If you squeeze together all the black in a regular black and white copy of a double-spaced letter, for example, its coverage will be 6 percent. However, color copying typically covers much more of the page than a standard letter. Instead of 6 percent coverage, you should base the estimated copy yield on somewhere between 25 percent and 35 percent coverage. Quite a few manufacturers quote coverage levels that are impractically low. Developer Toner needs to be mixed with another substance called developer to be magnetically attracted to the copier drum and then fused on to paper.
Most copiers require a separate developer for each toner color.
The yield of the developer is directly related to copy volume, so coverage isn't important. Developer usually has a yield that is twice that of the toner. When copies seem to get lighter, it's usually time to change the developer. When you run out of toner, on the other hand, you usually see an overall unevenness in color. One bottle will typically last twice as long as a bottle of developer.