MyCalcBuddy

Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law relationships.

Calculate

V = I x R | P = V x I
Enter any two values to calculate the others

Calculated Values

12.000
Voltage (V)
2.0000
Current (A)
6.00
Resistance (ohms)
24.000
Power (W)

Unit Conversions

Current2000.00 mA
Power24000.00 mW
Resistance0.0060 k ohms
Conductance0.166667 S

Ohm's Law Formulas

V = I x R
I = V / R
R = V / I
P = V x I
P = I^2 x R
P = V^2 / R

What is Ohm's Law?

Ohm's Law is the fundamental principle describing the relationship between voltage, current, and resistance in electrical circuits. Named after German physicist Georg Simon Ohm, this law states that current through a conductor is directly proportional to voltage and inversely proportional to resistance.

QuantitySymbolUnitUnit SymbolDescription
VoltageV or EVoltVElectrical potential difference
CurrentIAmpereAFlow of electric charge
ResistanceROhmΩOpposition to current flow
PowerPWattWRate of energy transfer
ConductanceGSiemensSInverse of resistance (1/R)

Ohm's Law Triangle

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

Where:

  • V= Voltage in Volts (electrical pressure)
  • I= Current in Amperes (electron flow)
  • R= Resistance in Ohms (opposition to flow)

Complete Ohm's Law Formulas

Ohm's Law can be combined with the power formula to create a complete set of electrical calculations. The power wheel shows 12 variations for finding any quantity.

FindFormula 1Formula 2Formula 3
Voltage (V)V = I × RV = P / IV = √(P × R)
Current (I)I = V / RI = P / VI = √(P / R)
Resistance (R)R = V / IR = V² / PR = P / I²
Power (P)P = V × IP = I² × RP = V² / R

Power Formulas

P = V × I (basic power formula) P = I² × R (power from current and resistance) P = V² / R (power from voltage and resistance)

Where:

  • P= Power in Watts
  • I²R= Current squared times resistance
  • V²/R= Voltage squared divided by resistance

Common Unit Prefixes in Electronics

Electronics work spans many orders of magnitude. Understanding metric prefixes is essential for practical calculations.

PrefixSymbolMultiplierExample Usage
MegaM×1,000,0001 MΩ = 1,000,000 Ω
Kilok×1,0004.7 kΩ = 4,700 Ω
(base)—×1100 Ω, 5 V, 1 A
Millim×0.00120 mA = 0.020 A
Microµ×0.000001100 µA = 0.0001 A
Nanon×0.00000000110 nA (very small current)

Conversion tip: When using Ohm's Law with mixed units, convert everything to base units (V, A, Ω) first, or use consistent prefixes (mA with kΩ gives V directly).

Series and Parallel Circuits

Ohm's Law applies differently to series and parallel circuits. Understanding how voltage and current behave in each configuration is crucial.

PropertySeries CircuitParallel Circuit
CurrentSame through all components (Itotal = I1 = I2)Divides between branches (Itotal = I1 + I2)
VoltageDivides across components (Vtotal = V1 + V2)Same across all branches (Vtotal = V1 = V2)
ResistanceRtotal = R1 + R2 + ...1/Rtotal = 1/R1 + 1/R2 + ...
Total resistanceGreater than largest RLess than smallest R
Failure behaviorOne open breaks circuitOne open doesn't break others

Two Resistors in Parallel

R_total = (R1 × R2) / (R1 + R2) For equal resistors: R_total = R / n

Where:

  • R1, R2= Individual resistor values
  • n= Number of equal resistors in parallel

Practical Applications of Ohm's Law

Ohm's Law is used in virtually every electrical and electronic calculation. Here are common practical applications.

ApplicationWhat You're CalculatingWhy It Matters
LED current limitingResistor value to limit LED currentPrevents LED burnout
Power dissipationHeat generated in resistorProper component sizing
Voltage dropVoltage lost across wire/componentWire gauge selection
Battery runtimeCurrent draw from loadBattery capacity planning
Fuse selectionMaximum expected currentCircuit protection
Motor currentCurrent under loadPower supply sizing

Limitations of Ohm's Law

While Ohm's Law is fundamental, it has limitations. It applies perfectly only to "ohmic" materials with constant resistance.

Component/SituationOhm's Law Applies?Reason
Resistors (metal film)YesConstant resistance over operating range
DiodesNoNon-linear V-I relationship
LEDsNoExponential current increase with voltage
TransistorsPartiallyComplex operating regions
Incandescent bulbsNo (heated)Resistance changes with temperature
SuperconductorsNoZero resistance below critical temperature

Note: For non-ohmic devices, use the device's characteristic curves or models instead of simple Ohm's Law calculations.

Worked Examples

Calculate Current Through a Resistor

Problem:

A 12V battery is connected to a 4.7kΩ resistor. What current flows through the circuit?

Solution Steps:

  1. 1Identify given values: V = 12V, R = 4.7kΩ = 4700Ω
  2. 2Use Ohm's Law: I = V / R
  3. 3Substitute values: I = 12V / 4700Ω
  4. 4Calculate: I = 0.00255 A = 2.55 mA
  5. 5Verify units: Volts / Ohms = Amperes ✓

Result:

2.55 mA (milliamperes) flows through the resistor

Find Resistance for Desired Current

Problem:

You need exactly 20mA to flow from a 5V supply. What resistor value is required?

Solution Steps:

  1. 1Identify: V = 5V, I = 20mA = 0.020A
  2. 2Rearrange Ohm's Law: R = V / I
  3. 3Substitute: R = 5V / 0.020A
  4. 4Calculate: R = 250Ω
  5. 5Find nearest standard value: 240Ω or 270Ω
  6. 6With 240Ω: I = 5/240 = 20.8mA
  7. 7With 270Ω: I = 5/270 = 18.5mA

Result:

250Ω (use 240Ω for slightly higher current or 270Ω for lower)

Calculate Power Dissipation

Problem:

A 100Ω resistor has 500mA flowing through it. How much power does it dissipate, and what wattage rating is needed?

Solution Steps:

  1. 1Identify: R = 100Ω, I = 500mA = 0.5A
  2. 2Use power formula: P = I² × R
  3. 3Substitute: P = (0.5)² × 100
  4. 4Calculate: P = 0.25 × 100 = 25W
  5. 5Rule of thumb: Use 2× rating for safety
  6. 6Required rating: 25W × 2 = 50W minimum

Result:

25 Watts dissipated; use at least a 50W resistor

Tips & Best Practices

  • ✓Remember the triangle: V = I × R. Cover what you need to find, and the remaining symbols show the formula.
  • ✓When mA and kΩ are used together, the result is in Volts directly (mA × kΩ = V).
  • ✓Always check units before calculating—convert everything to base units (V, A, Ω) when in doubt.
  • ✓For resistor power rating, use at least 2× the calculated dissipation for reliable operation.
  • ✓In parallel circuits, total resistance is always less than the smallest individual resistor.
  • ✓Use Ohm's Law to verify your circuit before building—calculate expected currents and voltages.
  • ✓A short circuit has near-zero resistance, resulting in very high current (dangerous!).

Related Calculators

Power Calculator

Calculate electrical power

Resistor Calculator

Resistor color codes and values

Voltage Divider Calculator

Voltage divider design

LED Resistor Calculator

Current limiting resistors

Capacitor Calculator

Capacitor calculations

Battery Life Calculator

Battery runtime estimation

Frequently Asked Questions

One Ohm (Ω) is the resistance that allows exactly 1 Ampere of current to flow when 1 Volt is applied. Think of it like the 'narrowness' of a pipe for water flow—more Ohms means harder for current to flow through. A typical LED resistor is hundreds to thousands of Ohms, while a short piece of wire might be fractions of an Ohm.
Electronics deals with resistance values spanning many orders of magnitude. Writing '4,700,000 Ohms' is cumbersome, so we use 4.7 MΩ instead. Similarly, 4,700 Ohms becomes 4.7 kΩ. This keeps numbers manageable. When calculating, remember to convert: 4.7 kΩ = 4,700 Ω and 4.7 MΩ = 4,700,000 Ω.
Ohm's Law works directly with AC for purely resistive loads. However, capacitors and inductors introduce 'reactance,' which combines with resistance to form 'impedance.' For AC circuits with reactive components, use V = I × Z (where Z is impedance), and calculations involve complex numbers or phase angles.
Ohmic components have a linear voltage-current relationship (graph is a straight line through the origin). Resistors, wires, and most passive components are ohmic. Diodes, LEDs, transistors, and temperature-sensitive devices are non-ohmic—their resistance changes with voltage, current, or temperature. Check the component's datasheet for V-I characteristics.
The resistor overheats. At moderate overload, it may discolor and its value may drift. At severe overload, it can smoke, catch fire, or fail open (breaking the circuit). Always calculate power dissipation (P = I²R or V²/R) and use a resistor rated for at least twice the expected dissipation for reliability.
Use the 'Ohm's Law Triangle': Draw a triangle with V at top, I and R at the bottom. Cover what you want to find—what remains is the formula. Cover V: see I×R. Cover I: see V/R (V over R). Cover R: see V/I. For power, add P to make a wheel with 12 formulas relating V, I, R, and P.

Sources & References

Last updated: 2026-01-22