Ohm's Law Calculator
Answer
V = I × R. Pick the quantity to solve, enter the other two, and keep units consistent: volts, amperes, ohms. Milliamps belong in the current field as 0.02 A, not 20, unless you convert first.
Calculator
How it works
Ohm’s law is the fastest sanity check on a bench measurement. Measure two of voltage, current and resistance on a single resistive element and you can find the third.
Choose what to solve, type the two known values, and read the third. Warnings call out tiny currents or sub-ohm values where lead resistance matters. For LED strings with forward voltage, switch to the LED series resistor calculator.
Parameters used on this page
| Parameter | Value | Source |
|---|---|---|
| Voltage | V = I R | Ohm's law |
| Current | I = V / R | Ohm's law |
| Resistance | R = V / I | Ohm's law |
Worked example
A 5 V supply feeding 20 mA through an LED resistor needs R = 5 / 0.02 = 250 Ω if the LED drops nothing (use the LED tool when VF matters). 12 V into 1 kΩ is 12 mA.
Pitfalls
- Mixing milliamps with volts without converting to amperes.
- Using cold resistance for a lamp or thermistor and wondering why the current is wrong at temperature.
- Forgetting that Ohm's law is for the resistor, not the whole loop — subtract diode drops first.
FAQ
How do I calculate resistance from voltage and current?
R = V / I with I in amperes.
What if I only know mA?
Divide by 1000 to get amperes, or use the unit converter on this site.
Can Ohm's law solve power?
Use the power calculator for P = V I once you know two of V, I and R.
Does this work for AC reactance?
Magnitude only if you treat X as R in a resistive-equivalent sketch; use the reactance calculator for Xc and Xl.
Related
Sources
- Ohm's law for linear resistive elements.