Here is the software I use - I think it is fairly old, but it is small, and works instantly:
http://www.joshmadison.com/softwareIt is is called "Convert", version 4.10
It converts variables for pressure, speed, temperature, time, torque, volume, volume-dry, acceleration, amount of substance, angle, area, computer, concentration, custom, density, distance, energy, flow, force, light, mass, and power (just reading the tabs - I haven't investigated all of them).
Hope this helps.
I'd love to include a screenshot, but can't.