Ohm's Law Calculator

Ohm's law ties together the three basic quantities in any circuit: voltage (E, volts), current (I, amperes) and resistance (R, ohms). Know any two and you can find the third. It shows up on its own on the exam and is a foundation for many calculation questions.

Procedure

1. Write down what you are given, with units. Convert first: milliamps to amps (divide by 1,000), kilohms to ohms (multiply by 1,000).
2. Pick the form that solves for the unknown: E = I × R, I = E ÷ R, or R = E ÷ I.
3. Substitute and calculate.
4. Sanity-check: more voltage across the same resistance means more current; more resistance on the same voltage means less current.

Worked example
A heating element measures 16 Ω and is connected to 120 V. What current does it draw?

• The unknown is current, so use I = E ÷ R.
• I = 120 V ÷ 16 Ω = 7.5 A.
• Check by going back the other way: 7.5 A × 16 Ω = 120 V, which matches the supply.

Most common mistake
Dividing the wrong way round, for example R ÷ E instead of E ÷ R. Picture the E-over-I×R triangle: E sits on top, so when you divide, voltage is always the number on top, never the divisor. A second trap is assuming the current stays the same when the voltage changes. For a fixed resistance it is the resistance that stays the same.

Power (P, in watts) is the rate at which a load uses energy. You can find it from any two of voltage, current and resistance, because Ohm's law links all three. The "power wheel" is just Ohm's law substituted into P = E × I.

Procedure

1. List the two quantities you are given.
2. Pick the matching form so you need no extra step:

• volts and amps: P = E × I
• amps and ohms: P = I² × R
• volts and ohms: P = E² ÷ R

1. To find a missing quantity from watts, rearrange: I = P ÷ E, R = E² ÷ P, E = √(P × R).
2. Convert kW to W (multiply by 1,000) before you start.

Worked example
A heating element is 19.2 Ω on 240 V. How many watts does it produce?

• Given volts and ohms, so P = E² ÷ R.
• 240² = 57,600.
• 57,600 ÷ 19.2 = 3,000 W.
• Check with current: I = 240 ÷ 19.2 = 12.5 A, and 240 × 12.5 = 3,000 W.

Most common mistake
Assuming a resistive load keeps its rated watts at a different voltage. The resistance is what stays fixed. Because power goes with voltage squared, half the voltage gives one quarter of the power, not one half. Also watch for squaring the wrong quantity: in I² × R only the current is squared.