Ohm's Law Calculator: Voltage, Current, Resistance & Power — Complete Electrical Guide

• Ohm's Law: V = IR. Three variables, three calculators in one — enter any two to find the third. Voltage (V) in volts, current (I) in amperes, resistance (R) in ohms.
• Resistors in series: R_total = R₁ + R₂ + ... (resistances add directly). In parallel: 1/R_total = 1/R₁ + 1/R₂ + ... (reciprocals add). Series increases resistance; parallel decreases it.
• Electrical power: P = VI = I²R = V²/R. A 100W bulb at 220V draws I = 100/220 = 0.455 A and has resistance R = 220/0.455 = 484 Ω.
• For JEE/NEET and Class 12: Kirchhoff's laws (KVL and KCL) combined with Ohm's law solve any circuit. Wheatstone bridge and potentiometer are the two most-tested circuit problems in Indian board exams.

Use our free Ohm's Law Calculator — enter any two of V, I, R, or P and instantly find all four electrical quantities.

OHM'S LAW: THE FOUNDATION

The Three Forms

V = IR → Find Voltage (V) when current and resistance are known I = V/R → Find Current (I) when voltage and resistance are known R = V/I → Find Resistance (Ω) when voltage and current are known

The Ohm's Law Triangle

` V / \ I × R `

Cover the quantity you want to find. The remaining two show the operation:

• Cover V → I × R
• Cover I → V ÷ R
• Cover R → V ÷ I

Ohm's Law Worked Examples

Example 1: A 12V battery drives current through a 6Ω resistor. Find I. I = V/R = 12/6 = 2 A

Example 2: A current of 0.5 A flows through a 220Ω resistor. Find V. V = IR = 0.5 × 220 = 110 V

Example 3: A kettle draws 5 A from a 240V supply. Find its resistance. R = V/I = 240/5 = 48 Ω

VOLTAGE CALCULATOR

EMF and Terminal Voltage

Example: A battery of EMF 12V and internal resistance 0.5Ω drives 2A. Terminal voltage V = 12 − (2)(0.5) = 11V

Voltage Divider

V_out = V_in × R₂/(R₁ + R₂)

Used to get a fraction of the supply voltage:

Example: V_in = 9V, R₁ = 300Ω, R₂ = 150Ω V_out = 9 × 150/(300+150) = 9 × 150/450 = 3V

Kirchhoff's Voltage Law (KVL)

The sum of all voltages around any closed loop = 0

Or equivalently: sum of EMFs = sum of voltage drops (IR)

Sign convention: Travelling in direction of current through a resistor = voltage drop (−IR); against current = voltage rise (+IR). Through EMF source from − to + = +EMF; from + to − = −EMF.

RESISTANCE CALCULATOR

Resistors in Series

R_total = R₁ + R₂ + R₃ + ...

• Total resistance > any individual resistor
• Same current through all
• Voltage divides proportionally

Example: 10Ω, 20Ω, 30Ω in series: R_total = 10 + 20 + 30 = 60Ω

Resistors in Parallel

1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...

For two resistors: R_total = R₁R₂/(R₁ + R₂) (product over sum)

• Total resistance < smallest individual resistor
• Same voltage across all
• Current divides inversely proportional to resistance

Example: 6Ω and 12Ω in parallel: R_total = (6 × 12)/(6 + 12) = 72/18 = 4Ω

Example — Three in parallel: 4Ω, 6Ω, 12Ω 1/R_total = 1/4 + 1/6 + 1/12 = 3/12 + 2/12 + 1/12 = 6/12 = 1/2 R_total = 2Ω

Mixed Series-Parallel Circuits

Method: Simplify from innermost parallel groups outward.

Example: ` 15V ─ 3Ω ─┬─ 6Ω ─┬─ └─ 12Ω ─┘ ` Parallel combination: (6 × 12)/(6 + 12) = 4Ω Total resistance: 3 + 4 = 7Ω Total current: I = 15/7 = 2.14A Voltage across 3Ω: V = 2.14 × 3 = 6.43V Voltage across parallel: 15 − 6.43 = 8.57V I through 6Ω = 8.57/6 = 1.43A I through 12Ω = 8.57/12 = 0.71A

POWER CALCULATOR

Four Forms of Electrical Power

The Power Triangle:

` P / \ V × I `

Worked Examples:

A 100W, 220V bulb: Current: I = P/V = 100/220 = 0.455 A Resistance: R = V/I = 220/0.455 = 484 Ω (hot) (Note: cold resistance is much lower — filament resistance increases with temperature)

A 5Ω resistor carrying 3A: P = I²R = (3)² × 5 = 9 × 5 = 45 W

Heat generated in 10 minutes: H = Pt = 45 × (10 × 60) = 27,000 J = 27 kJ

Electrical Energy and Units

Energy calculation: E (in kWh) = Power (kW) × Time (hours) A 2kW iron used for 30 minutes: E = 2 × 0.5 = 1 kWh (1 unit)

KIRCHHOFF'S CURRENT LAW (KCL)

The sum of all currents entering a node = sum of all currents leaving

Or: Σ currents at a node = 0 (with sign convention: entering = +, leaving = −)

Example: At a junction: 5A and 3A enter; I₁ and I₂ leave. 5 + 3 = I₁ + I₂ → I₁ + I₂ = 8A

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WHEATSTONE BRIDGE

Balanced condition: R₁/R₂ = R₃/R₄ (or R₁R₄ = R₂R₃)

When balanced: no current through galvanometer

Used to find an unknown resistance: R_x = R₂ × R₃/R₁

` A / \ R₁ R₂ / \ B─── G ───D \ / R₃ R₄ \ / C `

Balanced when: R₁/R₃ = R₂/R₄

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RESISTIVITY AND FACTORS AFFECTING RESISTANCE

R = ρL/A

Where: ρ = resistivity (Ω·m), L = length (m), A = cross-sectional area (m²)

Effect	Formula	Direction
Increase length	R = ρL/A	R increases
Increase area	R = ρL/A	R decreases
Increase temperature (metals)	R = R₀(1 + αT)	R increases
Increase temperature (semiconductors)	—	R decreases

Resistivity of common materials (at 20°C):

Material	Resistivity (Ω·m)	Type
Silver	1.59 × 10⁻⁸	Best conductor
Copper	1.72 × 10⁻⁸	Common conductor
Aluminium	2.82 × 10⁻⁸	Electrical wire
Iron	1.0 × 10⁻⁷	Moderate
Nichrome	1.0 × 10⁻⁶	Heating elements
Silicon	6.4 × 10²	Semiconductor
Glass	10¹⁰–10¹⁴	Insulator
Rubber	10¹³–10¹⁶	Insulator

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QUICK REFERENCE TABLE

Ohm's Law — Find Any Variable:

Find	Formula 1	Formula 2	Formula 3
V	V = IR	V = P/I	V = √(PR)
I	I = V/R	I = P/V	I = √(P/R)
R	R = V/I	R = V²/P	R = P/I²
P	P = VI	P = I²R	P = V²/R

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FAQ

What is Ohm's law and when does it not apply?

Ohm's law states that current through a conductor is directly proportional to voltage across it (V = IR) when temperature is constant. It applies to ohmic conductors (metals, most resistors). It does NOT apply to: semiconductor devices (diodes, transistors — non-linear I–V relationship), electrolytes, vacuum tubes, and incandescent bulbs (resistance changes with temperature significantly).

How do I calculate total resistance when resistors are both in series and parallel?

Work from the innermost/deepest part of the circuit outward. First simplify all parallel combinations into single equivalent resistances. Then add the series components. Redraw the simplified circuit at each step — this prevents errors in complex circuits.

What is the difference between EMF and terminal voltage?

EMF (electromotive force) is the battery's total voltage when no current flows (open circuit). Terminal voltage is the voltage across the battery terminals when current flows — always less than EMF because some voltage drops across the internal resistance (V = EMF − Ir). At short circuit (R = 0), terminal voltage = 0 and all EMF drops across internal resistance.

Why is household wiring connected in parallel, not series?

In parallel: each appliance gets the full supply voltage (220V) regardless of others. If one fails, others continue working. Current scales with power (a 2kW appliance draws 9A independently of a 100W bulb drawing 0.45A). In series, total resistance would add and voltage would divide unequally — a 2kW appliance would get almost all the voltage and a 100W bulb almost none, and switching one off would cut power to all.