The ISO standard
Cartridge yields are measured on standardized pages at exactly 4.5–5% coverage — a lightly-dressed memo. Real documents (emails with signatures, homework, maps) run 8–15%, and photos run 50%+. The box number is true for the test page and optimistic for your life — the 20% discount in these calculators models the gap.
The invisible consumption
Head-cleaning cycles consume ink directly; printers also "protect" nozzles by purging during idle periods. A printer used weekly spends little on maintenance; a printer used monthly spends it on cleaning instead of pages. Infrequent use is the hidden cartridge tax — which is why laser (powder, no nozzles) wins the low-volume reliability game.
Reading yields like a buyer
Compare cartridges within a printer family by cost per honest page (price ÷ ISO yield × 0.8) — high-yield cartridges almost always beat standard ones per page, often dramatically. And compare printer FAMILIES by the same number: the cartridge schedule is the real product spec the box does not print.