Why 1 kg vs 1 kg is the wrong way to compare materials

It is tempting to line up cradle-to-gate carbon factors and declare a winner: PP is 1.92, steel HRC 2.31, primary aluminium 14.8 kg CO₂e/kg. But comparing materials per kilogram answers a question almost nobody has. You do not buy "a kilogram of material" — you buy a part, a package or a structure that has to do a job. A kilogram of PP and a kilogram of aluminium do very different jobs.

Function, not mass

The fair basis is a functional unit: the amount of each material needed to deliver the same outcome. Package 100,000 litres of a drink and you might need ~20 g of PET or ~300 g of glass per bottle — so the honest comparison weighs 2 t of PET against 30 t of glass, not 1 kg against 1 kg. Only once you fix the function can the numbers mean anything, and even then you must supply the real per-unit masses; MatQuo will not invent a substitution ratio for you.

Boundaries and methods must match too

Even at equal function, two datasets can be incomparable. A per-kg value and a per-m² value are different dimensions. A cradle-to-gate number (raw materials to factory gate) and a cradle-to-cradle number (which already folds in recycling) describe different life-cycle slices. Different LCIA methods or allocation rules add further caveats. This is why MatQuo's comparison calculator checks a comparability matrix and will refuse a numeric comparison — for example flat glass (per m²) versus PP (per kg), or PET (cradle-to-gate) versus container glass (cradle-to-cradle) — rather than show false precision.

Do it properly

Use a shared function, supply the real masses, keep production and freight separate, keep Module D (end-of-life credit) out of the production figure, and read the caveats. The result is a defensible statement like "under these assumptions, option A has X% lower estimated production + freight emissions" — never "material A is greener." Try it in the material comparison, packaging comparator and lightweighting calculators.