I find I can't do without Master Converter, which is a shareware program you can download for free:

Savard Software - Master Converter

Ignoring the dialogue box in the following screen capture, you just enter the amount in one set of units on the left side of the screen, and the program will automatically give you the equivalent in other units on the right side of the screen. You can enter the units you have as either a number or a formula that the program will evaluate.



The program has 42 different catagories; everything from length to luminescence to kinematic viscosity to capacitance to data transfer rate.

When I go to the "volume" category of my version of Master Converter and punch in 28*16*18 and highlight "cubic inches" on the left side, it tells me the equivalent volume on the right side is:

34.9090909... US gallons
29.06789701... Canadian gallons
132145.2841 cubic centimeters
4468.363636... fluid ounces
8936.727272... US tablespoons
etc. etc. etc.

And, referring to the screen capture, there actually is a unit of length called the "twip", and it's 1/1440th of an inch.
http://en.wikipedia.org/wiki/Twip

It's a handy little program to keep on your desktop, and I find myself using it all the time.