What this Ohm's law calculator does
This tool applies Ohm's law V = IR in all three of its rearrangements. If you know current and resistance, it computes voltage. If you know voltage and resistance, it computes current. If you know voltage and current, it computes resistance. Every input is unit-aware: voltage in volts or kilovolts, current in amperes or milliamperes, resistance in ohms or kilo-ohms. The result panel includes step-by-step working, unit consistency checks, and real-world context — household mains voltage, USB voltages, battery voltages, and typical current ranges from LED indicator currents to appliance-scale loads — so you can tell immediately whether a computed value corresponds to a small electronic circuit or a household appliance.
Formula
Related formulas (from combining with P = VI)
Variable definitions
- V — potential difference across the conductor, in volts (V).
- I — current through the conductor, in amperes (A).
- R — resistance of the conductor, in ohms (Ω).
- P — power dissipated by the resistor, in watts (W).
Unit conversions you need to know
- 1 kV = 1000 V
- 1 mA = 0.001 A
- 1 kΩ = 1000 Ω
- 1 Ω = 1 V / 1 A
Worked examples
Example 1: Find voltage — a simple resistor circuit
- V = I × R = 0.5 × 12.
- V = 6 V.
- A current of 0.5 A through 12 Ω requires 6 V. This is a typical low-voltage electronic circuit value.
Example 2: Find voltage — household mains across an appliance
- V = 5 × 44.
- V = 220 V.
- 220 V is the standard household mains voltage in Pakistan. A 5 A current through 44 Ω gives the expected mains voltage.
Example 3: Find current — a bulb drawing current
- I = V / R = 220 / 500.
- I = 0.44 A = 440 mA.
- A 500 Ω bulb on 220 V mains draws 440 mA — around 100 W of power. Reasonable for a filament bulb.
Example 4: Find current — an LED indicator
- I = 5 / 470.
- I = 0.0106 A = 10.6 mA.
- This is the classic "USB LED with a 470 Ω resistor" circuit — a very typical low-current electronic application.
Example 5: Find resistance — identifying a component
- R = V / I = 12 / 0.024.
- R = 500 Ω.
- 500 Ω is a standard resistor value — often used for LED current-limiting at higher voltages.
Example 6: Find resistance — a heating element
- R = 220 / 6.8.
- R = 32.4 Ω.
- A 1500 W heating element on 220 V mains. The resistance is quite low, which is why heaters draw high current and why the element heats up.
Real-world reference values
Knowing typical voltages, currents and resistances helps you sanity-check any computed answer.
| Context | Typical value | Notes |
|---|---|---|
| AA battery | 1.5 V | Single dry cell |
| Mobile phone battery | 3.7 V | Lithium-ion single cell |
| USB port | 5 V | Standard USB-A supply |
| Car battery | 12 V | Lead-acid, 6 cells |
| Pakistani household mains | 220 V | AC RMS, 50 Hz |
| Industrial 3-phase | 380–400 V | Between phases |
| Transmission line | 11 kV – 500 kV | WAPDA grid tiers |
| LED indicator current | 5–20 mA | Small signal LED |
| Phone charger output current | 1–3 A | USB charging |
| Ceiling fan current | 0.3 A | ~70 W at 220 V |
| Refrigerator current | 0.5–1 A | Compressor cycle |
| Electric kettle current | 6–9 A | ~1500–2000 W |
| Air conditioner current | 4–8 A | Per unit |
| Typical LED resistor | 220–1000 Ω | Current-limiting at 5 V |
| Household wiring resistance | < 1 Ω | Copper, short run |
| Human body (dry skin) | 100 kΩ – 1 MΩ | Why dry-contact shock is usually survivable |
Common mistakes to avoid
- Mixing units silently. mA must be converted to A, kΩ to Ω, kV to V, before applying the formula. Using 500 mA as 500 A gives an answer 1000× too large. The calculator handles the conversion for you, but exam answers must be correct by hand.
- Setting resistance to zero. R = 0 would require infinite current for any nonzero voltage. Real circuits always have some resistance. The calculator treats R = 0 as a domain error when solving for current.
- Applying Ohm's law to non-ohmic devices. Filament bulbs, diodes, and thermistors do not obey V = IR at all temperatures or currents. Ohm's law is valid only for ohmic conductors at constant temperature.
- Confusing the resistor's colour code with its value. The bands on a resistor encode the resistance in ohms. Reading them incorrectly by one band gives a value 10× or 100× off. If the problem gives bands, decode them first, then use this calculator.
- Using the wrong sign for AC voltages. Household mains is quoted as an RMS value (220 V in Pakistan). The peak voltage is √2 × 220 ≈ 311 V. For resistance and power calculations, use RMS values; for peak withstand, use peak.
- Forgetting the power limit of resistors. A resistor rated ¼ W cannot safely dissipate 1 W — it will overheat. If a computation gives a power above the resistor's rating, the resistor is the wrong choice.
- Assuming series and parallel rules apply inside a single formula. Ohm's law applies to one component at a time. For combinations of resistors, compute the equivalent resistance first (using series/parallel rules), then apply V = IR.
- Using V = IR for capacitors and inductors directly. These components have reactance, and the correct relation uses impedance Z: V = IZ. Ohm's law applies only to resistors and resistive circuits.
FAQ
What is Ohm's law?
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across those points, and inversely proportional to the resistance between them: V = I × R. It applies to ohmic conductors at constant temperature.
What are the SI units in V = IR?
Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω). One ohm is the resistance that allows one ampere of current under one volt of potential difference. 1 Ω = 1 V / 1 A.
How do I calculate resistance from voltage and current?
Rearrange V = IR to get R = V / I. Divide the voltage by the current. For example, a 6 V source driving 0.5 A through a resistor gives R = 6 / 0.5 = 12 Ω.
Why does the calculator reject a resistance of zero?
A resistance of zero would require infinite current for any non-zero voltage (since I = V/R), which is unphysical. Real circuits always have some resistance — even a piece of copper wire has a small resistance. The calculator treats R = 0 as a domain error when solving for current.
What is the difference between an ohmic and a non-ohmic conductor?
An ohmic conductor obeys Ohm's law — its V-I graph is a straight line through the origin, and the resistance stays constant. A non-ohmic conductor does not: examples include filament bulbs (resistance increases with temperature) and diodes (current flows easily in one direction only). Ohm's law applies only to ohmic conductors.
Can Ohm's law be used for AC circuits?
For purely resistive AC circuits, Ohm's law applies using RMS values of voltage and current. For circuits with capacitors or inductors, the concept of impedance replaces resistance, and V = IZ must be used instead of V = IR. BISE class 9 and 10 problems mostly involve DC or purely resistive AC.
Related tools
Related Hira Academy resources
Revise the theory of current electricity with our Class 9 notes and Class 10 notes, and track term progress with the free Student Portal.