Ohm's Law Calculator — Solve Voltage, Current, Resistance, Power
Calculate voltage, current, resistance, and power instantly using Ohm's Law (V = IR) and the power equations. Select what you want to solve, enter two known values, and get the result with full step-by-step working.
Ohm's Law Calculator Tool
Results
Step-by-Step Solution
Calculate Current Of:
Pick a common appliance to instantly estimate its current draw. Every value in the popup — wattage, tonnage, and supply voltage — is editable, so you can model an 18W bulb, a 2-ton AC unit, or anything in between.
How This Calculator Works
This calculator uses two foundational electrical equations: Ohm's Law (V = IR) and Joule's first law of electrical heating (P = VI). From these two equations, twelve total formulas can be derived, covering every possible combination of voltage (V), current (I), resistance (R), and power (P).
You select the quantity you want to find, enter any two known values, and the calculator solves for the remaining two. Every result shows the formula used and the step-by-step substitution so you can verify the math yourself or use it in coursework.
All calculations follow SI units: voltage in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W).
The Twelve Ohm's Law Formulas Used by This Calculator
Every combination of solving for V, I, R, or P from two known quantities:
Solving for Voltage (V)
Solving for Current (I)
Solving for Resistance (R)
Solving for Power (P)
These twelve relationships form what electricians and engineers refer to as the "Ohm's Law Wheel," where any unknown can be determined from any two known values.
Electrical Units Supported
The calculator accepts inputs across the full range of practical electrical magnitudes used in real circuits.
Voltage
Millivolts (mV), volts (V), kilovolts (kV). A standard AA battery outputs 1.5V. Power transmission lines operate at 110kV to 765kV. Sensor circuits often deal in millivolts.
Current
Microamperes (μA), milliamperes (mA), amperes (A). Small sensors draw current in the μA range. Standard indicator LEDs run at 10–20mA. Household appliances draw 1–15A. Industrial motors can draw hundreds of amperes.
Resistance
Milliohms (mΩ), ohms (Ω), kilohms (kΩ), megohms (MΩ). Wire resistance is measured in milliohms per metre. Standard electronic resistors range from 1Ω to 10MΩ. Insulation resistance testing measures in hundreds of megohms.
Power
Milliwatts (mW), watts (W), kilowatts (kW), megawatts (MW). A smartphone charger delivers 5–20W. A household circuit breaker handles 1,800W (15A × 120V in the US) or 3,680W (16A × 230V in Europe). A large power station generates 500–1,000MW.
Common Reference Values for Checking Results
When validating calculator outputs, these real-world benchmarks help you spot errors immediately.
A standard USB 2.0 port supplies 5V at 0.5A, delivering 2.5W. USB-C Power Delivery can supply up to 5A at 20V (100W) under the latest specification. A typical US household outlet provides 120V at up to 15A on a standard 14-gauge circuit, giving a maximum of 1,800W. European outlets supply 230V at up to 13A (UK) or 16A (continental), delivering up to 2,990W or 3,680W respectively.
A standard 60W-equivalent LED bulb draws approximately 8–10W at 120V, with an effective hot resistance of about 1,440–1,800Ω. A 100W incandescent filament lamp (now largely phased out) drew 0.83A at 120V with a hot filament resistance of approximately 144Ω. A car battery outputs 12.6V fully charged, and a starter motor can draw 200–400A briefly, demanding 2,500–5,000W during ignition.
A Tesla Model 3 supercharger delivers up to 250kW at approximately 400V DC, drawing over 600A. Industrial arc furnaces used in steelmaking can draw 40,000–80,000A at voltages of 100–1,000V, consuming 10–100MW.
Ohm's Law Calculator for LED Circuits
LEDs require a current-limiting resistor to prevent damage. Without one, the LED draws excessive current from the supply and fails — often within seconds. The calculator finds the correct resistor value using a modified form of Ohm's Law.
where Vs is the supply voltage, Vf is the LED's forward voltage, and I is the target current.
LED forward voltage varies by colour and semiconductor material. Typical values: red LEDs 1.8–2.2V, yellow 2.0–2.2V, green 2.0–3.5V, blue and white 2.9–3.4V. Standard indicator LEDs operate at 15–20mA. High-power LEDs used in lighting can draw 350mA to over 1A.
Example
A 9V battery driving a red LED (Vf = 2.0V) at 15mA. The voltage across the resistor is 9V − 2V = 7V. R = 7V ÷ 0.015A = 467Ω. The nearest standard resistor value is 470Ω (from the E24 series). Power dissipated in the resistor: P = 7V × 0.015A = 0.105W. A standard ¼W (0.25W) resistor handles this comfortably.
Selecting a resistor below the calculated value pushes too much current through the LED and shortens its lifespan or destroys it instantly. Selecting a significantly higher value dims the LED noticeably. The global LED lighting market was valued at approximately $75 billion in 2023 and is projected to exceed $130 billion by 2030. Correct resistor sizing using Ohm's Law is fundamental to every one of those installations.
Ohm's Law Calculator for Voltage Drop
Every conductor has resistance, and every resistance carrying current produces a voltage drop (V = I × R). In long wire runs — automotive wiring, outdoor lighting, solar panel installations, and industrial plants — voltage drop reduces the voltage delivered to the load.
The Industry Standard
The National Electrical Code (NEC) in the United States recommends that voltage drop on branch circuits should not exceed 3% of the source voltage, and the total drop from the service entrance to the farthest outlet should not exceed 5%. In a 120V circuit, that means no more than 3.6V drop on a branch and no more than 6V total.
Example
A 12V automotive circuit pulling 10A through 15 metres of 16-gauge copper wire (resistance approximately 0.013Ω per metre, so 0.195Ω one way, 0.39Ω round trip). Voltage drop = 10A × 0.39Ω = 3.9V. That leaves only 8.1V at the load — a 32.5% drop, far beyond acceptable limits. The solution: use a thicker wire (lower gauge number, lower resistance) or reduce the cable length.
Wire resistance depends on material, cross-sectional area, and length. Copper has a resistivity of 1.68 × 10⁻⁸ Ω·m at 20°C. Aluminium, used in overhead power lines because of its lower cost and weight, has a resistivity of 2.65 × 10⁻⁸ Ω·m — about 60% higher than copper.
Ohm's Law Calculator for Series and Parallel Circuits
Series Circuits
Total resistance equals the sum of all individual resistances: R_total = R1 + R2 + R3. Current is the same through every component. Voltage divides across each resistor in proportion to its resistance. Once you find R_total, use this calculator to find total current from the supply voltage: I = V ÷ R_total.
Parallel Circuits
Total resistance is always less than the smallest individual resistor: 1/R_total = 1/R1 + 1/R2 + 1/R3. For two resistors, the shortcut formula is R_total = (R1 × R2) ÷ (R1 + R2). Voltage is the same across every branch. Current divides among branches in inverse proportion to resistance.
Example
Two resistors of 100Ω and 200Ω in parallel: R_total = (100 × 200) ÷ (100 + 200) = 66.7Ω. Connected to a 12V supply, total current = 12V ÷ 66.7Ω = 0.18A (180mA), with 120mA through the 100Ω branch and 60mA through the 200Ω branch.
Parallel resistor networks appear in LED strip wiring, speaker systems, load-balancing circuits, and voltage dividers used in sensor interfaces.
Ohm's Law Calculator for Speaker and Audio Systems
Speaker impedance, measured in ohms, describes how much a speaker resists current from an amplifier at a given frequency. The most common nominal impedances are 4Ω, 6Ω, and 8Ω. Professional PA speakers may be rated at 2Ω.
Amplifier-speaker matching is critical. Connecting speakers with impedance below the amplifier's minimum rating causes the amp to draw excessive current, overheat, and potentially fail.
Wiring two 8Ω speakers in series produces 16Ω total impedance — safe for any amplifier, but reduces power delivery to each speaker. Wiring two 8Ω speakers in parallel drops impedance to 4Ω — doubling the current draw from the amplifier compared to a single 8Ω speaker.
Industry Context
The global professional audio market was valued at approximately $8.9 billion in 2023. Impedance matching using Ohm's Law calculations is standard practice for every live sound engineer, recording studio designer, and home audio installer. Incorrect impedance matching is the leading cause of amplifier failure in touring sound systems.
Speaker impedance is frequency-dependent and includes reactive components (inductance and capacitance), so Ohm's Law provides an approximation at the nominal impedance point, not an exact value across all frequencies.
Ohm's Law Calculator for DC and AC Circuits
DC circuits maintain constant voltage and current direction. Batteries, power supplies, solar panels, and most electronic devices operate on DC. Ohm's Law applies directly: V = IR with fixed values.
AC circuits use voltage and current that reverse direction periodically — 50Hz in most of the world, 60Hz in North America, parts of South America, and some Asian countries. For purely resistive AC loads (incandescent bulbs, heating elements, electric kettles), Ohm's Law applies directly using RMS (root mean square) values.
A 120V AC outlet delivers 120V RMS, with a peak voltage of approximately 170V (120 × √2). A 230V AC outlet peaks at approximately 325V.
For circuits containing inductors (coils, motors, transformers) or capacitors, simple resistance (R) is replaced by impedance (Z), which accounts for the phase shift between voltage and current introduced by these reactive components. Impedance calculations require complex numbers and are beyond the scope of this Ohm's Law calculator, which is designed for resistive DC analysis.
Over 80% of the world's electricity is generated and distributed as three-phase AC power. Single-phase AC (what arrives at most household outlets) is derived from one phase of the three-phase system.
Ohm's Law Calculator for Automotive Circuits
Automotive electrical systems operate at 12V DC (nominally 12.6V from a fully charged lead-acid battery, rising to 13.5–14.5V when the alternator is running). Modern vehicles contain 2–3 miles (3–5 km) of wiring and between 150 and 300 electrical components.
Common Automotive Calculations
A headlight bulb rated at 55W at 12V draws I = 55 ÷ 12 = 4.58A and has a hot filament resistance of R = 12 ÷ 4.58 = 2.62Ω. A 12V heated seat element rated at 60W draws 5A.
Fuse Sizing
Automotive fuses protect circuits from overcurrent. A circuit drawing 4.58A should use a 7.5A or 10A fuse (fuses are typically rated at 125–150% of the expected maximum current). Using Ohm's Law to calculate the expected current before selecting a fuse prevents both blown fuses and fire hazards.
Electric vehicles operate at much higher voltages: the Tesla Model 3 battery pack operates at approximately 350–400V DC. The Porsche Taycan uses an 800V architecture to enable faster charging. At these voltages, the same power requires proportionally less current, allowing thinner cables and lighter wiring harnesses — a direct consequence of Ohm's Law.
Ohm's Law Calculator for Solar Panel Systems
Solar panels are DC power sources whose voltage and current vary with sunlight intensity, temperature, and load resistance. A typical residential solar panel produces 30–40V open-circuit voltage and 8–10A short-circuit current, yielding 300–400W at maximum power point.
String Sizing Example
Ten 400W panels wired in series produce a string voltage of approximately 370V (37V × 10) and maintain a string current of about 10.8A. The inverter's maximum input voltage must exceed the string's open-circuit voltage under cold conditions (voltage increases as temperature drops — approximately −0.3%/°C for crystalline silicon panels).
The global solar energy market surpassed $300 billion in 2023. Every solar installation — from a rooftop residential system to a 1GW utility-scale farm — relies on Ohm's Law for wire sizing, fuse selection, voltage drop calculations, and ensuring that conductor resistance losses do not exceed 1–2% of the system's total power output.
Ohm's Law Calculator for Power Dissipation and Resistor Sizing
When current flows through a resistor, electrical energy is converted to heat. The power dissipated is P = I² × R or equivalently P = V² ÷ R. If the power exceeds the resistor's rated wattage, the component overheats, changes value, and eventually fails — potentially causing a fire.
Standard Resistor Power Ratings
⅛W (0.125W), ¼W (0.25W), ½W (0.5W), 1W, 2W, 5W, and 10W for through-hole resistors. Surface-mount resistors are rated from 0.05W (0201 package) to 1W (2512 package).
Example
A 47Ω resistor carrying 200mA dissipates P = 0.2² × 47 = 1.88W. A standard ¼W resistor would fail catastrophically. You need at minimum a 2W resistor, and good practice is to derate by 50%, meaning you should use a 3W or 5W resistor for reliable operation.
Never operate a resistor above 50–60% of its rated power in continuous service. This extends component life and maintains resistance value stability. Military specifications (MIL-STD) typically derate to 50% of rated power at 70°C ambient temperature.
Ohm's Law Calculator for Industrial Power Conversions
Electrical power converts between several unit systems depending on the industry:
These conversions matter for motor nameplate ratings, HVAC load calculations, and generator sizing. A 5-horsepower industrial motor consumes approximately 3,730W (3.73kW) at full load. An air conditioning unit rated at 12,000 BTU/h (a "1-ton" unit) consumes approximately 3,517W.
The global electric motor market was valued at approximately $145 billion in 2023. Every motor specification sheet lists voltage, current, power, and efficiency — all quantities interconnected through Ohm's Law and the power equations.
Accuracy, Precision, and Limitations
This calculator computes results to full floating-point precision and displays up to 6 significant figures. No rounding is applied during intermediate steps — only the final displayed value is formatted.
All inputs must be positive numbers. Negative resistance, negative voltage, and negative current are not physically meaningful in the context of standard Ohm's Law calculations for passive components.
Where Ohm's Law Applies
Linear (ohmic) conductors at constant temperature — standard resistors, copper wire, aluminium conductors, nichrome heating elements, and carbon composition resistors across their rated operating range.
Where Ohm's Law Does Not Apply
Non-linear components including diodes (which follow the Shockley equation with exponential I-V curves), transistors, gas discharge tubes, varistors, and thermistors where resistance changes with temperature. Superconductors also violate Ohm's Law — below their critical temperature, resistance drops to exactly zero, and current flows without any voltage drop.
For a complete exploration of the physics behind Ohm's Law, including why it works at the electron level (the Drude model), when it breaks down, and its connection to Maxwell's equations, see our Ohm's Law — Complete Explanation.
Related Calculators and Tools
Calculators
Articles
Scientist
Frequently Asked Questions
What is an Ohm's Law Calculator?
An Ohm's Law Calculator is a tool that computes voltage, current, resistance, or power from any two known electrical values. It applies the Ohm's Law formula (V = IR) and the power formula (P = VI) automatically, removing the need for manual algebra. This calculator supports all twelve derived formulas covering every possible two-input combination.
How do I use this calculator?
Select what you want to solve for (voltage, current, resistance, or power), choose which two values you know from the dropdown, enter those values in the input fields, and click Calculate. The calculator returns the two unknown values with step-by-step working showing the formula used and the substitution performed.
Can this calculator solve for voltage drop?
Yes. Voltage drop equals current multiplied by resistance (V = I × R). Enter the wire or component resistance and the current flowing through it, and the calculator returns the voltage drop. For long wire runs, you need to account for the total round-trip resistance of the conductor — both the outgoing and return paths.
Does this calculator work for AC circuits?
This calculator is designed for DC and purely resistive AC circuits. For resistive AC loads — incandescent bulbs, heating elements, electric kettles — Ohm's Law applies directly using RMS voltage and current values. Circuits containing inductors or capacitors require impedance (Z) instead of simple resistance, which involves phase angles and complex numbers not covered by this tool.
How do I calculate the resistor for an LED?
Use the formula R = (Vs − Vf) ÷ I, where Vs is the supply voltage, Vf is the LED's forward voltage (typically 1.8–3.4V depending on colour), and I is the target current (usually 15–20mA for standard indicator LEDs). Enter the resulting voltage across the resistor (Vs − Vf) and the target current into this calculator to find the resistance and power dissipation.
What is the difference between ohms and Ohm's Law?
The ohm (Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. Ohm's Law is the mathematical relationship V = IR that describes how voltage, current, and resistance are connected. One ohm is defined as the resistance that produces a current of one ampere when one volt is applied.
Why does my resistor get hot?
When current flows through a resistor, electrical energy converts to heat at a rate of P = I² × R watts. If the power dissipated exceeds the resistor's rated wattage (typically ¼W for standard through-hole resistors), it overheats. Use this calculator to find the power dissipation, then select a resistor rated for at least double that value for reliable operation.
Can Ohm's Law calculate amps?
Yes. Current in amperes equals voltage divided by resistance: I = V ÷ R. Enter the voltage across a component and its resistance, and this calculator returns the current. If you know power and voltage instead, current equals I = P ÷ V.
Is Ohm's Law universal?
No. Ohm's Law applies to ohmic (linear) conductors under stable temperature conditions — standard resistors, metal wires, and similar materials. It does not hold for non-linear devices like diodes, transistors, thermistors, and gas discharge tubes, where resistance changes with applied voltage or temperature. Superconductors also violate Ohm's Law, as their resistance drops to exactly zero below a critical temperature.
What is the Ohm's Law formula wheel?
The Ohm's Law formula wheel (also called the Ohm's Law circle or pie chart) is a visual reference showing all twelve formulas for V, I, R, and P arranged in a circular diagram. You select the quantity you want to find in the centre, then read the formula for your two known values from the corresponding segment. This calculator implements all twelve formulas from the wheel automatically.
About Georg Simon Ohm
The Physicist Behind This Calculator
Georg Simon Ohm (1789–1854)
Every calculation on this page traces back to the relationship Georg Simon Ohm published in 1827: V = IR. Ohm's Law (V = IR), ohm unit of resistance, acoustic law.
Read his full biography →