Decaffeinated coffee is not non-caffeinated coffee. Decaffeinated beans go through a special roasting process which takes out about 50% of the caffeine. Most decaf beans also have chemicals thrown into this process, which gets the caffeine down to about 10-20% of the original amount of caffeine in the beans themselves.
A typical cup of coffee contains about 100mg of caffeine. You can find about the same amount of caffeine in a over-the-counter caffeine pill. Caffeine in its purest form is a white powder which is highly acidic and is deadly to inhale.
So if you get about 20% of the caffeine in a decaf cup of coffee, by the time you drink five cups, you have already had a cup of coffee for the day. You can't just keep drinking decaf thinking you aren't receiving caffeine all day at the office.
Tea has less caffeine than coffee, but not that much less. If you drink green tea, you will find yourself drinking about a third of the caffeine than black tea.
A typical cup of coffee on most brewing machines is six ounces. A typical cup of coffee used in this 100mg sample is an eight ounce mug. Be sure to know how much coffee your mug holds and how much caffeine you drink each day. Too much caffeine can lead to heart problems and many other health conditions.
Also, for you Starbucks advocates, Starbucks does not believe in adding chemicals to their decaf coffee beans. The decaf beans sold at Starbucks hold about 50% of the caffeine as regular beans.