Most dictionaries define the more common types of weights and measurements (kilos, feet, hectares, etc.). Most dictionaries also have a Weights and Measures chart that shows the relationship between the metric and classical systems. For a substantial list of conversion multipliers, check out http://www.wsdot.wa.gov/reference/metrics/factors.htm.
To convert almost any weight or measure to any other, use the MegaConverter. It covers: time, length, volume, weight, angles, area, distance, energy, heat, temperature, compound interest, force, mass, metric measurements, speed, power, pressure, wind chill and liquor quantities (among other things).