Estimate ink used for documents.
Quoted cartridge yields assume five percent page coverage, which is roughly a page of double-spaced text. Real documents with headers, tables or graphics run at eight to fifteen percent, and coverage scales the yield inversely: doubling coverage halves the pages you get. Correcting the rated yield for your actual coverage is what turns an optimistic 300-page claim into a realistic figure.
Coverage-adjusted yield
Real yield = rated yield x 5 / actual coverage percent
Running cost
Cost per page = cartridge price / real yield; annual cost = pages per month x 12 x cost per page
Two reasons: the rated yield assumes five percent coverage, and inkjets consume ink on start-up and cleaning cycles. Infrequent printing makes maintenance cycles a large share of total consumption.
Almost always on cost per page, often by thirty to fifty percent. They only lose if the ink dries out before you use it, which affects very low-volume users.