A litre of paint claiming 12-14 m² coverage means one thin, even coat on primed drywall — dark-to-light changes, bare plaster, or textured surfaces routinely need a second coat, doubling real consumption.
Manufacturer coverage figures are measured under ideal lab conditions — real walls absorb unevenly and colour changes need full opacity, not just an even film, which is why buying "exactly enough" paint so often falls short.
Estimate with the rule, then check it against the calculator that models it properly.
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