Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across those points. It is expressed as V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

What is the difference between AC and DC Ohm's Law?

For DC circuits, Ohm's Law (V = I × R) applies directly with resistance (R). For AC circuits, the equivalent is V = I × Z, where Z is impedance — a complex value that includes resistance and reactance from capacitors and inductors. This calculator covers DC and purely resistive AC loads.

How do I calculate power from resistance?

Power can be calculated from resistance in two ways: P = I² × R (if you know current and resistance) or P = V² / R (if you know voltage and resistance). These are derived by combining the basic power formula P = V × I with Ohm's Law V = I × R.

What are the units in Ohm's Law?

The units are: Voltage (V) in Volts, Current (I) in Amperes (A), Resistance (R) in Ohms (Ω), and Power (P) in Watts (W). Common prefixes include milli (m, ×10⁻³), kilo (k, ×10³), and Mega (M, ×10⁶).

Ohm's Law Calculator – Voltage, Current, Resistance & Power

Enter any two values — leave the other two empty

⚡ Voltage V

🔌 Current I

🔧 Resistance R

💡 Power P

Fill in exactly 2 fields, leave the other 2 empty.

Ohm's Law Formula Wheel

V = I × R | Ohm's Law Triangle

If you know I & R:

V = I × R
P = I² × R

If you know V & I:

R = V / I
P = V × I

P = V × I = I² × R = V² / R

What is Ohm's Law?

Ohm's Law is the fundamental relationship governing electrical circuits. Formulated by German physicist Georg Simon Ohm in 1827, it states that the voltage across a conductor is directly proportional to the current flowing through it, with the constant of proportionality being the resistance:

V = I × R

Where V is voltage in Volts (V), I is current in Amperes (A), and R is resistance in Ohms (Ω). This simple equation is the cornerstone of all electrical engineering and circuit analysis.

The Four Ohm's Law Formulas

By combining Ohm's Law with the power formula (P = V × I), we can derive 12 expressions covering every combination of two known variables:

Find Voltage (V):
V = I × R    V = P / I    V = √(P × R)

Find Current (I):
I = V / R    I = P / V    I = √(P / R)

Find Resistance (R):
R = V / I    R = V² / P    R = P / I²

Find Power (P):
P = V × I    P = I² × R    P = V² / R

Practical Examples

Example 1 – LED resistor: A 5 V supply, 20 mA (0.02 A) LED. R = V/I = 5/0.02 = 250 Ω. Power dissipated: P = V×I = 5×0.02 = 0.1 W.
Example 2 – Household appliance: A 1,200 W hair dryer on 230 V. I = P/V = 1200/230 ≈ 5.2 A. Resistance: R = V²/P = 230²/1200 ≈ 44.1 Ω.
Example 3 – Resistor check: A 10 kΩ resistor with 9 V across it. I = V/R = 9/10000 = 0.9 mA. Power: P = V²/R = 81/10000 = 8.1 mW.

Frequently Asked Questions

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across those points. Expressed as V = I × R (voltage equals current times resistance), it applies to any resistive circuit element under constant temperature. It was discovered by Georg Simon Ohm and published in 1827.

For DC circuits, Ohm's Law (V = I × R) applies directly using resistance (R). For AC circuits, resistance is replaced by impedance (Z) — a complex quantity that combines resistance with inductive reactance (X_L = 2πfL) and capacitive reactance (X_C = 1/(2πfC)). For purely resistive AC loads (heaters, incandescent bulbs), Ohm's Law works identically to DC. This calculator handles DC and resistive AC loads.

Power from resistance can be found two ways: if you know current and resistance, use P = I² × R. If you know voltage and resistance, use P = V² / R. Both formulas come from substituting Ohm's Law (V = I×R) into the base power formula P = V × I. For example, a 100 Ω resistor carrying 0.5 A dissipates P = 0.5² × 100 = 25 W.

The SI units are: Voltage (V) — Volts (V); Current (I) — Amperes (A); Resistance (R) — Ohms (Ω); Power (P) — Watts (W). Common prefixes: milli (m) = ×10⁻³, micro (μ) = ×10⁻⁶, kilo (k) = ×10³, Mega (M) = ×10⁶. So 4.7 kΩ = 4,700 Ω, and 22 mA = 0.022 A.