters. Do not mix millimeters and meters in the same multiplication without converting first.
Not every rectangular slab has one constant depth. Thickened edges, equipment pads, ramps, door thresholds and load-bearing zones can add substantial volume. The safest method is to calculate the main slab first, then calculate each additional thickened zone as a separate rectangular prism.
For example, if a 20 × 20 ft slab is 4 inches thick but contains a 4 × 6 ft machinery pad that is 8 inches thick, do not calculate the pad at the full 8 inches in addition to the slab unless you subtract the original 4-inch portion already counted. Instead, calculate the whole slab at 4 inches and add only the extra 4 inches of thickness within the machinery-pad footprint.
After the rectangular volume is calculated, translate the mathematical quantity into a practical order. Ready-mix producers can have minimum loads, accepted order increments, delivery charges and truck-capacity limits. A result such as 3.263 cubic yards may need to become 3.5 yards or another supplier-approved quantity.
For larger pours, compare the planned order with truck capacity and placement speed. Use the Concrete Delivery Calculator to estimate truck loads, partial final loads, delivery intervals and optional short-load fees. Before the first truck arrives, review access, forms, reinforcement, crew, pump or chute reach and curing materials with the Concrete Pouring Checklist.
The rectangular formula applies to many common construction elements beyond ordinary slabs. A parking pad, shed base, equipment foundation, sidewalk section, floor topping and rectangular footing can all be represented as rectangular prisms when their dimensions are constant.
| Example | Dimensions | Method | Planning Note |
|---|---|---|---|
| Shed Base | 10 × 12 ft × 4 in | Single rectangle | Check edge thickening |
| Parking Pad | 20 × 20 ft × selected depth | Single rectangle | Confirm design thickness |
| L-Patio | Two rectangles | Add sections | Do not overlap areas |
| Equipment Pad | Small thick rectangle | L × W × T | May need separate reinforcement |
| Strip Footing | Length × width × depth | Rectangular prism | Use Footing Calculator |
Visualize length, width and thickness — the three dimensions used to calculate rectangular concrete volume.
Answers about rectangular slabs, cubic yards, multiple sections, concrete bags, weight and cost.