Ohm's Law Calculator
Calculate voltage, current, resistance, power
Ohm's Law Calculator is built for calculate voltage, current, resistance, power — fast, free, and private. You provide Voltage (V) and Current (A); the tool does the rest in real time. Every answer includes a transparent breakdown you can repeat by hand. Great when you want certainty fast — no formulas to memorize, no apps to install. Everything runs in your browser — your inputs are not sent to our servers, and it works offline after the first visit (currency conversion needs a live connection). Searching for ohms law calculator voltage current resistance or free ohms law calculator? This tool covers it — free, fast, and private. Open Ohm's Law Calculator, enter your numbers, and you will have a trustworthy answer before you know it.
What does the page calculator do?
Ohm's Law Calculator works out the resistance from the Voltage and Current, following standard Science conventions — the page defaults produce a resistance of 6.00 Ω | Power: 24.00 W.
- Inputs: Voltage and Current.
- Output: the resistance, plus the intermediate steps behind it.
- Method: the standard Science formula, evaluated entirely in your browser.
Quick answer
With the default inputs (voltage of 12, current of 2), ohm's law calculator returns a resistance of 6.00 Ω | Power: 24.00 W. Assumptions and limits are summarized below.
How does it work?
Ohm's Law Calculator computes the resistance directly from your inputs — the Voltage and Current feed the formula. Nothing is uploaded: the math runs locally in your browser and the result appears as you type.
What This Calculator Really Does
Ohm's law ties together the three base quantities of every simple circuit: voltage (the push), current (the flow) and resistance (the opposition). Enter any two and this tool solves the third with the arithmetic shown, plus the power dissipated — often the number that decides whether a component survives.
It is the first formula every electronics course teaches because it is both exactly true for ohmic materials and a good working approximation far beyond them. A car light, a phone-charger cable, a heater element — all of them are V = IR problems once you know two of the three quantities.
The calculation runs locally in your browser. No inputs are uploaded, nothing is tracked, and the page works offline after the first visit.
Quick answer: 12 V across a 4 Ω resistor draws 3 A and dissipates 36 W — I = 12 ÷ 4, and P = V × I.
The Formula, Explained Plainly
Ohm's law: V = I × R, rearranged as I = V ÷ R and R = V ÷ I
- V — voltage in volts: the electrical pressure driving the flow.
- I — current in amperes: how much charge passes per second.
- R — resistance in ohms: how hard the material opposes that flow.
Power: P = V × I = I² × R = V² ÷ R, in watts. Power answers the practical question — how much heat will this produce?
How to Use It — In Order
- Leave blank the quantity you want — the tool solves for the empty field.
- Enter any two known values — volts, amps or ohms, in standard units.
- Read the solved value — with the division or multiplication shown.
- Check the power line — watts tell you if a resistor or wire can handle the load.
- Watch unit prefixes — milliamps and kilohms must be converted (mA ÷ 1000, kΩ × 1000) or results will be off by factors of a thousand.
- Sanity-check the direction — more resistance means less current at the same voltage.
Worked Example, Verified by Hand
A 12 V supply feeds a 4 Ω resistor:
- Current: I = V ÷ R = 12 ÷ 4 = 3 A.
- Power: P = V × I = 12 × 3 = 36 W — check via I²R: 3² × 4 = 36. Same answer two ways.
- Double the resistance to 8 Ω and current halves to 1.5 A, power drops to 18 W — the inverse relationship in action.
That second check (two power formulas agreeing) is the quickest error-catcher in bench electronics.
Common Mistakes to Avoid
- Unit-prefix slips — 470 Ω entered as 470 kΩ is a thousandfold error; convert milli and kilo prefixes before entering.
- Using Ohm's law alone for AC with reactance — impedance, not pure resistance, governs AC circuits with coils and capacitors.
- Ignoring power ratings — a resistor can satisfy V = IR and still burn up if the wattage exceeds its rating.
- Applying the law to non-linear devices — diodes and filaments do not keep constant resistance across voltages.
- Series vs parallel confusion — Ohm's law is per-element; combine resistances first for the circuit-level view.
Limitations
- Valid for DC and purely resistive AC; reactive circuits need impedance phasors.
- Assumes constant resistance; temperature-dependent elements drift as they heat.
- No tolerance modeling — real resistors vary several percent from their rating.
Expected Accuracy
Exact division and multiplication at full floating-point precision, rounded for display. Real-world agreement depends on component tolerance and measurement quality, not on the arithmetic.
Privacy — Your Data Never Leaves This Device
All computation is local: no circuit values, no tracking, nothing uploaded — the tool works offline too.
Related Tools & Guides
Natural next steps from Ohm's Law:
- All Science Calculators
- Voltage Drop Calculator — wire losses over length
- LED Resistor Calculator — the classic beginner application
- Power Factor Calculator — AC circuits beyond pure resistance
- Guide: Ohm's Law With Worked Examples
Sources & Standards
Definitions follow the International System of Units (SI).
- NIST — SI Units (volt, ampere, ohm definitions)
- BIPM — international measurement standards
Bottom Line
Two knowns give you the third — and the wattage tells you whether it is safe. Convert your prefixes, sanity-check power two ways, and V = IR covers most bench questions.
From Our Guides Library
Frequently Asked Questions
What is Ohm's law formula?
V = I × R: voltage equals current times resistance. Rearranged, I = V ÷ R gives current, and R = V ÷ I gives resistance.
How do I calculate current from voltage and resistance?
Divide voltage by resistance. Example: 12 V across 4 Ω gives 12 ÷ 4 = 3 amperes.
How do I calculate power with Ohm's law?
P = V × I, or equivalently I² × R or V² ÷ R. Example: 12 V and 3 A give 36 watts; check with I²R: 9 × 4 = 36.
Does Ohm's law work for AC circuits?
For purely resistive AC loads yes, but with inductors and capacitors you must use impedance, which includes reactance and phase.
Why did my resistor overheat even though the math worked?
The law was satisfied but the power exceeded the component's watt rating. Always compute P = V × I and pick a rating comfortably above it.
What units does Ohm's law use?
Volts for voltage, amperes for current, ohms for resistance, watts for power. Convert milli- and kilo-prefixes before calculating.
Is this calculator free and private?
Yes — it runs entirely in your browser, requires no sign-up, and no circuit values leave your device.
What does the tool calculate?
Ohm's Law Calculator keeps the whole calculation in front of you — the Voltage and Current, the formula, the intermediate steps, and a worked example you can reproduce line by line. Because the working is visible: Ohm's Law Calculator shows each operation behind the result in the steps panel, so you can verify the result instead of trusting a black box.
How is the resistance calculated?
The first steps are formula: v = i × r, p = v × i, then r = v / i = 12 / 2 = 6.00 ω. The calculation in Ohm's Law Calculator applies the standard Science method, keeping full precision internally and rounding only the final display.
What do I need to use the Ohm's Law Calculator?
The Voltage and Current it asks for, or the page defaults if you just want to see the calculation work. Each input maps directly to the formula, and changing any one of them recalculates the resistance instantly.
What does the result from the tool mean?
The main number the ohm's law calculator returns is the resistance for your exact inputs, and the supporting figures and step list give it context. Very large or very small inputs can push the resistance beyond what is practically meaningful — sanity-check extreme values before relying on them.
When is the page most useful?
Common scenarios for Ohm's Law Calculator: day-to-day planning, comparing scenarios side by side, and double-checking the resistance. The step list makes it equally useful for learning the method and for double-checking someone else's numbers. Run Ohm's Law Calculator twice with deliberately low and high inputs; the spread tells you how sensitive the result is, which a single run never shows.