How we calculate

Every number on CalcTheBuild should be something you can check. This page explains how our calculators work, where their data comes from and how we test them and the guides that use them.

How the calculators work

• One set of formulas, two unit systems. Every calculation runs internally in metric units (meters, millimeters, square and cubic meters, liters). Imperial inputs are converted on the way in and results on the way out, so the same formula gives the same answer in feet or meters.
• Standard geometry and published formulas. Areas and volumes use ordinary geometry. Roof surfaces use the pitch factor √(1 + (rise ÷ 12)²). Mortgage payments use the standard amortization formula. Each calculator page explains its formula in plain English.
• Manufacturer and industry data. Where a result depends on a product, such as bag yields for concrete and mortar, bundles per roofing square, paint coverage or insulation R-value per inch, we use published manufacturer figures or typical industry values and say so on the page.
• Explicit waste allowances. Where a waste allowance applies, it's a field you can change, and most material calculators show the quantity both with and without it.
• Default prices are placeholders. Prices pre-filled in the calculators are rough planning figures. Replace them with quotes from your local suppliers.

How we test them

• Before a calculator is published, we check its results against hand calculations for several examples in both imperial and metric units.
• Worked examples in our guides are produced by running the same calculator code with the same inputs. If a guide says a 10 × 10 ft slab needs 62 bags of 80 lb concrete, that's what the concrete calculator returns for those inputs.
• When a guide quotes a manufacturer's chart that differs from a calculator's typical value, for example a specific insulation product, we show both and explain the difference.

How we choose and check sources

• We prefer primary sources: building codes (such as the International Residential Code), government agencies (ENERGY STAR, the US Department of Energy, the Consumer Financial Protection Bureau, the Bureau of Labor Statistics), industry associations (such as the National Ready Mixed Concrete Association) and manufacturers' own data sheets.
• Cost figures come from published cost guides or price indexes and are labeled as estimates, with the date or period they apply to.
• Every external link is checked when an article is published, and each guide lists its sources at the end.
• Fast-changing figures, such as mortgage rates, are dated in the text so you know how current they are.

Limitations

Our calculators and guides are estimating tools for planning and budgeting. They are not engineering, architectural or financial advice. Site conditions, local building codes and specific products can change the right answer. For structural work, foundations and anything that needs a permit, check with a licensed professional and your local building department.

Corrections and updates

If you spot a mistake or a figure that has gone out of date, please email hello@calcthebuild.com or use our contact page. We review every report, fix confirmed errors promptly and update the page date when we make a substantive change.