Convert It! is probably the most comprehensive and customizable unit conversion utility available for Windows. It divides the unit conversions into 35 groups with up to 30 units per group. Convert metric to customary (or standard) and from customary to metric.
The groups are: temperature, distance, mass, volume, angle, area, power, energy, pressure, time, force, density, velocity, money, acceleration, angular acceleration, angular velocity, capacitance, charge, current, voltage, inductance, magnetic flux, illuminance, luminance, specific heat, thermal conductivity, torque, viscosity, coverage, SI units, radioactivity, radiation dose, absorbed radiation dose, and radiation density. Convert It! even allows you to add your own groups, up to 30.
You can customize the groups by turning off units and/or groups you don't need. You can even add your own custom groups. If you find a conversion factor that you want to change, you'll find that just a couple of mouse clicks get you there. Want to add a conversion factor? That feature is there too. Your needed metric conversion will always be just a couple of clicks away.