Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law. This calculator is useful for troubleshooting control circuits, sensors, resistive loads, voltage dividers, and 24 VDC instrumentation circuits.
Calculate
Enter any two of voltage, current, and resistance. The calculator will determine the remaining electrical values.
Understand Ohm's Law
Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit.
From that basic equation, you can rearrange the formula to calculate any one of the three variables:
Current: I = V ÷ R
Resistance: R = V ÷ I
V = I × R
What the variables mean
- V = voltage, measured in volts (V)
- I = current, measured in amps (A)
- R = resistance, measured in ohms (Ω)
In practical controls work, you'll often measure voltage and current and use those measurements to determine whether a resistance, load, or circuit is behaving as expected.
Power: The Other Important Relationship
Ohm's Law also gives us several useful ways to calculate electrical power.
P = I² × R
P = V² ÷ R
Power is measured in watts (W). This becomes especially important when selecting resistors because a resistor must be capable of safely dissipating the heat produced by the circuit.
Worked Examples
Example 1: Finding Current
A control circuit has a 24 VDC power supply and a 120 Ω load. What current should flow?
I = 24 ÷ 120
I = 0.20 A
The expected current is 200 mA.
Example 2: Finding Resistance
You measure 24 V across a load and 0.5 A flowing through it. What is the resistance?
R = 24 ÷ 0.5
R = 48 Ω
The calculated resistance is 48 Ω.
Example 3: 4–20 mA Signal
A 250 Ω resistor is installed in a 4–20 mA signal circuit. What voltage should appear across the resistor at 12 mA?
V = 0.012 × 250
V = 3 V
Therefore, 12 mA through a 250 Ω resistor produces 3 V.
This is why a 250 Ω resistor is commonly used when converting a 4–20 mA signal into a nominal 1–5 V signal.
24 VDC Controls Example
Many industrial control systems use 24 VDC. Ohm's Law can quickly help determine whether a circuit is behaving as expected.
| Voltage | Resistance | Expected Current |
|---|---|---|
| 24 V | 24 Ω | 1.00 A |
| 24 V | 48 Ω | 0.50 A |
| 24 V | 120 Ω | 0.20 A |
| 24 V | 240 Ω | 0.10 A |
| 24 V | 1,000 Ω | 24 mA |
Resistor Power
When current flows through resistance, electrical energy is converted into heat. The amount of power is important when determining whether a resistor is properly sized.
Example: 250 Ω Loop Resistor
At 20 mA, a 250 Ω resistor dissipates:
P = 0.020² × 250
P = 0.10 W
The resistor dissipates 0.10 W. A resistor with a suitable power rating should be selected rather than choosing a component solely from its resistance value.
Using Ohm's Law for Troubleshooting
Ohm's Law is particularly useful when measurements don't match what you expect from a circuit.
If voltage is correct but current is too low
- Check for excessive resistance in the circuit.
- Look for loose or corroded connections.
- Check for a partially open conductor.
- Verify the load resistance.
- Check whether the load is actually connected.
If voltage is correct but current is too high
- Check for an unintended short or low-resistance path.
- Verify that the correct load is installed.
- Check wiring against the control schematic.
- Look for damaged components.
If the calculated resistance seems wrong
- Verify that voltage and current were measured at the same operating condition.
- Check whether the load is nonlinear.
- Make sure the meter is measuring the intended circuit.
- Remember that an energized circuit should not normally have resistance measured across it with an ohmmeter.
Common Mistakes
- Mixing amps and milliamps: 20 mA = 0.020 A, not 20 A.
- Forgetting unit conversions: resistance may be expressed in kΩ or MΩ.
- Measuring resistance on an energized circuit: use the appropriate de-energized test procedure.
- Ignoring power: the correct resistance value does not automatically mean the resistor has the correct wattage rating.
- Assuming every load is purely resistive: motors, coils, electronic devices, and power supplies can behave differently from a simple resistor.
Quick Reference
| What You Know | What You Want | Formula |
|---|---|---|
| Current + Resistance | Voltage | V = I × R |
| Voltage + Resistance | Current | I = V ÷ R |
| Voltage + Current | Resistance | R = V ÷ I |
| Voltage + Current | Power | P = V × I |
| Current + Resistance | Power | P = I² × R |
| Voltage + Resistance | Power | P = V² ÷ R |
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Engineering Note
Ohm's Law is one of the most useful relationships in electrical troubleshooting, but real control systems often contain components that are not simple resistive loads. Motors, solenoids, electronic inputs, power supplies, and transmitters can have nonlinear or dynamic behavior.
Use Ohm's Law as a fundamental tool for understanding the circuit, then combine it with the equipment schematic, manufacturer's specifications, and actual measurements.