Many times, if units are involved in a GRE Math question, the question itself will supply the relative conversions. Sometimes, though, it expects you to have a few basic ones memorized. What should you know?
The very basic, very common conversions
Even the ones in this category, the test may supply, but sometimes it doesn’t. These are the ones you definitely should have memorized.
1 minute = 60 seconds
1 hour = 60 minutes
1 foot = 12 inches
1 mile = 5,280 feet
Also, the basic metric decimal conversions (all powers of ten)
1 m = 100 cm = 1000 mm
1 km = 1000 m
1 kg = 1000 gram
I would say: make sure you know these.
Some less common conversions
These are unit conversions used only in America, England, and some of the Commonwealth Nations. They are not used in the rest of the civilized world. It is highly unlikely that the GRE would expect you to know any of these, but because it’s even a remote possibility I will list them. If you want to be absolutely hyper-prepared in this particular category, if you believe that being over-prepared is always the lesser of two evils, then memorize these as well.
1 yard = 3 feet
1 pint = 16 ounces
1 quart = 2 pints
1 gallon = 4 quarts
Again, in all likelihood memorizing these will be absolutely unnecessary. If they appear at all, in all likelihood, the GRE will provide the conversion as part of the question. BUT, if you want to be hyper-prepared for every last hypothetical contingency, then go ahead and memorize them.
One unit that definitely will not appear without an explicit conversion factor is pounds, simply because of the sheer complexity of the issue —- there are avoirdupois pounds, troy pounds, London pounds, Merchants’ pounds, etc. etc. There are few people on the entire planet who have everything about pounds straight in their heads! The GRE will definitely not ask anything about “pound” without providing an explicit conversion!
Area and Volume Units
Here’s one thing you definitely need to know. They may expect you to know a common length unit conversion (1 ft = 12 in), or they may even give it to you, but the one thing they absolutely expect you to be able to do on your own: change a length-units conversion to a conversion for area-units or volume units. The basic idea is:
For example, 1 ft = 12 in, so that means:
and
Another example with metric units. We know 1 m = 100 cm. This means:
The GRE probably will tell you, but may expect you to know, that 1 cubic centimeter is the same thing as 1 milliliter. Therefore,
Again, the equivalence of 
is probably something the test will tell you if it appears at all, but there’s a small chance they could expect you to know it. You could memorize that if you want to be hypersafe.
By contrast, the GRE absolutely, unequivocally, expects you to be able to convert length units to area or volume units.
Practice question
1) http://gre.magoosh.com/questions/797
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