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Ohm's Law Calculator

Calculate V, I, R, and P

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Last Updated: March 2, 2026
avatarBy Viblaa Team

Calculate any variable

Power calculation

Formula display

You have a 12V power supply and a component rated for 20mA. What resistor do you need? Your circuit draws 2A through a 10Ω resistor—what's the voltage drop? The LED works but gets hot. Is something wrong with your calculations?

Ohm's Law is the foundation of all electrical work: V = I × R. Voltage equals current times resistance. Simple in theory, easy to mess up in practice. This calculator solves for any variable when you know the other two.

What is Ohm's Law?

Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in electrical circuits. It's fundamental to all circuit design, from simple LED projects to complex electronics.

The formulas:

V = I × R (Voltage = Current × Resistance)
I = V / R (Current = Voltage / Resistance)
R = V / I (Resistance = Voltage / Current)

Power also relates:
P = V × I = I²R = V²/R
Units Matter

Voltage in volts (V), current in amperes (A), resistance in ohms (Ω). Milliamps (mA) = A/1000. Kilohms (kΩ) = Ω × 1000.

Why People Actually Need This Tool

Every Circuit Uses This

Whether you're wiring LEDs, designing power supplies, or troubleshooting electronics, Ohm's Law applies. It's not optional knowledge.

  1. LED circuits — Calculate current-limiting resistors.

  2. Power supply design — Determine component ratings.

  3. Troubleshooting — Verify expected vs. measured values.

  4. Component selection — Choose appropriate resistors.

  5. Safety calculations — Ensure circuits don't overheat.

  6. Voltage dividers — Design reference voltage circuits.

  7. Current sensing — Calculate shunt resistor values.

How to Use the Ohm's Law Calculator

  1. Enter known values — Any two of: Voltage, Current, Resistance.

  2. Select units — V, mV, A, mA, Ω, kΩ, etc.

  3. Calculate — Get the third value plus power dissipation.

  4. Verify — Check that results make physical sense.

Known ValuesFormulaTypical Use Case
V and IR = V/IFind resistor for known current
V and RI = V/RFind current through resistor
I and RV = I×RFind voltage drop across component
Check Power Dissipation

Resistors have power ratings. A 1/4W resistor burning 1W will fail (possibly catching fire). Always check P = I²R.

Real-World Use Cases

1. The LED Current Limiter

Context: Powering 3.3V LED from 12V supply. LED needs 20mA.

Problem: What resistor value?

Solution: R = (12V - 3.3V) / 0.020A = 435Ω. Use 470Ω standard value.

Outcome: LED runs at safe current (18.5mA with 470Ω).

2. The Voltage Drop

Context: 100 feet of wire to shed. 10A load at 120V.

Problem: How much voltage lost in wire?

Solution: 100ft of 12AWG ≈ 0.16Ω. V = 10A × 0.16Ω = 1.6V drop each way = 3.2V total.

Outcome: Shed receives 116.8V—acceptable loss.

3. The Power Rating

Context: 100Ω resistor with 12V across it.

Problem: What wattage resistor needed?

Solution: P = V²/R = 144/100 = 1.44W. Need at least 2W resistor.

Outcome: Properly rated component prevents fire hazard.

4. The Current Measurement

Context: Measuring current in circuit. Only have voltmeter.

Problem: How to measure current indirectly?

Solution: Insert known 1Ω shunt. Measure voltage. I = V/1Ω = V directly.

Outcome: Current measured using Ohm's Law.

5. The Speaker Impedance

Context: Amplifier rated for 8Ω, speaker shows 4Ω.

Problem: How does this affect current?

Solution: At 10V output: 8Ω draws 1.25A. 4Ω draws 2.5A—double the current.

Outcome: Amplifier may overheat with 4Ω speaker. Add series resistor or use correct speaker.

6. The Battery Drain

Context: 9V battery powering 500mA device.

Problem: What's the effective load resistance?

Solution: R = 9V / 0.5A = 18Ω effective load.

Outcome: Understanding of circuit characteristics for battery life estimation.

7. The Parallel Resistors

Context: Need 33Ω but only have 100Ω resistors.

Problem: How to make 33Ω from 100Ω?

Solution: Three 100Ω in parallel: 1/R = 1/100 + 1/100 + 1/100 = 3/100. R = 33.3Ω.

Outcome: Custom resistance from standard values.

Common Mistakes and How to Avoid Them

Units Cause Most Errors

Mixing mA with A or kΩ with Ω causes 1000× errors. Double-check units before calculating.

Forgetting Unit Conversion
❌ The Mistake
Calculating R = 5V / 20mA as 0.25Ω instead of 250Ω.
✅ The Fix
Convert 20mA to 0.020A first. R = 5/0.020 = 250Ω.
Ignoring Power Dissipation
❌ The Mistake
Using tiny 1/8W resistor for 1W application because the resistance was correct.
✅ The Fix
Always calculate P = I²R and choose resistor rated 2× the calculated power.
Assuming Ideal Components
❌ The Mistake
Not accounting for wire resistance in long runs.
✅ The Fix
Wire has resistance. 100 feet of 14AWG is about 0.25Ω. Adds up in high-current circuits.
LED Forward Voltage
❌ The Mistake
Using supply voltage instead of voltage drop across resistor for LED calculations.
✅ The Fix
Subtract LED forward voltage. R = (Vsupply - Vled) / I.
Parallel Resistance Error
❌ The Mistake
Adding parallel resistances directly (100Ω + 100Ω = 200Ω parallel—wrong).
✅ The Fix
Parallel: 1/Rtotal = 1/R1 + 1/R2. Two 100Ω in parallel = 50Ω.

Privacy and Data Handling

This Ohm's Law Calculator operates entirely in your browser.

  • No calculations are sent to any server.
  • No data is stored.
  • No account required.
  • Works completely offline.

Your circuit designs stay private.

Conclusion

Ohm's Law is electrical engineering's most fundamental equation. Every circuit you build, test, or repair uses this relationship between voltage, current, and resistance.

This calculator eliminates arithmetic errors. Enter any two values, get the third plus power dissipation. Design circuits correctly the first time.

V = I × R. Simple law, countless applications.

Frequently Asked Questions