Ohm's Law Calculator
EverydayCalculate voltage, current, resistance, or power using Ohm's Law (V=IR, P=IV). Enter any two known electrical values to find all four quantities instantly.
Solve For
Known Values
Amps
Ohms
Formulae
V = I × R
I = V / R
R = V / I
P = V × I
Voltage
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Volts
All Values
VSOLVED
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Volts
I
—
Amps
R
—
Ohms (Ω)
P
—
Watts
Equation used: V = I × R
What is a Ohm's Law?
Ohm's Law is the most fundamental relationship in electrical engineering: the voltage across a conductor equals the product of the current flowing through it and its resistance (V = I × R). First formulated by German physicist Georg Simon Ohm in 1827, this simple equation underpins the analysis of every electrical circuit — from the wiring in your home to the transistors in your smartphone.
The Ohm's Law Calculator extends the basic formula to all four interrelated electrical quantities: Voltage (V), Current (I), Resistance (R), and Power (P). By entering any two known values, you can instantly calculate the remaining two. This is invaluable for students learning circuit theory, electricians sizing wires and fuses, hobbyists building electronics, and engineers troubleshooting electrical faults.
In the Indian context, Ohm's Law is directly relevant to understanding your household electrical system, which operates at 230 V AC / 50 Hz. Questions like "how much current does my 1,500 W geyser draw?" (answer: 1,500 ÷ 230 ≈ 6.5 A) or "what is the resistance of my 60 W bulb at operating temperature?" (answer: 230² ÷ 60 ≈ 882 Ω) are straightforward Ohm's Law calculations. The Voltage Drop Calculator builds on Ohm's Law to help you size electrical cables for safe, compliant installations.
How to use this Ohm's Law calculator
- Select what to Solve For — click one of the four coloured buttons: Voltage (V), Current (I), Resistance (R), or Power (W). This determines which unknown the calculator will find.
- Enter two known values — the calculator shows input fields for the remaining three quantities; provide any two of them. For example, if solving for Resistance, enter Voltage and Current (or Power and Voltage).
- Read all four results — the solved value is highlighted with a "SOLVED" badge. All four quantities are shown in a 2×2 grid so you can immediately see the complete electrical picture.
- Check the formula — a footer line shows which formula was applied (e.g., R = V ÷ I), useful for study or verification.
Formula & Methodology
The four interrelated formulas derived from Ohm's Law and the power equation P = V × I: Solving for Voltage: V = I × R; also V = P ÷ I; also V = √(P × R) Solving for Current: I = V ÷ R; also I = P ÷ V; also I = √(P ÷ R) Solving for Resistance: R = V ÷ I; also R = V² ÷ P; also R = P ÷ I² Solving for Power: P = V × I; also P = I² × R; also P = V² ÷ R Worked example: An electrician in Chennai needs to check whether a 1,500 W room heater connected to a 230 V supply is drawing safe current. - Solving for Current: I = P ÷ V = 1,500 ÷ 230 = 6.52 A - Resistance of the heating element: R = V ÷ I = 230 ÷ 6.52 = 35.28 Ω - Verification: P = I² × R = 6.52² × 35.28 = 42.51 × 35.28 ≈ 1,500 W ✓ The heater draws 6.52 A. A 15 A MCB protecting the circuit is adequate. A 10 A MCB would be marginal — acceptable but providing less headroom. The electrician knows to use wiring rated for at least 10 A (2.5 mm² copper conductor) for this circuit.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. It is the foundational relationship of basic circuit analysis, formulated by German physicist Georg Simon Ohm in 1827.
What is the formula for Ohm's Law?
The four interrelated formulas are: V = I × R (voltage), I = V ÷ R (current), R = V ÷ I (resistance), and P = V × I (power, also P = I²R = V²/R). Knowing any two of the four quantities — voltage, current, resistance, and power — allows you to calculate the other two.
How do I calculate power using Ohm's Law?
Power (W) = Voltage (V) × Current (A). This is derived from the basic power formula P = W/t combined with Ohm's Law. Equivalently, P = I² × R or P = V² ÷ R. The Ohm's Law Calculator computes all four values simultaneously once you provide any two known quantities.
What units are used in Ohm's Law calculations?
Voltage is measured in Volts (V), current in Amperes or Amps (A), resistance in Ohms (Ω), and power in Watts (W). For very large or small values, prefixes are used: milliamps (mA = 0.001 A), kilohms (kΩ = 1,000 Ω), megaohms (MΩ = 1,000,000 Ω), and kilowatts (kW = 1,000 W).
How do I use the Ohm's Law Calculator?
Select which quantity you want to solve for (V, I, R, or P) using the four mode buttons. Then enter any two of the remaining known values. The calculator instantly computes all four quantities and highlights the solved value. It also displays the specific formula used, so you can learn the relationship.
Is Ohm's Law applicable to AC circuits?
Ohm's Law in its basic V = IR form applies to DC (direct current) circuits and to purely resistive AC loads. In AC circuits with capacitors or inductors, the concept of impedance (Z) replaces resistance (R), and the formula becomes V = I × Z. For standard resistive appliances on Indian 230V AC supply, the basic Ohm's Law formulas give correct results.
What voltage does household electricity use in India?
Indian residential electricity uses 230 V AC at 50 Hz, as specified by the Bureau of Indian Standards. This is the standard across South Asia. North America uses 120 V / 60 Hz and 240 V for heavy appliances. Always verify the rated voltage on any appliance before connecting it — using a 120V device on a 230V Indian supply without a transformer will damage or destroy the device.
What is the difference between voltage, current, and resistance?
Voltage is electrical pressure — the force pushing electrons through a circuit, measured in Volts. Current is the rate of electron flow, measured in Amperes. Resistance is the opposition to that flow, measured in Ohms. A helpful analogy is water in a pipe: voltage is the water pressure, current is the flow rate, and resistance is the pipe's narrowness.
How do I find the resistance of an electrical appliance?
Use the formula R = V ÷ I. If an appliance rated at 230 V draws 2 A of current, its resistance is 115 Ω. Alternatively, R = V² ÷ P — a 230 V, 1,000 W appliance has a resistance of 230² ÷ 1,000 = 52.9 Ω. You can measure actual resistance directly with a multimeter (with the appliance unplugged).
Why is the power output of a device not always the same as input power?
Electrical power input (V × I) is what the device consumes from the supply. Some of that is converted to useful output (light, heat, mechanical work) and some is lost as heat in the wiring, motor windings, or other components. Efficiency = Useful Output ÷ Total Input. Most real-world devices are 60–95% efficient. Ohm's Law describes the electrical input side, not the mechanical output efficiency.
What is a safe current for standard Indian home wiring?
Standard Indian household wiring uses copper conductors of 1.5 mm² (for lighting circuits, rated up to 15 A) and 2.5 mm² (for power sockets, rated up to 20 A). Using more current than the wire's rated capacity causes overheating and fire risk. Always use properly rated wires, fuses, or MCBs (miniature circuit breakers) to protect circuits. See the [Voltage Drop Calculator](/voltage-drop-calculator/) for wire sizing checks.