Ohm's Law Calculator for Voltage, Current and Resistance
Calculate voltage, current or resistance with V = IR. Choose independent metric prefixes, see the equivalent SI value, estimate ideal resistive power and review every substitution step.
- Solve V, I or R
- Exact prefix conversion
- Derived power result
- Open and short checks
Online Ohm's Law Calculator
Select the unknown electrical quantity, enter the other two magnitudes and choose each unit. The tool converts values to volts, amperes and ohms before applying the equation.
Ideal voltage across the entered resistance at the entered current.
Equivalent Result Units
| Unit | Equivalent value |
|---|---|
| millivolts (mV) | 12000 mV |
| volts (V) | 12 V |
| kilovolts (kV) | 0.012 kV |
Calculation Steps
- Convert the current to 2 A and resistance to 6 Ω.
- Apply V = IR = 2 × 6 = 12 V.
- Calculate ideal resistive power: P = VI = 12 × 2 = 24 W.
- Convert the voltage answer to the selected display unit.
The result assumes a passive, linear resistor whose resistance stays constant. It does not account for tolerance, self-heating, wiring losses or source limits.
How to Use This Ohm's Law Calculator
- Choose the unknown. Select voltage, current or resistance as the value you want to calculate.
- Enter the first known magnitude. Use a nonnegative decimal or scientific-notation value and select its prefix.
- Enter the second known magnitude. Check uppercase and lowercase prefix letters carefully, especially m for milli and M for mega.
- Choose answer units. Select the preferred unit for the unknown and a separate unit for the derived power result.
- Set displayed precision. Choose up to four, six, eight or ten significant digits. This changes display rounding, not the exact internal calculation.
- Calculate and review. Check the SI values, equation, substitutions, zero-case note and model assumptions before using the result.
What Is Ohm's Law?
Ohm's law relates the potential difference across an ohmic element, the electric current through it and its resistance. The common equation is V = IR. Here, V is voltage in volts, I is current in amperes and R is resistance in ohms.
For a fixed resistance, current is directly proportional to applied voltage. Doubling the voltage doubles the current when the component remains ohmic and its temperature and other physical conditions stay effectively unchanged. Rearranging the same equation lets you solve for any one of the three quantities.
Ohm's law is an empirical relationship, not a rule followed by every electrical device at every operating point. An ordinary resistor often behaves approximately linearly over a useful range. Diodes, lamps, thermistors, batteries and many semiconductor devices have nonlinear or condition-dependent voltage-current behavior.
Ohm's Law and Electrical Power Formulas
Choose the rearranged equation whose left side is the unknown. Use coherent units, or let the calculator convert each selected prefix before solving.
| Find | Formula | Required values | SI result |
|---|---|---|---|
| Voltage | V = IR | Current and resistance | volt (V) |
| Current | I = V/R | Voltage and nonzero resistance | ampere (A) |
| Resistance | R = V/I | Voltage and nonzero current | ohm (Ω) |
| Power | P = VI = I2R; when R > 0, P = V2/R | A complete valid V-I-R state | watt (W) |
The power result is secondary. The calculator first solves the requested Ohm's-law quantity, then uses P = VI. For a passive resistor under the magnitude-only model, this is the ideal rate at which electrical energy is converted, usually into heat.
Worked Ohm's Law Examples
Example 1: Find current
A 12 V potential difference is applied across a 4 Ω resistor. Rearrange Ohm's law:
The ideal resistive power is P = VI = 12 × 3 = 36 W. A real component needs an appropriate power rating and thermal design; the calculated 36 W is not a recommended resistor rating.
Example 2: Find resistance
A circuit has 5 V across an element and a measured current of 20 mA. First convert 20 mA to 0.020 A:
The same calculation returns P = 5 × 0.020 = 0.10 W, or 100 mW.
Example 3: Find voltage with mixed prefixes
A current of 15 mA flows through a 2.2 kΩ resistance. Convert both prefixes or multiply them consistently:
The ideal power is 33 × 0.015 = 0.495 W. Resistance change caused by self-heating is outside this fixed-resistance calculation.
Example 4: High resistance and small current
A 3.3 V signal appears across 1 MΩ. The ideal current is 3.3 microamperes because 1 MΩ equals 1,000,000 Ω. The ideal power is 10.89 microwatts. In a real measurement, meter input resistance, leakage and noise may become important.
Voltage, Current, Resistance and Power Units
The coherent SI units are the volt, ampere, ohm and watt. One ohm equals one volt per ampere. SI prefixes scale a unit by exact powers of ten, so a prefix error may change an answer by thousands or millions.
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| pico | p | 10-12 | 1 pA = 0.000000000001 A |
| nano | n | 10-9 | 1 nV = 0.000000001 V |
| micro | µ | 10-6 | 1 µA = 0.000001 A |
| milli | m | 10-3 | 1 mΩ = 0.001 Ω |
| kilo | k | 103 | 1 kΩ = 1000 Ω |
| mega | M | 106 | 1 MΩ = 1,000,000 Ω |
| giga | G | 109 | 1 GV = 1,000,000,000 V |
| tera | T | 1012 | 1 TΩ = 1,000,000,000,000 Ω |
Unit symbols are case-sensitive. The calculator keeps each input unit independent, which lets you enter combinations such as milliamperes and kiloohms without converting them by hand.
How the Calculator Handles Zero Values
Zero values need physical interpretation. They are not all ordinary division cases.
- Zero current through a finite resistance gives zero voltage and zero power.
- Zero voltage across a positive resistance gives zero current and zero power.
- Zero voltage with nonzero current gives zero resistance in the ideal magnitude model, a short-circuit result.
- Nonzero voltage divided by zero resistance has no finite current in this ideal equation.
- Nonzero voltage with zero current implies no finite resistance, an ideal open-circuit limit.
- Zero voltage and zero current do not determine a unique resistance because every finite resistance satisfies V = IR at that single point.
The tool reports these cases directly. It does not replace an undefined result with Infinity, NaN or an arbitrary large number.
Electrical Power and Resistor Ratings
For a purely resistive load, ideal dissipated power may be written as P = VI or P = I2R. When resistance is greater than zero, P = V2/R is also equivalent. The condition matters because V2/R is undefined for the calculator's ideal zero-voltage, zero-resistance short-circuit state, while VI and I2R correctly return zero.
A calculated power value is not a component recommendation. A real design also considers rated power, ambient temperature, airflow, enclosure conditions, mounting, pulse duration, duty cycle, surge energy, derating, tolerance and allowed surface temperature. Source current limits and wiring resistance may stop the assumed voltage from appearing across the component.
When Ohm's Law Applies
A component is ohmic over a range when its current-voltage relationship is linear and its resistance is effectively constant there. Metallic resistors often approximate this model under controlled conditions. Resistance may still change with temperature, material, dimensions, mechanical strain and frequency.
Calculating R = V/I at one operating point gives a ratio. It does not prove the device has constant resistance. A filament lamp heats as current rises, a diode has a nonlinear curve, and a thermistor is designed to change resistance with temperature. Those devices need their characteristic data or a more suitable model.
DC and AC use
For DC, enter steady values across and through the resistor. For a purely resistive sinusoidal AC load, the same equations work with RMS voltage and RMS current, and P = VrmsIrms. Do not mix peak and RMS values.
Reactive AC loads require impedance and phase information. Inductors and capacitors introduce reactance, and general real power includes power factor. This calculator does not solve impedance, apparent power, reactive power or phase angle.
Improve Calculation and Measurement Accuracy
- Check prefixes first. A 2.2 kΩ resistor is 2200 Ω, while 2.2 MΩ is 2,200,000 Ω.
- Use the same element. Voltage must be measured across the element carrying the entered current.
- Include tolerance. A marked resistance is nominal. Its actual value may differ within the specified tolerance.
- Account for temperature. Resistance and power dissipation may change as a component heats.
- Consider the measuring instrument. Meter input resistance, lead resistance, burden voltage, contact resistance and calibration uncertainty may affect a reading.
- Match precision to evidence. Extra output digits do not make an imprecise measurement more accurate.
For uncertainty analysis, compare the measured and expected values with the Percent Error Calculator. For manual rearrangement or a symbolic check, use the Equation Calculator.
Electrical Safety Limits
This calculator is an educational equation tool. It does not determine whether a voltage, current, component or installation is safe. Do not use a calculated body resistance to predict safe current through a person. Human impedance varies with voltage, frequency, path, moisture, contact area, skin condition and exposure time.
Do not work on energized mains, panels, exposed conductors or unfamiliar equipment from a calculator result. De-energize and verify the circuit using approved procedures and suitable test equipment. Energized electrical work belongs to qualified persons using the required controls, protective equipment and current standards.
Related Calculators
Frequently Asked Questions
What is the formula for Ohm's law?
Ohm's law is V = IR, where V is potential difference in volts, I is current in amperes and R is resistance in ohms. Rearranged forms are I = V/R and R = V/I.
How do I calculate current from voltage and resistance?
Divide voltage by resistance: I = V/R. For example, 12 V across 4 ohms gives 3 A. Resistance must be greater than zero for a finite current result.
How do I calculate resistance from voltage and current?
Divide voltage by nonzero current: R = V/I. For example, 5 V divided by 0.020 A gives 250 ohms. Zero voltage and zero current do not determine one unique resistance.
What is one ohm?
One ohm is one volt per ampere. A resistance of 1 ohm has a current of 1 ampere when a potential difference of 1 volt is applied under the stated ohmic conditions.
Does this calculator also find electrical power?
Yes. After finding a valid voltage-current-resistance state, it calculates ideal resistive power with P = VI. The result is not a resistor wattage or thermal-design recommendation.
Can I enter milliamps and kiloohms together?
Yes. Each input has an independent unit selector. The calculator converts both values to amperes and ohms exactly before applying the formula, then converts the answer to your selected unit.
Why is uppercase M different from lowercase m?
SI prefix symbols are case-sensitive. Lowercase m means milli, or 10 to the power of -3. Uppercase M means mega, or 10 to the power of 6. Therefore 1 MΩ is one billion times 1 mΩ.
Does Ohm's law work for AC circuits?
It works with RMS voltage and current for a purely resistive AC load. Reactive loads require impedance and phase or power-factor analysis. Do not mix RMS and peak values.
Why might a measured value differ from the result?
Real results may differ because of resistor tolerance, temperature, self-heating, wiring resistance, source limits, meter loading, contact resistance, noise and measurement uncertainty.
Can I use this calculator for live electrical work?
No. It is an educational formula tool, not a safety or code-compliance assessment. De-energize circuits and leave energized work, mains systems and consequential designs to qualified persons following current requirements.
Method and Review Basis
The voltage-current-resistance relationship and its limits were checked against OpenStax Ohm's law guidance. The resistive power equations were checked against OpenStax electrical energy and power. The RMS and power-factor distinction was checked against OpenStax AC power guidance. SI units and prefix relationships were checked against NIST electrical-unit guidance and the BIPM SI Brochure. Electrical-work precautions were reviewed against NIOSH electrical safety guidance, while the electrical-shock interpretation scope was checked against IEC 60479-1.
Educational and electrical-safety disclaimer: This calculator provides equation-based estimates for a passive linear resistor. It does not verify a circuit, component rating, installation, protection method, electrical code or safe work condition. Confirm measurements, units, tolerances, thermal limits and standards independently. Only qualified persons should work on or near energized electrical equipment.