Ohm's Law
Calculator
Enter any two of Voltage, Current, Resistance, or Power — instantly calculate all four. Includes unit scaling, power rating check, and step-by-step formulas.
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12
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Watt
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Ohm's Law Calculator
Enter any 2 values — solve for all 4
Enter any 2 values below to solve for all 4.
VISUAL REFERENCE
Ohm's Law Wheel — All 12 Formulas
Each quadrant shows the three ways to calculate V, I, R, or P from the other variables.
- V = Voltage (Volts) → IR · √(PR) · P/I
- I = Current (Amps) → V/R · P/V · √(P/R)
- R = Resistance (Ω) → V/I · V²/P · P/I²
- P = Power (Watts) → VI · I²R · V²/R
ALL 12 FORMULAS
Ohm's Law & Watt's Law — Complete Reference
Every formula derived from V = IR and P = VI.
FIND V
V = I × R
from I and R
FIND V
V = P / I
from P and I
FIND V
V = √(P × R)
from P and R
FIND I
I = V / R
from V and R
FIND I
I = P / V
from P and V
FIND I
I = √(P / R)
from P and R
FIND R
R = V / I
from V and I
FIND R
R = V² / P
from V and P
FIND R
R = P / I²
from P and I
FIND P
P = V × I
from V and I
FIND P
P = I² × R
from I and R
FIND P
P = V² / R
from V and R
The Water Analogy for Ohm's Law
Think of electricity like water in a pipe: Voltage is the water pressure that pushes electrons through the circuit. Current is the water flow rate — how many electrons move past a point per second. Resistance is the pipe's diameter — a narrower pipe (more resistance) restricts flow for the same pressure, exactly the way a higher-resistance component reduces current for the same voltage. Power is pressure × flow — how much work the water (or the circuit) does every second.
Power Rating — Why It Matters
Every resistor has a maximum power rating. Standard ratings are 1/8W, 1/4W (most common), 1/2W, 1W, 2W, 5W, and 10W. Exceeding the rating causes overheating and eventually failure.
Rule of thumb: always choose a resistor rated at roughly 2× the calculated dissipation — a 1Ω resistor with 1A flowing through it dissipates 1W, so use a 2W resistor. SMD resistors are typically rated 1/10W to 1/4W due to their small size.
DC vs AC Circuits
Ohm's Law (V = IR) applies directly to DC circuits and purely resistive AC circuits. For AC circuits with capacitors or inductors, resistance becomes impedance (Z) — a complex quantity combining resistance (R) and reactance (X).
V = I × Z, where Z = √(R² + X²). For purely resistive circuits (no capacitors or inductors), impedance equals resistance R and standard Ohm's Law applies at any frequency.
Ohm's Law — All Formulas Explained
Ohm's Law and Watt's Law together provide 12 formulas for computing voltage, current, resistance, and power from any two known quantities. These are the most-used formulas in all of electronics and electrical engineering.
The Core Relationships
V = I × R (Ohm's Law — voltage)
I = V / R (Ohm's Law — current)
R = V / I (Ohm's Law — resistance)
P = V × I (Watt's Law — power from V and I)
P = I² × R (power from I and R)
P = V² / R (power from V and R)
Units: V = volts, I = amperes, R = ohms (Ω), P = watts
Example: 12V source, 100Ω resistor → I = V/R = 12/100 = 0.12A = 120mA → P = I²R = (0.12)² × 100 = 1.44W
Derived Formulas (solving for each variable)
Solving for Voltage (V): V = I × R (from I, R) · V = P / I (from P, I) · V = √(P × R) (from P, R)
Solving for Current (I): I = V / R (from V, R) · I = P / V (from P, V) · I = √(P / R) (from P, R)
Solving for Resistance (R): R = V / I (from V, I) · R = V² / P (from V, P) · R = P / I² (from P, I)
Solving for Power (P): P = V × I (from V, I) · P = I² × R (from I, R) · P = V² / R (from V, R)
Unit Conversions
Voltage: 1 kV = 1,000 V · 1 mV = 0.001 V
Current: 1 A = 1,000 mA · 1 mA = 0.001 A
Resistance: 1 kΩ = 1,000 Ω · 1 MΩ = 1,000,000 Ω
Power: 1 kW = 1,000 W · 1 mW = 0.001 W
Always convert to base units (V, A, Ω, W) before calculating.
Example: 5V, 20mA → 0.020A → R = V/I = 5/0.020 = 250Ω → P = V×I = 5×0.020 = 0.1W = 100mW