Ohm's Law Calculator
Ohm's Law calculator (V = IR): solve for voltage, current or resistance in SI units and get the power. Curriculum-aligned.
Ohm's Law Calculator
Enter any two values and leave the third blank — the calculator solves for it and works out the power. Results are in SI units.
- Curriculum
- English (global) — Cambridge International + IB
- Built against
- Cambridge IGCSE Physics 0625 + IB Diploma (2023–2025) — Electricity
- Unit system
- SI primary; US/imperial readout below
- First published
- 24 May 2026
- Last updated
- 1 Jun 2026
View authoritative scientific sources
- NIST SP 811 (2008), §3 — units & conversions
- BIPM SI Brochure, 9th edition (2019)
- CODATA 2018 recommended values
- Ohm's Law — Encyclopædia Britannica
⚠️ Educational use only — see full disclaimer
EDUCATIONAL USE DISCLAIMER
This calculator is provided for educational and reference purposes only. It is not a substitute for instruction from a qualified teacher, your prescribed textbook, or your school's official curriculum materials.
When preparing for examinations, always cross-check our calculations and notation against your current syllabus and your teacher's guidance. Syllabus conventions and accepted notation vary between curricula and may change between examination years.
If you believe any calculation, notation, or curriculum reference in this tool is inaccurate, please let us know via the feedback button. We review feedback promptly and update tools when verified corrections are needed.
RECATOOLS accepts no liability for academic, examination, professional, or research outcomes arising from use of this tool.
How to Use the Ohm's Law Calculator
Pick your curriculum
Use the curriculum pills above to match your syllabus (Cambridge, SEAB, SPM, SBMPTN, 高考, 學測 or HKDSE). Terminology, the worked example and the whole page follow your selection.
Enter any two values
Type two of voltage, current and resistance — leave the one you want to find blank. Each field has a unit selector (V, mV, kV; A, mA; Ω, kΩ, MΩ).
Read the SI result
The answer is shown in SI units — volts (V), amperes (A), ohms (Ω). Power P = VI is worked out automatically in watts.
Check against your syllabus
The Tool Information block shows exactly which syllabus this is built against. Spot something off? Use the feedback button — we review every report.
Ohm's Law, in Your Curriculum's Words
Ohm's Law
Example: A 4.0 Ω resistor is connected across a 12 V supply. The current through it is:
Given: V = 12 V, R = 4.0 Ω. Using I = V / R:
I = 12 ÷ 4.0 = 3.0 A
Ohm's law states that the potential difference across an ohmic conductor is directly proportional to the current through it: V = IR. Voltage is in volts (V), current in amperes (A), resistance in ohms (Ω) — the SI units every curriculum here teaches. Rearranged, the same law gives current (I = V / R) or resistance (R = V / I), which is why this tool solves for whichever value you leave blank.
Electrical units have no imperial equivalents, so the result is always SI only. The calculator also works out the electrical power P = VI. All calculation happens in your browser — nothing is uploaded.
One ohm is the resistance that lets one ampere flow when one volt is applied across it. The law is the same everywhere; only the words around it change.
10 Facts About Ohm's Law
V = IR — Ohm's law — was published by Georg Ohm in 1827.
One ohm lets one ampere flow under one volt — the SI definition.
It holds for ohmic conductors — metals at constant temperature.
Non-ohmic parts (diodes, filament lamps) give a curved V–I graph.
The ohm symbol is the Greek letter omega (Ω), after Georg Ohm.
Electrical power has three equivalent forms: P = VI = I²R = V²/R.
Across a fixed resistor, doubling the voltage doubles the current.
The resistance of most metals rises as temperature increases.
Resistors in series add directly; in parallel their reciprocals add.
This calculator runs in your browser — your working stays private.
Frequently Asked Questions
- V = IR — voltage equals current times resistance. Rearranged, I = V / R and R = V / I. This calculator solves for whichever of the three you leave blank, and also works out the electrical power P = VI in watts.
- SI units: volts (V), amperes (A), ohms (Ω). Electrical units have no imperial equivalents, so the result is always SI only — there is no imperial readout like other science tools. You can enter prefixes such as mV, kV, mA, kΩ or MΩ; the tool converts them to SI before calculating.
- Yes. Enter any two of voltage, current and resistance and leave the third blank — the calculator rearranges V = IR and solves for the missing value.
- Yes. Once all three quantities are known, the calculator works out the electrical power P = VI automatically and shows it in watts. This is handy for circuit questions that ask for the power dissipated in a resistor.
- The physics — V = IR in SI units — is identical worldwide. What changes is the terminology, worked-example style and notation. For example, mainland Chinese physics writes the law as I = U/R because voltage is denoted U, while HKDSE and Cambridge use V. The selector matches those to your syllabus.
- An ohmic conductor is one whose current is directly proportional to the voltage at constant temperature, so its V–I graph is a straight line through the origin. A metal wire is a common example; diodes and filament lamps are non-ohmic.
- The smaller of your inputs' significant figures, capped at five, switching to scientific notation for very large or very small numbers — standard exam practice.
- The Tool Information block lists the exact syllabus for your selected curriculum (e.g. SEAB Pure Physics 6091 or SPM Fizik 4531). It is a study aid, not a substitute for your official syllabus or teacher.
- No. Every calculation runs in your browser; nothing you type is uploaded. It works offline once the page has loaded.
- Completely free, no account or usage limit. It runs entirely in your browser and collects no data.
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