About FreeToolbox24 & how we calculate

FreeToolbox24 is a small set of free calculators for home improvement and DIY projects. Each one answers a single practical question – how much paint, how many tiles, how many bags of concrete – before you spend money.

Our method

The FreeToolbox24 team maintains the calculator definitions and their guides. We use a team attribution and do not claim a named engineer or a professional certification for these estimates.

How to verify a calculation

The last tested date on a calculator records an automated comparison of its JavaScript result with the expected values in a saved test case. For paint, the metric example has a paintable area of 35.3 m² and a rounded paint requirement of 7.8 L. Unit conversion checks compare the forward calculation with a separate Python definition and check the inverse calculation.

The report is linked to the calculator's inputs, formula, instructions and expected results. Rebuilding the site does not advance the test date. Changing those definitions invalidates an old result. These are calculation checks, not on-site measurements, structural inspections, manufacturer guarantees or tests of every browser. When no matching report exists, no test date is displayed.

You can reproduce a worked example by entering the stated values, calculating each step by hand and comparing the unrounded estimate with the displayed result. Check product coverage, dimensions and yields against your own label before using an estimate for purchasing.

Formulas and assumptions

• Formulas shown on every page. Each calculator states the exact formula, the assumptions (such as the area of a standard door) and a worked example you can check by hand.
• Metric and imperial. Every calculator accepts metres and feet. Conversions use exact factors (1 ft = 0.3048 m, 1 US gallon = 3.785 L, 1 yd³ = 0.7646 m³).
• Waste margins. Real projects lose material to cuts, breakage and uneven surfaces. Where it matters we add a stated margin (usually 5–15%) and tell you what it is.
• Manufacturer data first. Coverage rates, bag yields and product sizes are typical values from common products. Your product label always wins – most calculators let you enter your own figure.

Limits

Results are estimates for planning and shopping. They are not structural, electrical or engineering advice. For load-bearing structures, roofs, retaining walls over 1 m (3 ft) and anything covered by local building codes, check with a qualified professional and your local authority.

Corrections

If an estimate differs from your measurements, compare the stated assumptions and product values first. Calculation test dates describe saved automated checks, not a promise of regular manual review.