Ohm's Law is the basic formula for electronics. With it, we may calculate Resistance (Ohms), Voltage (Volts) or Current (Amps) by knowing any two of the three values.
How to Calculate Milliamps
A milliamp is one one-thousandth of an amp. By calculating the value in amps and dividing by one thousand, you will have a value of current in milliamps.
Ohm's Law is Voltage = Resistance X Current. The derivations are: Resistance = Voltage / Current Current = Voltage / Resistance
Calculate Current using your known Voltage and Resistance as shown in Step 2. Current (I) = Voltage (V) divided by Resistance (R) I = V/R Ex: if your voltage is 12V and resistance is 200 Ohms I = V/R = 12 / 200 = 0.06 Amps
Knowing your Current in Amps, multiply by 1000 to see the value in milliamps Ex: 0.06 Amps x 1000 = 60 milliamps
Given a known Voltage and Current, you may also derive Power (Watts). Power = Voltage times Current ( P = V x I ) Ex: 12V x 0.06A = 0.72W or 720 milliwatts