How to measure
Measure the finished slab length and width, then record depth in several locations where the base may vary. Split steps, thickened edges, grade beams, or irregular outlines into separate shapes and add their volumes.
Concrete calculator
Concrete Slab Calculator
Estimate concrete volume and ready-mix requirements for a rectangular slab. Switch between US customary and metric units, adjust the assumptions, and keep every step visible.
ReviewedNovember 2, 2026
ScopeFormula, defaults & sources
Report an issue ↗The calculation
A visible formula, not a black box.
Volume & bags · Mathematical derivationvolume = length × width × depth × (1 + waste allowance)
Inputs are converted to a common base unit before calculation. Purchase quantities are rounded only at the final buying boundary.
Example inputDefaultStatus
Slab length18 ftReplace with measurement
Slab width12 ftReplace with measurement
Slab depth4 inPlanning default — verify
Waste allowance10 %Planning default — verify
Worked example
Example: an 18 × 12 ft pad
At 4 in deep with a 10% planning allowance, the calculator returns about 2.93 yd³. Confirm thickness and order increments before purchase.
Use this to understand the workflow—not as a recommendation for your project.Sources & scope
What supports this calculation.
Manufacturer dataQUIKRETE Concrete Mix technical data ↗
Published package yields and the named product's slab-use scope; it does not set a universal slab thickness.
Mathematical derivationNIST SI conversion factors ↗Exact length, area, and volume unit conversions; the project formula is derived from standard geometry.
Common mistakes
Check the job, not just the number.
- 01Leaving the example concrete slab inputs unchanged instead of entering measured project values.
- 02Treating an editable coverage, size, density, or allowance as universal instead of checking the selected product label, project documents, and local requirements.
- 03Ordering from the rounded result without reviewing exclusions: Use finished dimensions. For a sloped or irregular slab, split the project into simple sections and estimate each separately.
Questions, answered
Concrete slab FAQ
What does the Concrete Slab Calculator estimate?
Estimate concrete volume and ready-mix requirements for a rectangular slab. The primary output is volume & bags, with the intermediate assumptions kept visible.
How is the concrete slab result calculated?
The visible formula is: volume = length × width × depth × (1 + waste allowance). Inputs are converted to a common base unit before the final purchase boundary is rounded.
Are the default values recommendations?
No. Dimensions are worked examples and product-dependent presets are planning defaults. Replace them with the selected product label, project documents, and local requirements.
Is the result ready to order from?
Use it as a checked planning estimate, then confirm packaging, minimum orders, installation instructions, site conditions, and any code or professional-design requirements.
Project notes
Measure, adjust, and verify.
Details specific to concrete slab planning.
What the result includes
The calculator converts dimensions to one base unit, multiplies length × width × depth, applies the chosen allowance, and rounds the suggested ready-mix quantity to the displayed buying increment.
When to adjust allowance
Use an allowance that reflects excavation tolerance, base variation, spillage, and supplier minimums. Do not increase depth merely to make the material estimate feel safer; thickness must come from the project design.
Limitations
The calculation excludes reinforcement, subbase, vapor barriers, joints, forms, pump or chute access, truck capacity, and structural design. Slopes and nonuniform thickness require separate sections or an average-depth method checked on site.