A worked circuit: 500 mA, 12 V
A circuit with a current of 500 mA and a voltage of 12 V has a resistance of 24 ohms — but only after converting the current to its standard unit first: 500 mA converts to 0.5 A, and Ohm's law (R = V ÷ I) gives 12 ÷ 0.5 = 24. That 24-ohm result, converted to kilo-ohms, comes out to exactly 0.024 kΩ.
What goes wrong without the conversion step
Plugging the raw 500 (milliamperes) directly into R = V ÷ I instead of converting to 0.5 (amperes) first would give 12 ÷ 500 = 0.024 — a resistance off by a factor of 1,000 from the correct 24 ohms, purely from skipping the unit conversion.
A second circuit, starting from millivolts instead
A circuit already given as 2 A and 5,000 mV needs the voltage converted first: 5,000 mV converts to 5 V, giving a resistance of 5 ÷ 2 = 2.5 ohms. Whichever quantity arrives in a non-standard unit, it has to be converted before Ohm's law is applied.
Why Ohm's law itself has no idea what unit you used
R = V ÷ I is a relationship between volts, amps, and ohms specifically — it doesn't know or care whether the numbers fed into it were originally in millivolts or microamperes. Getting a meaningful answer out requires making sure the numbers going in are actually in those standard units first.
The practical rule
Before applying any physics formula that combines multiple quantities, convert every input to its standard unit first — current to amperes, voltage to volts — then compute, and convert the result to whatever unit is actually useful afterward.