The sievert (Sv) is the System International or SI unit for dose equivalent (H). The rem is the same, but is an older conventional unit still used at an operational level in some countries (e.g., the United States). The conversion ratio is: 1 Sv = 100 rem.
When converting units, be careful of the “prefix” or “symbol” on the unit, i.e., “M” for mega, “m” for “milli,” “u” for “micro,” etc. A prefix, or its symbol, connotes a certain order of magnitude with the numerical value shown. For example, milli implies one thousandth; thus, 1 mSv = 0.001 Sv, and micro implies one millionth; 1 uSv = 0.000001 Sv. This is a convenient way to express a smaller value of a measurement quantity without writing out many zeroes, and it applies to any unit, e.g., uCi for microcurie, mL for milliliter, etc.