Ohm's Law Calculator
Solve V, I, R & Power Instantly
The most accurate free Ohm's Law calculator online. Enter any two known electrical values — Voltage, Current, Resistance, or Power — and instantly calculate the remaining quantities using Ohm's Law (V = IR).
What is Ohm's Law?
The Foundation of Every Electrical Circuit
Ohm's Law states that the voltage (V) across a conductor equals the current (I) flowing through it multiplied by its resistance (R). Expressed as V = IR, this fundamental law of electronics governs how electrical circuits behave — from a simple LED to a complex PCB design. Named after German physicist Georg Simon Ohm, it is the single most important formula in electrical engineering.
🔋 Voltage (V) — Volts
The electrical potential difference that drives current through a circuit. Think of it as the pressure that pushes electrons through a conductor.
⚡ Current (I) — Amperes
The flow rate of electric charge through a conductor. Higher current means more electrons are moving through the circuit per second.
🔌 Resistance (R) — Ohms (Ω)
The opposition a material offers to the flow of current. Resistors, traces, and components all contribute resistance to a circuit.
💡 Power (P) — Watts
The rate at which electrical energy is consumed or produced. Power determines heat dissipation and component ratings in PCB design.
Ohm's Law Calculator
Enter any two values and click Calculate to solve for the remaining quantities.
Ohm’s Law Formulas
Ohm's Law Formulas
What Are the 3 Ohm's Law Formulas?
Ohm's Law is expressed through three primary formulas relating Voltage (V), Current (I), and Resistance (R). Together they allow you to solve any DC circuit when two values are known.
V = I×R | I = V÷R | R = V÷I
Voltage Formula
Voltage equals Current multiplied by Resistance. Use this when you know the current flowing through a component and its resistance, and need to find the voltage drop across it.
Current Formula
Current equals Voltage divided by Resistance. Use this to calculate how much current flows through a circuit or resistor when you know the supply voltage and resistance value.
Resistance Formula
Resistance equals Voltage divided by Current. Use this to find the resistance of a component, trace, or material when you measure its voltage drop and the current through it.
By combining Ohm's Law (V = IR) with the Power formula (P = VI), three additional equations let you calculate Power (Watts) from any two of the four quantities.
Step-by-Step Guide
How to Calculate Ohm's Law — 5 Simple Steps
Whether you are solving a circuit manually or using our Ohm's calculator, follow these five steps to get accurate results every time.
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1
Identify the Known Values
Determine which two electrical quantities you already know — Voltage (V), Current (I), Resistance (R), or Power (P). You need at least two values to solve for the remaining unknowns.
💡 Check your circuit diagram, component datasheet, or measurement device. -
2
Convert to Standard Units
Ensure all values are in standard SI units before calculating: Volts (V) for voltage, Amperes (A) for current, Ohms (Ω) for resistance, and Watts (W) for power. Convert milliamps to amps (÷1000) or kΩ to Ω (×1000) as needed.
💡 Example: 250mA = 0.25A | 4.7kΩ = 4700Ω -
3
Select the Correct Formula
Choose the Ohm's Law formula that solves for your unknown using the two known values. Use the triangle method: cover the unknown quantity to reveal its formula automatically.
💡 V=IR | I=V/R | R=V/I | P=VI | P=I²R | P=V²/R -
4
Substitute and Calculate
Insert the known values into the chosen formula and perform the arithmetic. Alternatively, enter your two known values into our V IR calculator above and click Calculate — all four results are instantly displayed.
💡 Example: V=12V, R=6Ω → I = 12÷6 = 2A → P = 12×2 = 24W -
5
Verify and Apply
Cross-check your result using a second formula to confirm accuracy. Then apply the calculated values to select appropriate components — resistors, wires, fuses, or PCB traces — ensuring they are rated above the calculated current or power.
💡 For PCB design, ensure trace width handles calculated current without overheating. See our PCB Layout Design services.
This table shows all six Ohm's Law formula combinations. Find the row where your two known values match and read off the formula for the unknown quantity.
| Known Values | Find Voltage (V) | Find Current (I) | Find Resistance (R) | Find Power (P) |
|---|---|---|---|---|
| V & I known | — | — | R = V ÷ I | P = V × I |
| V & R known | — | I = V ÷ R | — | P = V² ÷ R |
| V & P known | — | I = P ÷ V | R = V² ÷ P | — |
| I & R known | V = I × R | — | — | P = I² × R |
| I & P known | V = P ÷ I | — | R = P ÷ I² | — |
| R & P known | V = √(P×R) | I = √(P÷R) | — | — |
| 20A & 12Ω | V = 20×12 = 240V | — | — | P = 20²×12 = 4800W |
Why It Matters
Benefits of Using an Ohm's Law Calculator
A reliable Ohm's calculator saves time, eliminates manual errors, and ensures safe, accurate circuit design for professionals and students alike.
Instant Calculations
Solve V, I, R, and P simultaneously in under a second — no manual formula switching or multi-step arithmetic required.
Error-Free Results
Eliminates arithmetic mistakes common in manual calculations, especially when dealing with decimal values or multiple unit conversions.
Component Selection
Quickly determine correct resistor values, wire gauge, fuse ratings, and PCB trace widths based on actual circuit current and power.
PCB Design Support
Calculate power dissipation and trace resistance for thermal management in PCB design, ensuring reliability under load.
Education & Learning
Ideal for electronics students to verify hand calculations, understand circuit relationships, and build intuition around Ohm's Law.
All 6 Formula Modes
Supports every combination of two known inputs — any pair from V, I, R, and P — covering all practical Ohm's Law calculation scenarios.
Follow these professional guidelines to get safe, accurate results when applying Ohm's Law in real circuit design.
- ✓Always convert units to standard SI before calculating (mA→A, kΩ→Ω).
- ✓Add a 20–25% safety margin above calculated current and power values.
- ✓Verify with a second Ohm's Law formula to cross-check your answer.
- ✓For AC circuits, account for impedance (Z) rather than pure resistance (R).
- ✓Consider temperature effects — resistance increases with temperature in most metals.
- ✓For PCB traces, also calculate trace width using the IPC-2221 standard.
FAQ
Frequently Asked Questions — Ohm's Law Calculator
Clear, direct answers to the most common questions about Ohm's Law, its formulas, and practical applications in electronics and PCB design.
To calculate Ohm's Law, identify any two known electrical values from Voltage (V), Current (I), Resistance (R), or Power (P), then apply the appropriate formula. The three core Ohm's Law formulas are:
V = I × R I = V ÷ R R = V ÷ I
For example, if you know Current (I = 3A) and Resistance (R = 4Ω), multiply them to find Voltage: V = 3 × 4 = 12 Volts. Then calculate Power: P = V × I = 12 × 3 = 36 Watts. Use our free Ohm's Law calculator above to solve all four values instantly by entering any two known quantities.
The three primary Ohm's Law formulas are derived from the fundamental relationship V = IR:
Formula 1 — Voltage: V = I × R — Voltage equals Current multiplied by Resistance. Used to find the voltage drop across a component or circuit section.
Formula 2 — Current: I = V ÷ R — Current equals Voltage divided by Resistance. Used to determine how much current flows through a circuit given the supply voltage and resistance.
Formula 3 — Resistance: R = V ÷ I — Resistance equals Voltage divided by Current. Used to calculate the resistance of a component when its voltage drop and current are measured. Additionally, three Power formulas — P = VI, P = I²R, and P = V²÷R — extend Ohm's Law to energy calculations.
When Current (I) = 20 Amperes and Resistance (R) = 12 Ohms, you can calculate Voltage and Power using Ohm's Law:
Voltage: V = I × R = 20 × 12 = 240 Volts
Power: P = I² × R = 20² × 12 = 400 × 12 = 4,800 Watts (4.8 kW)
This scenario is typical in industrial motor circuits and heavy-duty electrical systems. At 4,800W power dissipation, careful PCB trace sizing and thermal management are critical. For high-current PCB design requirements, explore our PCB Layout Design services.
Ohm's Rule (also called Ohm's Law) states that in a linear electrical conductor, the current flowing through it is directly proportional to the voltage applied across it, provided temperature and other physical conditions remain constant. Mathematically expressed as V = I × R, it was formulated by German physicist Georg Simon Ohm in 1827.
The rule applies to ohmic conductors — materials that maintain a constant resistance regardless of the applied voltage (metals, resistors, wires). It does not apply directly to non-ohmic devices such as diodes, transistors, LEDs, or thermistors, where resistance changes with voltage or temperature.
In practice, Ohm's Rule is the single most important principle in circuit analysis, used for everything from basic LED current-limiting resistor calculations to complex multi-layer PCB impedance control design.
A V IR calculator is an electronic tool that applies Ohm's Law (V = IR) to solve for unknown circuit values. It is used by electronics engineers, PCB designers, electricians, and students to quickly calculate Voltage (V), Current (I), Resistance (R), and Power (P) from any two known inputs.
Common use cases include: selecting current-limiting resistors for LEDs, calculating wire gauge for safe current capacity, determining power dissipation in resistors and ICs, sizing fuses for protection circuits, verifying PCB trace resistance and voltage drops, and troubleshooting electrical faults in production systems.
Ohm's Law in its basic form (V = IR) applies directly to DC circuits and purely resistive AC circuits. For general AC circuits, the formula is extended using impedance (Z) instead of resistance: V = I × Z, where Z accounts for resistance, capacitive reactance, and inductive reactance.
For purely resistive loads on AC supply (heaters, incandescent bulbs), the standard Ohm's Law calculator gives accurate results using RMS voltage and current values. For circuits with capacitors, inductors, or motors, you should use the complex impedance form of Ohm's Law with phasor analysis.
