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Ohm's law calculator

Enter any two of voltage, current, resistance and power to get the other two. Below are calculators for voltage dividers, resistors in series and parallel, and the resistor for an LED.

Ohm's law and power

Fill in any two boxes. The others are calculated from the two you changed most recently.

Voltage divider

Output voltage––
Current––

Resistors in series and parallel

Separate with commas. 4k7 and 1M style values work too
In series–
In parallel–

LED series resistor

Resistor––
Resistor power––

Ohm's law

Ohm's law connects voltage (V), current (I) and resistance (R), and the power formula adds power (P). Know any two and you can find the rest:

V = I × R I = V ÷ R R = V ÷ I
P = V × I = I² × R = V² ÷ R

For example, a radio drawing 20 A from a 13.8 V supply uses 276 W, and 100 W into a 50 Ω dummy load means 70.7 V RMS across it and 1.41 A through it. These are the sums that come up in every amateur radio exam, and they're worth practising by hand.

Voltage dividers

Two resistors in series divide a voltage in proportion to their values: Vout = Vin × R2 ÷ (R1 + R2). Anything connected to the output draws current and pulls the voltage down, so enter a load to see its effect. Keep the divider's current at least ten times the load current, or use a regulator instead, if the output needs to stay steady.

Series and parallel

In series, resistances add. In parallel, the reciprocals add, so the total is always less than the smallest resistor: 1 ÷ R = 1 ÷ R1 + 1 ÷ R2 + … Two equal resistors in parallel give half the value of one, which is a handy way to make a non-standard value or share the power between two resistors, as in a home-made dummy load.

LED resistors

An LED needs a resistor to limit its current. The resistor drops the difference between the supply and the LED's forward voltage: R = (Vsupply − VLED) ÷ I. Modern LEDs are bright at 2 to 10 mA, so there's no need to run them at 20 mA as a panel indicator. Choose the next standard value up, and check the resistor's power rating.

Questions

How do I calculate current from voltage and resistance?

Divide the voltage by the resistance. 12 V across 4 Ω gives 3 A.

What voltage is across a 50 Ω load at 100 W?

About 70.7 V RMS, from V = √(P × R) = √(100 × 50). The peak voltage is 100 V.

What resistor do I need for an LED on 12 V?

For a red LED at 10 mA from 12 V, (12 − 2) ÷ 0.01 = 1,000 Ω, so 1 kΩ. From 13.8 V, the nearest standard value up is 1.2 kΩ.

Why is the parallel resistance less than the smallest resistor?

Each resistor adds another path for current, so the total current rises and the overall resistance falls.