Concrete Slab Volume Calculator
Use this estimator to calculate the concrete volume needed for a slab and to approximate material counts and cost. Enter length and width in feet and slab thickness in inches; the tool applies a user-configurable waste allowance to support ordering and planning.
Outputs include cubic feet, cubic yards, and cubic meters, with optional bag-count approximations for 60 lb and 80 lb premixed bags, an estimated in-place weight, and a cost estimate per cubic yard. These approximations use standard engineering conventions for planning—final ordering should reference ready-mix supplier tickets and project specifications.
Inputs
Results
Volume (cubic feet)
36.6667
Volume (cubic yards)
1.358
Volume (cubic meters)
1.0383
60 lb bags (approx.)
81.4815
80 lb bags (approx.)
61.1111
Estimated weight (pounds)
5,500
Approximate concrete cost
$169.75
| Output | Value | Unit |
|---|---|---|
| Volume (cubic feet) | 36.6667 | ft³ |
| Volume (cubic yards) | 1.358 | yd³ |
| Volume (cubic meters) | 1.0383 | m³ |
| 60 lb bags (approx.) | 81.4815 | bags |
| 80 lb bags (approx.) | 61.1111 | bags |
| Estimated weight (pounds) | 5,500 | lb |
| Approximate concrete cost | $169.75 | USD |
Visualization
Methodology
Volume calculation follows standard geometric volume: area × thickness, using inches → feet conversion for slab thickness. Waste allowance multiplies the computed volume by (1 + waste_percentage / 100).
Bag counts are approximated from typical bag yields (60 lb ≈ 0.45 ft³; 80 lb ≈ 0.6 ft³). Estimated weight uses a common nominal unit weight for normal-weight concrete of 150 lb/ft³; individual mixes or lightweight mixes will vary.
Cost is estimated by multiplying cubic yards by the user-provided cost per cubic yard. For project procurement, confirm final quantity and price with suppliers and factor delivery minimums and slumps per specification (see ACI/ASTM references below).
Expert Q&A
How accurate are bag-count estimates?
Bag-count outputs use typical bag yields (60 lb ≈ 0.45 ft³; 80 lb ≈ 0.6 ft³). These are approximate; actual yield varies by mixing water, compaction, and batch accuracy. For critical pours use volume in cubic yards or meters and consult supplier yields or mix design.
Why does the tool use 150 lb/ft³ for weight?
150 lb/ft³ is a widely used nominal unit weight for normal-weight concrete and is suitable for planning. Actual unit weight depends on aggregate type and moisture; consult project specifications or material data sheets for precise structural calculations.
How should I choose waste allowance?
Typical waste allowances range from 5% for small, controlled pours up to 15% for complex forms or variable site conditions. Recommended practice: 5–10% for simple slabs, 10–15% for irregular shapes or when pumping/placement waste is expected.
Can I rely on the cost estimate for bidding?
This cost is a planning-level estimate using the entered cost per cubic yard. It does not include delivery minimums, admixtures, finishing labor, reinforcement, formwork, or disposal. Use supplier quotes and project cost breakdowns for bids.
What factors require adjustment beyond this tool?
Adjust for reinforcing steel, structural toppings, subbase compaction, pump waste, slump and admixtures, temperature or cold-weather measures, and safety/inspection requirements. Structural design decisions should reference ACI codes and an engineer of record.
Sources & citations
- American Concrete Institute (ACI) — Concrete practices and design guidance — https://www.concrete.org
- ASTM International — Concrete and concrete aggregates standards — https://www.astm.org
- National Institute of Standards and Technology (NIST) — Units and conversion references — https://www.nist.gov
- Occupational Safety and Health Administration (OSHA) — Construction industry safety guidance — https://www.osha.gov
- Federal Highway Administration (FHWA) — Concrete material guidance and specifications — https://www.fhwa.dot.gov
- MIT OpenCourseWare — Materials and structures educational resources — https://ocw.mit.edu