Methodology
This page explains how each number on the site is calculated, which constants it uses and where they come from, so you can check our work.
Principles
- One formula engine drives every calculator, guide, chart and size page.
- Lengths are converted to feet first, exactly (inches ÷ 12, centimeters ÷ 30.48, meters ÷ 0.3048), and rounding happens only at display.
- Constants live in one data file, each with a source. Constants with no reliable source are not published.
- Results are estimates. Order quantities are rounded up.
Formulas
| Shape | Formula |
|---|---|
| Slab, wall, strip footing | L × W × T × n (wall: minus openings) |
| Round column, sonotube, round slab | π × (D ÷ 2)² × H × n |
| Tube | π × (R² − r²) × L × n |
| Stairs | W × run × rise × N(N + 1) ÷ 2, plus landing L × W × T |
| Curb and gutter | Cross-section area × run length |
| Post hole | π × (hole D ÷ 2)² × depth, minus the post |
| Blocks | Wall area × 1.125 (8×16 in face) |
| Rebar | Bars = floor(span ÷ spacing) + 1 in each direction; weight = length × lb/ft |
| Gravel | Area × thickness, × (1 + compaction), × tons per yd³ |
| Mix | Dry volume = wet × 1.54, split in proportion to the ratio |
Stair volume assumes solid steps resting on the ground. Stairs on a sloped slab or with an open underside use less. The wall, footing and slab formulas assume a uniform cross-section.
Waste
Waste defaults to 10% and can be set from 0 to 25%. Typical advice is 5 to 10%, and more for trenches and uneven ground. The order quantity is the net volume × (1 + waste). Bag counts are rounded up to whole bags.
Constants and where they come from
| Constant | Value | Source |
|---|---|---|
| Bag yield 80 / 60 / 50 / 40 lb | 0.6 / 0.45 / 0.375 / 0.3 ft³ | QUIKRETE data sheet |
| Bags per pallet 80 / 60 / 50 lb | 42 / 56 / 64 | Retail and distributor listings |
| Concrete density | 150 lb/ft³ (range 140 to 150) | PCA; NRMCA CIP 36 |
| Rebar weight #3 / #4 / #5 / #6 | 0.376 / 0.668 / 1.043 / 1.502 lb/ft | ASTM A615 |
| Rebar stick length | 20 ft | Common stock length |
| Blocks per ft², 8×16 face | 1.125 | NCMA TEK 2-1B; CMHA TEK 4-2A |
| Gravel tons per yd³ | 1.4 (editable) | Typical value; ask your supplier |
| Dry-volume factor | 1.54 | Common range 1.52 to 1.57 |
| Loose cement density | 1440 kg/m³ | Common value |
| Joint spacing | 24 to 36 × thickness, max about 15 ft | NRMCA CIP 6; ACI 302.1R |
Home page calculator
The calculator on the home page is the original five-module version (slab or square, cylinder, tube, curb and stairs). It is metric internally and uses a density of 2,130 kg/m³ (about 133 lb/ft³), and it counts bags as weight divided by bag weight. The calculators on the tool pages use 150 lb/ft³ and the bag yields from the manufacturer. As a result, the two can differ by a few percent for the same shape. Both are estimates; use the one that suits you, and check the quantity with your supplier.
Prices
Prices are shown as ranges, never as a single number, and are labeled with the date they were last verified (2026-10-04). Ready-mix, delivery and short-load figures come from published price guides listed below. The 80 lb bag range is anchored on retail listings seen on 2026-10-04; the ranges for 40, 50 and 60 lb bags are estimates scaled from it. Prices vary by region, supplier, season, strength and order size. Treat them as a sanity check and get a local quote.
We review the figures at least quarterly, and replace them when a reliable newer source is available. Pages that show prices carry the verification date.
Limitations
- We estimate the volume of idealized shapes. Real forms, bases and soils differ.
- Reinforcement, base, labor and extras are not part of the concrete volume.
- Local codes and engineering requirements override anything on this site.
- Results do not replace the advice of an engineer or a qualified contractor.
Corrections
Found an error? Email hello@concretecalculator.org. Confirmed errors are fixed and the page’s updated date changes.
Sources
- QUIKRETE Concrete Mix No. 1101 data sheet (bag yields)
- Retail/distributor listings for QUIKRETE 1101-80 (42 per pallet) and 1101-60 (56 per pallet)
- PCA, Design and Control of Concrete Mixtures - Aggregates for Concrete (normal-weight concrete 140-150 lb/ft3)
- NRMCA CIP 36 - Structural Lightweight Concrete (90-120 lb/ft3 vs 140-150 lb/ft3)
- ASTM A615/A615M, Table 1 - nominal weights of deformed bars
- ACI 302.1R, Guide to Concrete Floor and Slab Construction (joint spacing 24-36 x thickness)
- ACI 308R-16, Guide to External Curing of Concrete
- NCMA TEK 2-1B, Typical Sizes and Shapes of Concrete Masonry Units (nominal 8 x 16 in face, 3/8 in joint)
- CMHA TEK 4-2A, Estimating Concrete Masonry Materials (119 units per 100 ft2 including 5% waste)
- NRMCA CIP 6, Joints in Concrete Slabs on Grade (24-36 x thickness, 4 in slab about 10 ft, 15 ft maximum)
- PCA, Design and Control of Concrete Mixtures, Chapter 12: Curing Concrete (7-day minimum above 40 F)
- VRMCA Technical Bulletin 6, Residential flatwork: IRC 3,500 psi and ACI 332 4,000 psi, air entrainment, joint spacing
- NIST Guide to the SI, conversion factors (1 ft = 0.3048 m exactly)
- Concrete Calculate - How Much Does Concrete Cost Per Yard Delivered? (2026)
- Concrete Network - concrete cost overview (2025 average about $180 per cubic yard)
- AllConcreteCalculator - concrete delivery cost and short-load fees (May 2026)
- Home Depot - Quikrete 80 lb. Concrete Mix, model 110180