Ohm Law Calculator

This calculator solves for voltage, current, resistance, or power. Type in any two known values, and the tool computes the other two automatically using the Ohm's Law formula and the power formula.

The calculator works for DC circuits, resistive AC circuits, and any application where Ohm's Law applies: LED resistor sizing, voltage divider design, speaker impedance matching, and voltage drop over long wire runs.

Calculate Voltage, Current, Resistance, or Power Using Ohm's Law

The calculator uses Ohm's Law to calculate voltage, current, or resistance from any two known quantities. To calculate voltage, multiply current by resistance. To calculate current (also called amps), divide voltage by resistance. To calculate resistance, divide voltage by current. To calculate power (also called wattage), multiply voltage by current.

Enter the two values you know, and the tool solves for current flow, voltage, resistance, or power without manual math.

Ohm Law Calculator Formula

Ohm's Law states that current through a conductor is proportional to voltage and inversely proportional to resistance. The Ohm Law Calculator formula for resistance is R = V/I. The ohm's law formula to calculate voltage is V = I × R. These two equations, plus the current equation I = V/R, form the base of every calculation the tool performs.

Ohm's Law Equations Explained

There are 4 core equations behind the calculator:

The power formula comes from Joule's Law, not Ohm's Law directly. Combining the two lets the calculator find electrical power in watts from voltage and current alone, without a separate wattmeter reading.

How to Use Ohm Law Calculator

Enter two known values into the calculator, and it returns the other two automatically. The tool detects which fields are filled and picks the correct formula on its own.

Enter Known Electrical Values

Users can enter any two of these four values: voltage, current, resistance, or power. If you know the battery voltage and the resistor value, the calculator finds current and power. If you know current and power, it finds voltage and resistance. This flexibility covers most circuit design and troubleshooting scenarios.

Electrical Units Supported by Ohm Law Calculator

The calculator supports SI units and their common multiples:

• Voltage: millivolts (mV), volts (V), kilovolts (kV)
• Current: milliamperes (mA), amperes (A)
• Resistance: ohms (Ω), kilohms (kΩ), megohms (MΩ)
• Power: watts (W), kilowatts (kW), megawatts (MW), and conversions to horsepower (hp) and BTU per hour

Ohm Law Calculator for Circuits

The calculator solves resistance, voltage, current, and power for series circuits, parallel circuits, and combination circuits with multiple resistors.

In a series circuit, current stays the same through every component, and resistances add directly: R_total = R1 + R2 + R3. In a parallel circuit, voltage stays the same across every branch, and total resistance drops below the smallest individual resistor.

Ohm Law Calculator for Parallel Circuits

For 2 resistors in parallel, total resistance equals (R1 × R2) / (R1 + R2). For 3 or more resistors, use 1/R_total = 1/R1 + 1/R2 + 1/R3. Once you find the total resistance, apply Ohm's Law to find total current draw from the source voltage.

Parallel resistor networks appear in speaker wiring, LED strips, and load-balancing circuits, where multiple components share the same voltage source.

Ohm Law Calculator for Multiple Resistors

The calculator handles 2 resistors or larger combination networks. For resistors in series, add the values directly. For resistors in parallel, use the reciprocal formula above. Mixed networks require solving each series or parallel section first, then combining the results into one total resistance value before applying Ohm's Law.

Ohm Law Calculator for AC and DC Circuits

Direct current (DC) flows in one direction at a constant voltage, like current from a battery. Alternating current (AC) reverses direction periodically, like household mains power. Ohm's Law applies directly to resistive circuits in both cases, using RMS (root mean square) voltage and current values for AC.

Ohm's Law applies to purely resistive AC loads the same way it applies to DC loads: V = I × R. Circuits with inductors or capacitors need impedance (Z), measured in ohms but including a phase component, instead of plain resistance. Impedance calculations require complex numbers and a separate reactance formula.

Ohm Law Calculator for AC Circuits

AC voltage is usually expressed as an RMS value, since the instantaneous voltage constantly changes. A 120V AC household outlet delivers 120V RMS, with a peak voltage near 170V. For resistive AC loads, such as incandescent bulbs and heating elements, Ohm's Law calculates current and power directly from the RMS voltage and the resistance value.

Ohm Law Calculator for DC Circuits

DC circuits keep a fixed voltage and current direction. Battery circuits, power supplies, and most electronic devices run on DC. A 9V battery connected to a 100-ohm resistor draws 90 milliamperes of current and dissipates 0.81 watts of power, all calculated directly with Ohm's Law.

Ohm Law Calculator for LED Circuits

LEDs need a current-limiting resistor to prevent damage. The calculator finds the correct resistor value from the supply voltage, the LED forward voltage, and the target current.

LED forward voltage varies by color and material, typically between 1.8V and 3.4V. LED current usually runs between 10mA and 20mA for standard indicator LEDs. Selecting a resistor below the calculated value pushes too much current through the LED and shortens its life or destroys it instantly.

LED Resistor Calculation Using Ohm's Law

The resistor formula for LED circuits is:

Vs is the supply voltage, Vf is the LED forward voltage, and I is the target current in amps. A 9V supply driving a red LED (Vf = 2V) at 15mA needs a 467-ohm resistor, rounded up to a standard 470-ohm value.

Ohm Law Calculator for Speakers and Audio Systems

Speaker impedance, measured in ohms, describes how much a speaker resists current from an amplifier at a given frequency. Common speaker impedances are 4, 6, and 8 ohms.

Amplifier matching depends on total speaker impedance in the circuit. Wiring 2 speakers in parallel lowers the total impedance and increases the current draw on the amplifier. Wiring 2 speakers in series raises the total impedance and lowers current draw. Resistance describes a fixed opposition to DC current; impedance describes a frequency-dependent opposition that includes the speaker's inductance and capacitance, so Ohm's Law gives an approximation for audio circuits, not an exact figure across all frequencies.

Ohm Law Calculator for Three Phase Systems

Three-phase electrical systems deliver power through 3 alternating currents offset by 120 degrees, used across industrial electrical applications, motors, and large-scale power distribution. Three-phase power calculations use line voltage and line current, with an added factor of √3 (about 1.732) compared to single-phase formulas: P = √3 × V_line × I_line × power factor.

Line voltage measures the potential difference between any two of the three phase conductors. Line current measures the current flowing through each conductor. Industrial motors and large HVAC equipment run on three-phase power because it delivers more power per conductor size than single-phase systems.

Ohm Law Calculator for Voltage Drop

Voltage drop equals current multiplied by resistance: V = I × R.

Every wire carries some resistance, so voltage drops along its length as current flows. Long wire runs, automotive wiring, and electrical installations all need voltage drop calculations to keep the delivered voltage close to the source voltage.

A 12V automotive circuit pulling 10 amps through a wire with 0.1 ohms of resistance loses 1V along the wire, leaving 11V at the load. Voltage drop over 3% of the source voltage is generally considered acceptable for most branch circuits; drops beyond that reduce equipment performance and increase heat in the wiring.

Ohm Law Calculator for Vape Applications

Vape coil resistance, battery voltage, and power output follow the same Ohm's Law relationship as any other resistive circuit. Lower coil resistance draws higher current from the battery and produces more power (and vapor) at the same voltage. Higher coil resistance draws less current and produces less power. Users match battery output and coil resistance within the manufacturer's rated limits to avoid excess current draw.

Ohm Law Calculator for 555 Timer Circuits

555 timer circuits use resistors and a capacitor to set timing intervals. Resistance in these circuits controls the charge and discharge rate of the capacitor, which sets the output pulse width and frequency. Higher resistance values slow the charge and discharge rate and lower the current flowing into the capacitor; lower resistance values speed up timing and raise current flow through the resistor-capacitor (RC) network.

Ohm Law Calculator for Power and Horsepower Conversion

Electrical power converts between watts, kilowatts, megawatts, horsepower, and BTU per hour. One horsepower equals about 746 watts. One kilowatt equals about 1.341 horsepower. One watt equals about 3.412 BTU per hour.

These conversions matter for motor nameplate ratings, generator sizing, and HVAC load calculations, where power shows up in different unit systems depending on the industry and the country.

Explore additional electrical calculators designed to simplify resistor, voltage, and circuit calculations.

Voltage Divider Calculator

A voltage divider splits input voltage across 2 resistors in series, producing a lower output voltage at the midpoint. The formula is:

Vin is the input voltage, R1 and R2 are the two resistors in the series network, and Vout is the reduced voltage measured across R2. Voltage dividers appear in sensor circuits, where a variable resistor (like a thermistor or photoresistor) forms one half of the divider and produces a voltage signal that changes with temperature or light level.

Calculate Output Voltage Using Two Resistors

A 12V input through R1 = 1kΩ and R2 = 2kΩ produces an output voltage of 8V across R2. Changing either resistor value shifts the output voltage without changing the input source.

Resistor Color Code Calculator

Resistors use colored bands to mark their resistance value, tolerance, and sometimes a temperature coefficient. Each band's color and position represents a digit, a multiplier, or a tolerance percentage.

Decode Resistor Values Using Color Bands

4-band resistors use 2 significant digits, 1 multiplier band, and 1 tolerance band. 5-band resistors add a third significant digit for tighter precision. 6-band resistors add a temperature coefficient band on top of the 5-band structure. Reading the bands from left to right, starting with the band closest to a resistor lead, gives the resistance value and its tolerance range.

Why Use Our Electrical Calculators?

These calculators serve students learning circuit theory, engineers designing electrical systems, electricians sizing wire and verifying voltage drop, and hobbyists building electronics projects. Each tool removes manual algebra from routine electrical calculations and returns results in the unit format the user needs, whether that's millivolts for a sensor circuit or kilowatts for a power system.

What Is Ohm's Law?

Ohm's Law states that current through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance between them. German physicist Georg Simon Ohm published this relationship in 1827.

Voltage pushes current through a circuit. Resistance opposes that current flow. Raise the voltage, and current rises with it, if resistance stays fixed. Raise the resistance, and current drops, if voltage stays fixed. This relationship forms the base for nearly every circuit calculation in electronics and electrical engineering, from sizing a single resistor to designing a power distribution network.

Ohm's Law Explained: Relationship Between Voltage, Current, and Resistance

Voltage, current, and resistance connect through a single fixed ratio in any resistive circuit: V/I = R. For the full plain-language explanation of Ohm's Law, including worked examples for beginners, Doubling the voltage across a fixed resistor doubles the current through it. Doubling the resistance while holding voltage constant cuts the current in half. This proportional relationship holds for ohmic conductors, materials like copper wire and standard resistors, across a wide range of operating conditions.

Go Deeper: The Physics Behind Ohm's Law

Ohm's Law is a macroscopic empirical relationship, but the physics behind it runs deep. Electrons colliding with a crystal lattice, Drude's free-electron model, and the microscopic point-form equation all explain why V = IR holds for ohmic materials — and why it breaks down for others.

• Read why Ohm's Law actually works, derived from electron physics — the Drude model, electron drift velocity, and resistivity explained.
• Explore the point form of Ohm's Law, J = σE — used in real device design and electromagnetic field theory.
• Discover why superconductors break Ohm's Law entirely — resistance drops to zero and Cooper pairs replace classical electron scattering.
• Understand why diodes don't follow Ohm's Law — exponential I-V curves, the Shockley diode equation, and non-linear semiconductor physics.

Ohm's Law Formula and Equations Explained

The Ohm's Law formula for calculating resistance is R = V/I. The formula for voltage is V = I × R. The formula for current is I = V/R. Each version of the equation isolates one unknown value from the other two known quantities.

Ohm's Law Triangle Calculator

The Ohm's Law triangle places V at the top and I and R at the bottom, split by a horizontal line. Covering V shows I × R underneath, giving the voltage formula. Covering I shows V over R, giving the current formula. Covering R shows V over I, giving the resistance formula. Click any letter in the triangle to see its formula rearranged from the other two.

Ohm's Law and Electrical Power (Joule's Law)

Joule's Law relates electrical power to current and resistance, and combines with Ohm's Law to give three interchangeable power formulas:

• P = V × I
• P = I² × R
• P = V² / R

Ohm's Law and Joule's Law together let the calculator find power in watts from any two known values among voltage, current, and resistance, without needing all three measured directly.

Electrical Power Units Explained

Power scales from watts (W) up to kilowatts (kW), then megawatts (MW), with horsepower (hp) and BTU per hour as parallel unit systems used in mechanical and thermal engineering. A 100-watt light bulb draws 0.1 kilowatts. A power plant generator might output 500 megawatts, equal to roughly 670,500 horsepower.

Conductance and Its Relationship With Resistance

Conductance measures how easily current flows through a material, the inverse of resistance: G = 1/R. Conductance is measured in siemens (S).

Low resistance means high conductance, and high resistance means low conductance. A thick copper wire has low resistance and high conductance; a thin nichrome wire has higher resistance and lower conductance. Engineers use conductance instead of resistance in circuits involving multiple parallel paths, since conductance values add directly across parallel branches.

Electrical Units Used in Ohm's Law

The International System of Units (SI) defines the base units behind every Ohm's Law calculation: the volt (V) for voltage, the ampere (A) for current, and the ohm (Ω) for resistance. Power uses the watt (W), derived from voltage and current.

Voltage Units

Voltage scales from millivolts (mV) up to volts (V) and kilovolts (kV). A standard AA battery outputs 1.5V, or 1,500mV. Power transmission lines carry voltage in the tens or hundreds of kilovolts.

Current Units

Current scales from microamperes (µA) up to milliamperes (mA) and amperes (A). Small electronic sensors draw current in the microampere range. Household appliances typically draw between 1 and 15 amperes.

Resistance Units

Resistance scales from milliohms (mΩ) up to ohms (Ω), kilohms (kΩ), and megohms (MΩ). Wire resistance is often measured in milliohms per meter. Standard electronic resistors range from a few ohms to several megohms.

Applications of Ohm's Law in Real Electrical Systems

Ohm's Law governs current flow in electronics, batteries, DC power systems, and large-scale electric power distribution.

Ohm's Law in Electronics

Engineers designing ICs, transistor biasing circuits, and PCB traces rely on the point form of Ohm's Law used in real device design (J = σE). Circuit designers use Ohm's Law to size resistors, predict current draw through components, and protect sensitive parts like LEDs from excess current. Every resistor value on a circuit board traces back to an Ohm's Law calculation matching the supply voltage to the current the component needs.

Ohm's Law in Batteries and DC Systems

Battery voltage, current draw, and load resistance follow Ohm's Law directly. A battery with a fixed voltage delivers current based on the resistance of whatever it powers. Connecting a lower-resistance load to the same battery draws more current and drains the battery faster.

Ohm's Law in Electrical Power Systems

Power utilities apply Ohm's Law to voltage distribution and current flow across transmission and distribution networks. Raising transmission voltage lowers current for the same power delivered, which reduces resistive losses in the lines. This principle, high voltage and low current for long-distance transmission, keeps power grids efficient across hundreds of miles of wire.

Limitations of Ohm's Law

Ohm's Law applies to ohmic conductors under constant temperature conditions. Materials and components that keep a fixed resistance regardless of applied voltage, like standard resistors and copper wire, follow the law closely across a wide operating range.

Ohm's Law breaks down for non-linear devices. Diodes, transistors, and other semiconductor components change their resistance based on the voltage applied, so a single fixed R value cannot describe their behavior. Temperature changes also shift resistance in most materials, which introduces error into Ohm's Law calculations for components running hot, like incandescent filaments or high-power resistors under heavy load.

History of Ohm's Law and Georg Simon Ohm

Georg Simon Ohm, a German physicist, published the relationship between voltage, current, and resistance in 1827, in his work "Die galvanische Kette, mathematisch bearbeitet." Ohm derived the law through direct experiments measuring current through wires of different lengths and thicknesses under varying voltage.

The scientific community met Ohm's work with skepticism at first. Recognition came later in his career, and the ohm (Ω), the SI unit of electrical resistance, now carries his name. Ohm's Law remains one of the base principles taught in electrical engineering and electronics programs worldwide.

Ohm Law Example Calculation

This ohm law example calculates the current flowing through a 470 ohm resistor connected to a 9 volt battery source:

Citation and Physics Context

The core statement of Ohm's Law is that current through a linear conductor is directly proportional to voltage and inversely proportional to resistance. Electrical power in a resistive circuit equals the square of the current multiplied by the resistance. Ohmic behavior occurs only when temperature remains constant and the electrical carrier density does not shift under voltage stress.