By Christopher Donahue • Updated Mar 24, 2022

Ohm’s Law is the foundation of electrical engineering. With it, you can calculate resistance, voltage, or current when you know any two of the three.

Calculating Milliamps Step by Step

1. Understand the Unit

A milliamps (mA) is one‑thousandth of an ampere. Once you know the current in amperes, simply divide by 1,000 to express it in milliamps.

2. Apply Ohm’s Law

Ohm’s Law: V = I × R. From this you can derive the other two equations: I = V ÷ R and R = V ÷ I.

3. Compute the Current in Amps

Insert the known voltage and resistance into I = V ÷ R. For example, with 12 V and 200 Ω:

I = 12 V ÷ 200 Ω = 0.06 A

4. Convert to Milliamps

Multiply the current in amperes by 1,000. 0.06 A × 1,000 = 60 mA.

5. Optional: Calculate Power

Power (W) equals voltage times current: P = V × I. Using the same values, 12 V × 0.06 A = 0.72 W, or 720 mW.

What You’ll Need

• Known voltage value
• Known resistance value