How every number on this site is computed

The chain

Every calculator follows the same chain: shape → quantities → local prices → line items → total → confidence band. Each step is deterministic, so the same inputs always produce the same output, and every step can be checked.

Step 1: Quantities

For a slab the engine computes:

• Area = length × width.
• Concrete volume = area × thickness ÷ 27 (cubic yards), with a waste allowance added separately (10% for bagged mix, 5% for ready-mix delivery).
• Bag counts assume 0.60 cubic feet of yield per 80-lb bag and 0.45 cubic feet per 60-lb bag, rounded up.
• Truckloads assume 10 cubic yards per truck; orders below a configurable threshold are flagged as likely to carry a short-load fee.
• Rebar on an 18-inch grid: linear feet = (length ÷ 1.5 + 1) × width + (width ÷ 1.5 + 1) × length. Wire mesh is priced by area.
• Gravel base tonnage = area × (depth ÷ 12) ÷ 27 × 1.4.

Project presets (foundation, driveway, patio, shed, garage) change defaults and add items such as footings, vapor barrier, thicker edges, aprons or demolition.

Step 2: Local prices

Each line item has a material cost per unit and a labor time per unit. Labor time is multiplied by the hourly wage for the relevant occupation from the Bureau of Labor Statistics Occupational Employment and Wage Statistics survey, then by 1.35 for crew costs and by 1.20 for overhead and profit. Those two multipliers are our estimates. Your ZIP code is mapped to its metro area and the metro wage is used; where BLS publishes no metro wage, the state wage is used, then the national one. The page tells you which level was used; we never fall back silently.

Ready-mix concrete is the one sourced material price: the National Ready Mixed Concrete Association's 2023 average, carried forward each month with the BLS producer price index for ready-mix (details on the data sources page). Every other material price is our estimate and is labeled "estimated" wherever it appears. Estimates are replaced as prices get sampled or as tradespeople correct them.

Step 3: Range and confidence

The engine produces a midpoint and a band around it. The band is about ±15% when a metro wage is used, ±20% for a state wage and ±25% for the national one. The confidence level is shown as a badge next to the result.

Step 4: Sanity guards

A check in our test suite prices a plain 4-inch broom-finish slab at the national level and compares it with the per-square-foot ranges published by HomeGuide, Concrete Network and Homewyse. If our number falls outside those ranges, the check fails and we correct the estimates.

Quote checks

When you check a quote, the engine compares your price with the range for your inputs and returns a verdict: unusually low, fair, slightly high or high. It also lists the included and excluded items that most often explain a gap. Quote checks are stored without any personal data. They count toward whether a city or state has enough local data for a page of its own; they don't change prices.

What we do not claim

An estimate is not a bid. Site conditions, access, permits and contractor availability move real prices. Use our numbers to plan, to compare quotes and to ask better questions, not as a contract price.

See the data sources page for the datasets, their release dates and licenses, and the editorial policy for how pages are made and corrected.