National Council of Teachers of Mathematics
The Math Forum: Mass vs. Weight
This site gives an explanation that distinguishes mass from weight. It distinguishes between the two types of mass (inertial and gravitational), and gives the equations for computing both.
Primary Resources
Primary Resources: Measures
A huge collection of teacher resources to aide in the teaching of all types of metric measurement: length, distance, mass, weight, capacity, and volume. Activities include measuring, estimating, converting, investigating, and finding...
Ducksters
Ducksters: Kids Math: Units of Measurement Glossary and Terms
Math glossary and terms on units of measurement for kids. Learn the definitions used in this mathematics subject such as time, temperature, length, and distance.
Other
Online Conversion
Conversion calculators galore! Convert just about anything from one measureable unit into another measureable unit here with over two dozen calculators. Each of the links is specific to a measureable quantity. Some examples include...