Primary formula
V = I x R; I = V / R; R = V / I; P = V x I.
Physics calculator
Ohm's Law Calculator
Calculate voltage, current or resistance using Ohm's Law, with a supporting electric-power result for the same circuit branch.
Last reviewed: July 21, 2026Physics formula engine v3.0.0
Physics calculator
Ohm's Law Calculator
Enter values, select units and calculate locally. Results use SI internally and show rounded display values.
Decimal.js physics math
Mode
Find voltage from current and resistance.
Result and visual stay hidden until you choose Calculate.
Formula and assumptions
Input assumptions
Resistance is greater than zero when used as a divisor. Values describe one steady circuit branch. The component behaves approximately ohmically.
Precision note
Physics calculators convert compatible inputs into SI units before calculating. Decimal.js is used for arithmetic where practical; trigonometric angle factors are rounded for display.
Physics calculation flow
Parse
Read labelled inputs, selected units and calculation mode from the calculator card.
Convert
Convert compatible values to SI base or derived units without calling external services.
Calculate
Apply the selected deterministic mechanics formula and validate against NaN, Infinity and invalid divisors.
Label
Display the result, supporting unit rows, formula steps, warnings and dynamic visual below the input card.
What Ohm's Law Calculator does
Use this calculator for simple ohmic circuit relationships where two of voltage, current and resistance are known.
The calculator is deterministic and runs from manual inputs only. It is a physics formula tool, so it should not be confused with a unit converter that only changes labels after a value is already known.
Formula and variables
V = I x R; I = V / R; R = V / I; P = V x I.
The calculator converts supported units into SI base or derived units for the actual calculation, then formats the result in the selected display unit.
- V is voltage in volts
- I is current in amperes
- R is resistance in ohms
- P is electric power in watts
Worked examples
Use the examples below to check expected order of operations and unit handling before applying the tool to your own values.
- 2 A x 5 ohm = 10 V and P = 20 W
- 12 V / 6 ohm = 2 A and P = 24 W
- 9 V / 0.03 A = 300 ohm
Unit conversions
Length, time, mass, area, volume, speed, force, pressure, density, frequency, energy and power units reuse the central NexaCalc conversion registry. Acceleration, angle and spring-constant units use the same SI-first convention where applicable.
The displayed conversion rows are supporting context. They do not change the physical assumptions of the selected formula.
Common mistakes
Most errors come from mixing physical quantities that sound similar in everyday language or choosing a unit that belongs to a different dimension.
- Using milliamps as amps without changing the unit selector.
- Treating resistance as zero in a divider.
- Using Ohm's Law for a non-ohmic part without checking the device behavior.
Assumptions and limitations
This page is intended for educational and planning calculations, not professional engineering certification.
This calculator uses idealized physics formulas and the input values you provide. Real systems may require additional effects such as friction, drag, rotation, deformation, losses or measurement uncertainty.
- Resistance is greater than zero when used as a divisor.
- Values describe one steady circuit branch.
- The component behaves approximately ohmically.
- Real circuits can include heat, tolerance, alternating-current impedance and device nonlinearity.
- This tool does not check wiring safety or component ratings.
Frequently asked questions
What does Ohm's Law calculate?
Ohm's Law Calculator uses V = I x R; I = V / R; R = V / I; P = V x I. The result is calculated from the values and units entered in the form.
How do I calculate voltage from current and resistance?
Ohm's Law Calculator uses V = I x R; I = V / R; R = V / I; P = V x I. The result is calculated from the values and units entered in the form.
How do I calculate current from voltage and resistance?
Ohm's Law Calculator uses V = I x R; I = V / R; R = V / I; P = V x I. The result is calculated from the values and units entered in the form.
How do I calculate resistance from voltage and current?
Ohm's Law Calculator uses V = I x R; I = V / R; R = V / I; P = V x I. The result is calculated from the values and units entered in the form.
Why does the result also show electric power?
Use this calculator for simple ohmic circuit relationships where two of voltage, current and resistance are known. Resistance is greater than zero when used as a divisor.
What units does Ohm's Law use?
Ohm's Law Calculator converts compatible inputs to SI internally, then displays the selected output unit. V is voltage in volts; I is current in amperes; R is resistance in ohms; P is electric power in watts.
Can resistance be zero in this calculator?
Ohm's Law Calculator allows signed results only where the physics quantity can carry direction or reference-sign meaning. Divisors such as time, mass and gravity are validated before calculation.
Does Ohm's Law work for every component?
Use this calculator for simple ohmic circuit relationships where two of voltage, current and resistance are known. Resistance is greater than zero when used as a divisor.
How is this different from Electric Power Calculator?
Use this calculator for simple ohmic circuit relationships where two of voltage, current and resistance are known. Resistance is greater than zero when used as a divisor.
When should I use a circuit simulator instead?
Ohm's Law Calculator is formula-based. Use the related calculator when the question changes physical quantity, and use Conversion tools only when the value is already known.
References
- NIST Special Publication 811, Guide for the Use of the International System of Units. Relevance: SI unit symbols and derived-unit naming. Last verified: July 21, 2026. Source.
- BIPM, The International System of Units SI Brochure. Relevance: SI base and derived units used in mechanics, fluids, waves and optics. Last verified: July 21, 2026. Source.
- OpenStax University Physics Volume 1. Relevance: introductory formulas for motion, force, work, energy, momentum, fluids, waves and optics. Last verified: July 21, 2026. Source.
- NIST CODATA internationally recommended values. Relevance: physical constants for electricity, modern physics, gravitation and radiation formulas. Last verified: July 21, 2026. Source.
- OpenStax University Physics Volume 2. Relevance: electric circuits, capacitance, magnetic force and electromagnetic waves. Last verified: July 21, 2026. Source.
- OpenStax University Physics Volume 3. Relevance: photons, nuclear decay and blackbody radiation concepts. Last verified: July 21, 2026. Source.
Physics references verified for SI units, mechanics, fluids, waves, sound and optics formulas on July 21, 2026.
Educational disclaimer
This calculator provides mathematical results from the values, conventions and methods you enter. Verify important academic, engineering or professional work independently.