Ohm’s Law
Calculator
Calculate Voltage, Current, Resistance, or Power in one click. Built-in formula guide, animated circuit, and all unit conversions included.
Ohm’s Law Calculator
// V=IR · P=VI · All Units Supported · 2025
What Is Ohm’s Law?
Ohm’s Law is one of the most important rules in electronics and electrical engineering. It describes the relationship between three fundamental electrical quantities. Those quantities are Voltage, Current, and Resistance.
The law is named after Georg Simon Ohm, a German physicist who discovered it in 1827. He found that the current flowing through a conductor is directly proportional to the voltage across it — as long as the temperature stays constant.
In simple terms: if you increase the voltage, more current flows. If you increase the resistance, less current flows. This simple relationship forms the foundation of all electrical circuit design.
The Ohm’s Law Formula
The core formula is simple. You can express it three ways depending on what you want to find.
Where:
- V = Voltage in Volts (V) — the electrical pressure pushing current through the circuit
- I = Current in Amperes (A) — the flow of electrical charge through the conductor
- R = Resistance in Ohms (Ω) — the opposition to current flow
The easiest way to remember this is the VIR triangle. Cover the value you want to find. The remaining two show you the calculation. Cover V → you see I × R. Cover I → you see V ÷ R. Cover R → you see V ÷ I.
The Power Formula
Power is the rate at which electrical energy is transferred or converted. It is measured in Watts (W). The power formula connects all four electrical quantities together.
These three expressions are all equivalent. They just use different combinations of the known values. This calculator automatically picks the right one based on what you enter.
Real-World Examples of Ohm’s Law
Ohm’s Law is used in thousands of everyday situations. Here are some practical examples you can verify with the calculator above.
| Application | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| LED indicator light | 3 V | 20 mA | 150 Ω | 60 mW |
| Smartphone charger | 5 V | 2 A | 2.5 Ω | 10 W |
| Car headlight bulb | 12 V | 4.2 A | 2.86 Ω | 50 W |
| Household lamp | 120 V | 0.5 A | 240 Ω | 60 W |
| Electric kettle (UK) | 230 V | 10.9 A | 21.1 Ω | 2,500 W |
Understanding Voltage, Current, and Resistance
Voltage (V)
Voltage is often compared to water pressure. It is the electrical potential difference between two points. Higher voltage means more force pushing current through the circuit. Common voltages include 1.5V (AA battery), 5V (USB), 12V (car), 120V or 230V (mains).
Current (I)
Current is the flow of electric charge. The letter I comes from the French word “intensité” (intensity of current). It is measured in Amperes (amps). One amp equals one coulomb of charge passing a point per second. Smaller devices use milliamps (mA).
Resistance (R)
Resistance is the opposition to current flow. Every material has some resistance. Conductors like copper have very low resistance. Insulators like rubber have very high resistance. Resistors are components specifically designed to introduce a fixed amount of resistance into a circuit.
// Key Takeaways
- Ohm’s Law: V = I × R — the foundation of all electrical circuit analysis.
- Current increases when voltage increases. Current decreases when resistance increases.
- Power (P = V × I) tells you how much electrical energy is consumed or converted per second.
- Use the VIR triangle to remember all three formulas. Cover the unknown to see the calculation.
- milli (m) = 0.001, kilo (k) = 1,000, Mega (M) = 1,000,000. Know your unit prefixes.
- Ohm’s Law assumes constant temperature and resistive (ohmic) materials. Diodes and transistors are non-ohmic.
- In series circuits, resistance adds up. In parallel circuits, the reciprocal of total resistance is the sum of reciprocals.
Frequently Asked Questions
How to Use This Calculator
This calculator has four modes. Each mode solves for a different unknown value.
- Calculate Voltage — Enter Current and Resistance. The calculator finds Voltage using V = I × R.
- Calculate Current — Enter Voltage and Resistance. The calculator finds Current using I = V ÷ R.
- Calculate Resistance — Enter Voltage and Current. The calculator finds Resistance using R = V ÷ I.
- Calculate Power — Enter any two of Voltage, Current, or Resistance. The calculator finds Power and all other values.
Each input supports multiple units. For voltage you can choose mV, V, or kV. For current you can choose mA, A, or kA. For resistance you can choose mΩ, Ω, kΩ, or MΩ. For power you can choose mW, W, kW, or MW. The calculator converts everything automatically before calculating.
For more on electrical fundamentals and circuit theory, see the Wikipedia article on Electrical Resistance ↗ and Wikipedia: Electric Power ↗.
Ohm’s Law Calculator: Calculate Voltage, Current, Resistance & Power (Free Tool)
You are working on an electronic circuit. You have a 12V battery. You have a resistor. You need to know how much current will flow.
Maybe you are building a project. Maybe you are troubleshooting a device. Maybe you are studying for an exam.
You need Ohm’s Law.
Ohm’s Law is the most important formula in electronics. It relates voltage, current, and resistance.
Today, I give you a free Ohm’s Law Calculator.
You enter any two values. The calculator finds the other two.
It shows:
- Voltage (V) in volts
- Current (I) in amps
- Resistance (R) in ohms
- Power (P) in watts
Let me explain Ohm’s Law and how to use it.
What is Ohm’s Law? (Very Simple Explanation)
Ohm’s Law states that the current through a conductor between two points is directly proportional to the voltage across the two points.
In plain English:
- More voltage = more current
- More resistance = less current
The formula:
V = I × R
Where:
- V = Voltage (volts)
- I = Current (amps)
- R = Resistance (ohms)
The power formula (Joule’s Law):
P = V × I
Where:
- P = Power (watts)
Why This Ohm’s Law Calculator Matters
Here is why you need an Ohm’s Law calculator.
Reason 1: Design circuits correctly
You need to choose the right resistor for your LED. Too much current burns the LED. Too little and it stays dark. Ohm’s Law tells you the exact resistor value.
Reason 2: Troubleshoot problems
A device is not working. You measure voltage. You measure current. Ohm’s Law tells you if the resistance is correct.
Reason 3: Calculate power consumption
How much power does your circuit use? What size battery do you need? Ohm’s Law gives the answers.
Reason 4: Save components
Using the wrong resistor can damage expensive parts. Calculate first. Solder second.
Reason 5: Understand electricity
Ohm’s Law is the foundation of all electronics. Once you understand it, everything else becomes easier.
The Ohm’s Law Formula (Detailed)
The three basic forms:
V = I × R (Voltage = Current × Resistance)
I = V ÷ R (Current = Voltage ÷ Resistance)
R = V ÷ I (Resistance = Voltage ÷ Current)
Power formulas:
P = V × I (Power = Voltage × Current)
P = I² × R (Power = Current squared × Resistance)
P = V² ÷ R (Power = Voltage squared ÷ Resistance)
LIVE Ohm’s Law Calculator
Enter any two values. The calculator finds the other two instantly.
⚡ Ohm’s Law Calculator
⚡ Voltage (V)
🔌 Current (I)
🔄 Resistance (R)
💡 Power (P)
⚡ Ohm's Law Calculator
Enter any two values to calculate the rest
⚡ Voltage (V)
🔌 Current (I)
🔄 Resistance (R)
💡 Power (P)
📊 CALCULATED VALUES
V = I × R
Enter any two values above. ⟳ Reset
💡 Ohm's Law: Voltage = Current × Resistance • Power = Voltage × Current
How to Use This Ohm's Law Calculator
Follow these 3 simple steps.
Step 1: Enter any two values
You can enter:
- Voltage (V) and Current (I)
- Voltage (V) and Resistance (R)
- Current (I) and Resistance (R)
- Voltage (V) and Power (P)
- Current (I) and Power (P)
- Resistance (R) and Power (P)
Step 2: Leave the other two fields empty
The calculator will fill them automatically.
Step 3: Read the results
The calculator shows the calculated values and the formulas used.
Real Examples: Different Scenarios
Example 1: LED resistor calculation
You have a 9V battery. Your LED needs 2V and 20mA (0.02A).
Voltage across resistor = 9V - 2V = 7V
Current = 0.02A
Resistance = 7 ÷ 0.02 = 350 ohms
Result: Use a 350Ω resistor (or closest standard value: 330Ω or 360Ω).
Example 2: Heater power calculation
A space heater draws 10A from a 120V outlet.
Power = 120V × 10A = 1,200 watts (1.2 kW)
Example 3: Current calculation
A 100W light bulb on 120V.
Current = 100W ÷ 120V = 0.833A
The Ohm's Law Triangle
A simple way to remember Ohm's Law:
V
┌┴┐
│I│
└─┘
R- To find V: V = I × R
- To find I: I = V ÷ R
- To find R: R = V ÷ I
Understanding Each Quantity
Voltage (V) – measured in volts
Voltage is the electrical "pressure" that pushes electrons through a circuit. Higher voltage = more pressure.
Analogy: Voltage is like water pressure in a pipe. More pressure = more water flow.
Current (I) – measured in amperes (amps)
Current is the flow rate of electrons. Higher current = more electrons flowing per second.
Analogy: Current is like the amount of water flowing through the pipe.
Resistance (R) – measured in ohms (Ω)
Resistance opposes the flow of current. Higher resistance = less current for the same voltage.
Analogy: Resistance is like a narrower pipe. Less water flows at the same pressure.
Power (P) – measured in watts (W)
Power is the rate at which electrical energy is converted to another form (heat, light, motion).
Common Ohm's Law Applications
LED current limiting:
LEDs need a current-limiting resistor. Without one, they draw too much current and burn out.
Step 1: Find LED forward voltage (typically 2-3V for red, 3-4V for blue/white)
Step 2: Find desired current (typically 20mA = 0.02A)
Step 3: Calculate resistor value
Example: 12V supply, 3V LED, 20mA.
Resistor voltage = 12V - 3V = 9V
Resistance = 9V ÷ 0.02A = 450Ω
Fuse sizing:
A device draws 5A. The fuse should be slightly higher: 6-7A.
Wire sizing:
For a given current, you need thick enough wire to prevent overheating.
Frequently Asked Questions (FAQs)
1. What does Ohm's Law state?
Current is directly proportional to voltage and inversely proportional to resistance (V = I × R).
2. Can I use Ohm's Law for AC circuits?
Yes, but for AC with reactive components (capacitors, inductors), you need impedance (Z) instead of resistance (R).
3. What is the unit of resistance?
The ohm (Ω). 1 ohm = 1 volt per amp.
4. What is a short circuit?
A short circuit is a path with very low resistance (near zero ohms). It causes very high current.
5. What is an open circuit?
An open circuit has infinite resistance. No current flows.
6. Can resistance be negative?
No. Passive components always have positive resistance.
7. Does Ohm's Law apply to all materials?
No. It applies to "ohmic" materials (metals, resistors). Semiconductors and diodes are non-ohmic.
8. How do I calculate power from voltage and resistance?
P = V² ÷ R
9. How do I calculate power from current and resistance?
P = I² × R
10. What is the difference between AC and DC for Ohm's Law?
For purely resistive loads, the same formulas apply. For AC, you may need RMS values.
Common Mistakes
Mistake #1: Confusing voltage and current
Voltage is pressure, not flow. Use the triangle or formula to keep them straight.
Mistake #2: Using the wrong units
Voltage must be in volts. Current in amps. Resistance in ohms. Power in watts.
Convert mA to A: 20mA = 0.02A. Convert kΩ to Ω: 2.2kΩ = 2,200Ω.
Mistake #3: Forgetting the voltage drop across components
In a series circuit, voltage divides. In an LED circuit, the resistor only drops the difference between supply and LED voltage.
Mistake #4: Ignoring power ratings
A resistor can overheat if too much power passes through it. Calculate power and choose a resistor with a higher rating.
Mistake #5: Using Ohm's Law for non-ohmic components
Diodes, LEDs, and transistors do not follow Ohm's Law linearly. Calculate LED current using the resistor.
Resistor Power Ratings
A resistor's power rating tells you how much power it can dissipate without overheating.
Common power ratings:
- 1/8 watt (0.125W) – Small surface mount
- 1/4 watt (0.25W) – Most common through-hole
- 1/2 watt (0.5W) – Larger resistors
- 1 watt (1W) – Power resistors
- 5+ watts – High-power resistors
Example: A 100Ω resistor with 5V across it:
P = V² ÷ R = 25 ÷ 100 = 0.25W. Use at least a 1/4 watt resistor.
Rule of thumb: Use a resistor with a power rating at least double the calculated power.
Resistor Color Codes
Resistors use colored bands to indicate resistance.
| Color | Digit | Multiplier |
|---|---|---|
| Black | 0 | ×1 |
| Brown | 1 | ×10 |
| Red | 2 | ×100 |
| Orange | 3 | ×1,000 |
| Yellow | 4 | ×10,000 |
| Green | 5 | ×100,000 |
| Blue | 6 | ×1,000,000 |
| Violet | 7 | ×10,000,000 |
| Gray | 8 | ×100,000,000 |
| White | 9 | ×1,000,000,000 |
| Gold | – | ×0.1 (tolerance 5%) |
| Silver | – | ×0.01 (tolerance 10%) |
Example: Red-Red-Brown-Gold = 22 × 10 = 220Ω, 5% tolerance.
The History of Ohm's Law
Georg Ohm (1789-1854) was a German physicist.
In 1827, he published his findings on the relationship between voltage, current, and resistance.
The scientific establishment ignored him at first. His work was too mathematical.
Later, his findings were confirmed. The unit of resistance was named the "ohm" in his honor.
Today, Ohm's Law is taught in every physics and electronics class worldwide.
Series and Parallel Circuits
Series circuits:
- Same current through all components
- Voltage divides across components
- Total resistance = R1 + R2 + R3 + …
Parallel circuits:
- Same voltage across all components
- Current divides through branches
- Total resistance = 1/(1/R1 + 1/R2 + 1/R3 + …)
Example: Two 100Ω resistors in series = 200Ω. Two 100Ω resistors in parallel = 50Ω.
Final Thoughts
Ohm's Law is simple. But it is powerful.
My Ohm's Law Calculator gives you:
- Instant calculation of voltage, current, resistance, and power
- Working formulas displayed
- Support for any two known values
Bookmark this page. Use it for circuit design, troubleshooting, and studying.
The next time you need to know "what resistor do I need for this LED?", you will have the answer.
Disclaimer: This is an educational tool. Always verify calculations before building circuits. Use appropriate safety precautions when working with electricity.
External Links (Authority Backlinks):
- Wikipedia – Ohm's law{:target="_blank" rel="noopener noreferrer"}
- Wikipedia – Electrical resistance{:target="_blank" rel="noopener noreferrer"}
- Wikipedia – Electric power{:target="_blank" rel="noopener noreferrer"}
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