6.5ml is equivalent to 1.375 US teaspoons.
5ml = 1 teaspoon 1ml = 1/4 teaspoon 0.5ml = 1/8 teaspoon
6.5ml : 1 teaspoon + 1/4 (0.25) teaspoon + 1/8 (0.125) teaspoon = 1.375 teaspoon
Milliliters is a metric unit of volume equal to one thousandth (0.001) of a liter. Teaspoons have several different kinds of measurement available - US, UK and Metric.
You can also check Metric Conversions if you need to convert more.
The metric system originated in France in 1799 following the French Revolution although decimal units had been used in many other countries and cultures. There have been many different measurements and the definitions of the units have been revised and the official system of measurements of most countries is the modern form of the metric system which is known as the "International System of Units".