BioDeviceHub

Ohm's Law Calculator

Enter any two of voltage, current, resistance and power to calculate the other two, with units from microvolts and microamps up to kilovolts and megohms.

Formula

V=I RV = I\,R
P=V IP = V\,I
P=I2R=V2RP = I^2 R = \dfrac{V^2}{R}
VV
voltage, in volts
II
current, in amperes
RR
resistance, in ohms
PP
power, in watts

How it works

For a resistive element the voltage across it is the current through it multiplied by its resistance. The power it dissipates is the product of voltage and current. Any two of the four quantities fix the other two.

On the bench the same relationship underlies leakage-current limits, fuse and resistor ratings, and the load a defibrillator analyzer presents to the device under test.

Worked example

12 V is applied across a 470 Ω resistor.

  1. I = V / R = 12 V / 470 Ω = 0.02553 A.
  2. P = V² / R = 144 / 470 = 0.3064 W.

The resistor carries 25.53 mA and dissipates 306.4 mW, so a 0.25 W part would be overloaded.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • A purely resistive load with steady direct current, or the RMS values of an alternating current.
  • Resistance is constant. It changes with temperature in real components.

Common mistakes

  • Applying V = I × R to reactive loads without accounting for impedance and phase.
  • Mixing prefixes, for example milliamps with kilohms, without converting. The unit selectors do this for you.
  • Choosing a resistor with a power rating below the calculated dissipation.

Related tools

Nyquist Sampling Rate CalculatorMinimum sampling rate and alias frequencyOpen calculatorDevice Battery Life CalculatorRun time, battery size, and energy use of a deviceOpen calculatorDecibel CalculatorRatios to dB, dB to ratios, and dBm to powerOpen calculator

Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.