For finding equivalent sizes of units within the category named

Length 
Area 
Volume 
Mass 
Temperature 
Feet & Inches 
Angles 
Pounds & Ounces 
 Units of Alcohol 

Density 
Pressure & Stress 
Speed 
Fuel Consumption 
Power 
or ONE calculator just for Changing Prefixes 
Energy (Work)  Flow Rate by Mass by Volume 
Force 
Torque 
Specific Energy
by Mass
by Volume (Calorific Value)  Spread Rate by Mass by Volume (including Rainfall) 
Concentration 
Line Density (inc. Textiles) 
Area Density 
Acceleration 
Viscosity Dynamic Kinematic 
Specific Heat Capacity
by Mass
by Volume 
Heat Flux Density

Thermal Conductivity

Thermal Conductance 
For a comprehensive account concerning units of measurement see the Dictionary of Units 