Ohms Law Calculator Online — Free Ohms Law Calculator Tool

CodexFuse Team · 2026-11-02T13:34:25.572Z · 10 min read min read

Solve V, I or R instantly with the free Ohms Law Calculator: enter any two values and get the third, with a worked 12 V example.

An ohms law calculator turns two numbers you already know into the one you need. Enter any two of voltage, current and resistance, and it solves the third instantly, so you stop guessing resistor values, fuse sizes and wire ratings.

Most small electrical mistakes are arithmetic mistakes. A resistor picked from memory overheats. A fuse chosen by feel blows for no reason. One quick calculation, done before you connect anything, prevents all three.

What is an ohms law calculator?

An ohms law calculator is a free online tool that solves Ohm's law, V = I × R, from any two known values. You enter voltage in volts, current in amps and resistance in ohms, leave the unknown blank, and it returns all three values, so you can check a design, a repair or a purchase in seconds.

Georg Simon Ohm showed that, for a simple resistive conductor, current rises in step with voltage and falls as resistance rises. The calculator is just that relationship, rearranged three ways. It does not measure your circuit. It checks whether the numbers you plan to use belong together.

The three formulas you actually use

One law, three rearrangements. You only ever need the version that isolates your unknown.

Keep the units consistent. Volts, amps and ohms belong together. Milliamps and kilohms must be converted first: 20 mA is 0.02 A, and 4.7 kΩ is 4,700 Ω. Mixing units is the most common reason a correct formula gives a wrong answer.

How to use the ohms law calculator without guessing units

The Ohms Law Calculator on CodexFuse keeps the workflow deliberately short. There are no unit pickers to misread, which is a strength once you know the one rule: enter plain numbers in base units only.

1. Pick your two knowns. A measured supply voltage, a rated current, a marked resistor value. Leave the third field blank.
2. Enter them in volts, amps and ohms. Convert milliamps to amps and kilohms to ohms before typing anything in.
3. Click Calculate. The tool shows all three values to four decimal places, so small differences stay visible.
4. Copy the result as plain text. The copy button gives you a line like V=12, I=2, R=6, ready for a parts list, a quote or a job note.
5. Sanity-check before you build. If the answer looks far from expected, recheck units first. A factor-of-1,000 error almost always means milliamps were entered as amps.

Unsure where to start? Try a preset on the ohms law calculator page: 12 V with 2 A gives 6 Ω, 5 V across 10 Ω gives 0.5 A, and 3 A through 4 Ω gives 12 V. Confirm the maths matches, then enter your own numbers.

Worked example: sizing a 12 V garden lighting circuit

Here is a clearly labelled fictional scenario, worked end to end with the tool.

The situation. Imran, a self-employed electrician, is fitting a small 12 V garden lighting run. The supply is a 12 V transformer, and the test load he plans to verify the run with is rated at 6 Ω. Before choosing a fuse and checking the cable, he needs the current the run will draw.

The inputs. Voltage: 12 V. Resistance: 6 Ω. Current: unknown, left blank.

The steps. Imran opens the Ohms Law Calculator, types 12 into voltage and 6 into resistance, leaves current blank, and clicks Calculate.

The outputs. The tool returns V=12, I=2, R=6, each to four decimals. The current is therefore 2.0000 A. Power, worked out separately as P = V × I = 12 × 2 = 24 W, can then be checked against the amps-to-watts converter before finalising the fuse rating.

The decision. With a 2 A draw confirmed, Imran can choose a fuse rated above normal running current but well below the cable's capacity, and show the client the copied result line as the working behind the quote. One measured supply, one rated load, one calculation, one documented answer.

The same tool handles the opposite problem. A 5 V supply driving an LED at 20 mA needs R = 5 ÷ 0.02 = 250 Ω, so Imran picks the nearest standard resistor at or above that value. The LED resistor calculator guide covers that choice with forward voltage included.

Where this calculation saves real money

For tradespeople, the value is not the formula. It is the avoided rework, the same logic behind the maths electrical contractors live by: a small, checked number up front beats an expensive correction later.

• Resistor orders stop being trial and error. You order the value the maths demands, not a bag of near-misses.
• Fuse and wire choices get a paper trail. A copied result line in the job note shows why that rating was chosen.
• Fault-finding gets faster. If a circuit draws far more than V ÷ R predicts, you have a short, a wrong component or a misread label.
• Quotes get tighter. Known current and power lead straight to the parts list, cable size and labour allowance.

What the ohms law calculator does not do

Honest limits matter, because a tool trusted beyond its range causes the very mistakes it was meant to prevent.

• It does not calculate power. Power is P = V × I, worked out from the answers. A 12 V, 2 A circuit is 24 W, but the tool solves voltage, current and resistance only.
• It does not handle AC impedance. Circuits with capacitors or inductors need impedance, phase and frequency, not plain resistance.
• It does not accept milli or kilo units. No mA, mV or kΩ pickers. Convert first, or the answer will be off by a factor of 1,000.
• It does not accept one or three values. Enter exactly two, or the tool shows an "Enter any two" prompt instead of an answer.
• It does not reject bad measurements. Zero or negative values are refused, but a wrong meter reading entered confidently still gives a confident-looking wrong answer. Measure twice.

For the power side of a job, pair the result with the amps-to-watts converter or an electric current converter when readings arrive in milliamps. For running costs, the electricity bill calculator turns watts and hours into a bill estimate.

Moving the business side to CodexFuse, step by step

Trades and repair businesses often run a fragmented setup: hosting with one provider, email with another, backups nowhere. When any piece fails, the customer cannot reach you. CodexFuse replaces that patchwork with one managed platform on credits.

1. Bring your domain and site over. Point the domain to CodexFuse hosting and migrate the existing site, keeping the pages and contact forms customers already use.
2. Move business email with it. Set up matching mailboxes on the same platform, so quotes come from your own domain instead of a free inbox clients trust less.
3. Pay with credits, only for what you use. Top up credits and draw them down as hosting and email run. No subscription tier to outgrow or cancel when work goes quiet.
4. Let the platform handle the babysitting. Updates, monitoring and backups sit with the managed service, so a failed drive is handled before it becomes a lost weekend of client calls.

Frequently Asked Questions

What is Ohm's law in plain terms?

Ohm's law says voltage equals current multiplied by resistance, V = I × R. Raise the voltage and current rises; raise the resistance and current falls. The calculator rearranges that one relationship to solve for whichever value you do not know.

How do I use an ohms law calculator?

Enter any two of voltage, current and resistance, leave the third blank, and click Calculate. Use base units only: volts, amps and ohms. The tool returns all three values to four decimals, with a copy button for your notes.

What if I only know one value?

One value is not enough, because countless circuits share a single voltage, current or resistance. Measure or look up a second value, such as the supply voltage from the adapter label, then calculate.

Can I enter milliamps or kilohms directly?

No. This calculator takes plain numbers in base units only, with no mA or kΩ pickers. Divide milliamps by 1,000 to get amps, and multiply kilohms by 1,000 to get ohms, or the answer will be wrong by a factor of 1,000.

Does the calculator work for AC circuits?

Only for simple resistive AC cases. Circuits with motors, transformers, capacitors or inductors involve impedance and phase, so a plain V = I × R answer will mislead you. Use an impedance calculation for those.

Why does the tool reject zero or negative values?

Zero resistance would imply infinite current, and negative resistance or current has no meaning in a simple passive circuit. The tool refuses those inputs so a typo cannot silently produce a nonsense answer that looks plausible.

Check the number, then fix the setup around it

An ohms law calculator will not make you a better electrician on its own. It will make your numbers checkable, your quotes defensible and your component choices repeatable, and that stops the small errors that eat margin. Run the calculation before you order, copy the result into the job note, and treat any surprising answer as a prompt to remeasure.

Then apply the same discipline to the business behind the bench. Move your hosting and business email to CodexFuse today: one fully managed platform, pay-as-you-go credits, no subscriptions and no server babysitting, so the next client finds a working website and a working inbox, not a dead link. Get started at CodexFuse now, and let the platform carry the infrastructure while you carry the tools.