Methodology and sources

This page explains exactly how every calculator on Free Concrete Calculator works: the formulas, the constants and where they come from, how results are rounded, and how we test them. If a number on this site ever disagrees with this page, the page is the error. Please tell us.

1. How every calculation is done

All calculators share one tested library of functions. Every dimension you enter is first converted to feet, the volume is computed in cubic feet, and that single figure is converted to every unit we show. Because one volume feeds every result, the cubic yards, cubic meters, bag counts, tons and weights on a page always agree with each other.

• Units per field: each dimension on the concrete calculator has its own unit menu (in, ft, yd, cm or m), so you can mix units, for example length in feet and thickness in inches. The US/Metric switch only sets the starting units.
• Feet and inches: inches ÷ 12 = feet; yards × 3 = feet.
• Metric input: meters × 3.28084 = feet; centimeters ÷ 100 × 3.28084 = feet.
• Cubic yards: cubic feet ÷ 27 (one cubic yard is 3 ft × 3 ft × 3 ft).
• Cubic meters: cubic feet × 0.0283168.
• Shortcut used in our guides: length (ft) × width (ft) × thickness (in) ÷ 324 = cubic yards, because 12 × 27 = 324.

2. Shape formulas

Steps are treated as solid concrete on grade. On a monolithic slab the edge depth is the extra depth below the slab, and the edge is measured along the inside of the outline so the corners are counted once. The curb and gutter shape is a rectangular curb plus a flat gutter pan; real curb profiles with sloped faces or rounded edges will differ, so use the dimensions from your plan. Post holes subtract the part of the hole the post occupies, using the actual size of the post (a 4×4 is 3.5 × 3.5 in).

3. Bags of premixed concrete

Bag counts divide the volume in cubic feet by the yield of one bag and always round up to a whole bag. Yields are the figures published by QUIKRETE for its standard Concrete Mix (No. 1101) on its product data sheet:

Source: QUIKRETE Concrete Mix data sheet.

On the Sakrete calculator page, the calculator uses the yields from the SAKRETE High-Strength Concrete Mix data sheet instead: 40 lb 0.30 ft³, 60 lb 0.45 ft³, 80 lb 0.60 ft³ and 90 lb 0.66 ft³ (18.7 L; 27 ÷ 0.66 ≈ 40.9, so 41 bags per cubic yard). That data sheet lists no 50 lb bag, so the Sakrete calculator does not offer one. Other products, including fast-setting mixes, toppings and self-leveling mixes, have different yields, so check the bag.

Mortar: the mortar calculator uses the QUIKRETE Mortar Mix data sheet figure of up to 37 standard bricks or 13 standard 8 × 8 × 16 in blocks per 80 lb bag (source).

4. Concrete weight

Wet-weight results use 150 lb per cubic foot for normal-weight concrete, which gives about 4,050 lb per cubic yard (about 2,400 kg per cubic meter). Real normal-weight concrete typically weighs about 140 to 155 lb/ft³ (2,240–2,480 kg/m³) (FHWA). For bridge dead loads, WSDOT uses 150 lb/ft³ for plain concrete and 155 lb/ft³ for reinforced concrete. QUIKRETE lists about 140 lb/ft³ for its bagged Concrete Mix. The concrete calculator lets you switch the density to 150, 145 or 140 lb/ft³. Where weight matters (trailer loads, structural checks), use your supplier's figure.

5. Gravel, stone, soil, sand and mulch

Tons are cubic yards multiplied by a typical loose bulk density for the material. Densities vary with moisture, grading and compaction. The gravel, stone, soil and mulch figures below are typical planning values, not published standards. The dry-sand figure comes from the QUIKRETE Mason Sand data sheet: a 50 lb bag holds about 0.5 ft³, which is about 100 lb/ft³ or 1.35 tons per yd³. The concrete and asphalt rows are explained in section 4 and below the table. Each calculator shows the density it used, and you should use your supplier's figure when they give one.

The asphalt figure of 145 lb/ft³ (108.75 lb per square yard per inch) falls inside the 144–160 lb/ft³ (108–120 lb per square yard per inch) range for compacted hot-mix asphalt given in the Pennsylvania Asphalt Pavement Association's Constructing Quality Asphalt Pavements checklist (2025, hosted by Penn State). It is not a measured value for your mix; actual density depends on the mix design and compaction, so use the density on your plant's quote when you have it.

Bagged materials (mulch, soil, sand, gravel) are counted by dividing cubic feet by the bag size and rounding up.

6. Masonry, rebar and mix design

• Concrete blocks: a standard 8 × 8 × 16 in block has a nominal face of 16 × 8 in (including a ⅜ in mortar joint), so a wall needs 1.125 blocks per square foot of face, less any openings.
• Rebar: bar weights are the nominal values published on the Gerdau straight rebar data sheet (ASTM A615 bar sizes): #3 0.376, #4 0.668, #5 1.043, #6 1.502, #7 2.044 and #8 2.67 lb per foot. Grid layouts are bars each way at your spacing, inset by your edge cover, plus a lap allowance.
• Mix ratios: the volume method is a rough planning estimate. It uses two calculator assumptions that we have not verified against a standard or a manufacturer data sheet: a dry-volume factor of 1.54, and about 1 ft³ for one 94 lb bag of Portland cement. For structural work, use a mix design from your supplier or engineer.

7. Rounding, waste and ordering

• Waste allowances are added before rounding. The concrete calculator's waste slider runs from 0 to 20% and starts at 10%.
• This calculator rounds ready-mix orders up to the next ¼ cubic yard. Ask your plant what increment and minimum load it sells.
• Bag counts are always rounded up to whole bags.
• Displayed values are rounded for reading (usually two decimals); calculations keep full precision.

8. Prices

We do not publish national average prices as facts. Prices for concrete, delivery, aggregates and labor vary by region, season, mix and order size. Cost calculators use the prices you enter from your own supplier and contractor quotes, and the guides explain what drives the price and what to ask.

9. How we test

• Every formula has automated tests built from hand-worked examples (for example, a 12 × 12 ft slab 4 in thick = 48 ft³ = 1.78 yd³ = 80 bags of 80 lb), and the tests are run before changes are published.
• Worked examples, tables and FAQ figures in the guides are checked against the same library.
• Formulas and reference tables are reviewed by our technical reviewer, M Bilal, before publishing.

10. Limitations

Results are estimates for planning and ordering. They do not account for every site condition: forms that bulge, uneven bases, over-excavation, compaction of aggregates, or material that stays in the truck and tools. Structural elements (footings, foundations, retaining walls, suspended slabs) must be designed to your local building code. See the disclaimer.

Last reviewed 2026-10-09. Questions about a calculation: hello@freeconcretecalculator.com.