Ω
Ohm’s Law Calculator
Enter any two values to calculate the rest (Voltage, Current, Resistance). Power (W) is also calculated automatically.
Enter values (any two)
All results update as you type
Tip: You only need two fields. For example:
V + R → I, or V + I → R, or I + R → V.
Optional: LED resistor helper
Quick common use-case
Calculates R = (Vs − Vf) ÷ I and then fills values into the main calculator.
Ohm’s Law + Power Formulas
Most-used equations
V = I × R
I = V ÷ R
R = V ÷ I
P = V × I (power, watts)
P = I² × R
P = V² ÷ R
Units:
V (volts),
A (amps),
Ω (ohms),
W (watts).
And: mA = A ÷ 1000, kΩ = Ω × 1000.
Real-World Examples
Practical use cases
| Scenario | Given | Result |
|---|---|---|
| 12V device with 6Ω load | V=12V, R=6Ω | I=2A, P=24W |
| USB 5V drawing 0.5A | V=5V, I=0.5A | R=10Ω, P=2.5W |
| LED resistor (5V, Vf=2V, 20mA) | Vs=5V, Vf=2V, I=20mA | R≈150Ω, P≈0.06W |
| Heating warning example | V=24V, R=2Ω | I=12A, P=288W (hot!) |
Reminder: High power usually means heat. For resistors, choose a watt rating with margin (e.g., 2×).
FAQ
Quick answers
Do I need all three (V, I, R)? No. Any two is enough.
Why does power matter? Power tells you heat and whether parts may burn.
mA vs A? 1000 mA = 1 A. Use mA for small electronics.
kΩ vs Ω? 1 kΩ = 1000 Ω. 1 MΩ = 1,000,000 Ω.
AC vs DC? This calculator is for basic DC/resistive calculations (rule-of-thumb).
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