AmpSizerOhm's law calculator
Ohm's law calculator
60.00 ohms
V = I × R and P = V × I, so two of the four always give the other two
| Quantity | Symbol | Value |
|---|---|---|
| Voltage | V | 120.00 volts |
| Current | I | 2.00 amps (2,000 mA) |
| Resistance | R | 60.00 ohms |
| Power | P | 240.00 watts (0.240 kW) |
Pick which two you know first — the labels on the boxes change with it, so the numbers never end up in the wrong slot.
A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.
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MonthlyOne thing a month worth measuring, testing or recalculating on a home electrical system — the part of sizing that happens after the install. No product pitches, and nothing that needs a live panel.
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What is Ohm's law?
Ohm’s law is V = I × R: voltage equals current times resistance. The AmpSizer result for 120 volts across a 2 amp load is 60 ohms and 240 watts.
Paired with the power law P = V × I, those two expressions generate every other formula in the subject. Twelve arrangements are usually drawn as a wheel, and all twelve are algebra on the same two statements.
What makes it useful in real work is the substitution. Combine them and you get P = I² × R, which says power in a conductor rises with the square of the current — the reason a 20% overload heats a wire 44% harder, and the reason ampacity tables exist at all.
The twelve formulas, arranged by what you are solving for. This is the wheel, written out.
| To find | From V and I | From V and R | From I and R | From P and V | From P and I | From P and R |
|---|---|---|---|---|---|---|
| Volts | — | — | V = I × R | V = P ÷ I | V = P ÷ I | V = √(P × R) |
| Amps | — | I = V ÷ R | — | I = P ÷ V | — | I = √(P ÷ R) |
| Ohms | R = V ÷ I | — | — | R = V² ÷ P | R = P ÷ I² | — |
| Watts | P = V × I | P = V² ÷ R | P = I² × R | — | — | — |
Everything in that table is V = IR and P = VI rearranged. There is nothing else to learn.
The two that matter on a job site
I = P ÷ V turns an appliance nameplate into a circuit. A 1,500 W heater on 120 V is 12.5 A, and now you know which breaker it belongs on.
P = I² × R turns a bad connection into a temperature. It is the formula behind every burnt terminal, every scorched receptacle and every ampacity table in the code book — and the one worth understanding rather than looking up.
Where it stops being enough
Ohm’s law describes resistance. Real alternating-current circuits also contain inductance, from motor windings and transformers, and capacitance, from long cable runs and correction gear. Those store energy and give it back a fraction of a cycle later, so current and voltage no longer peak together.
At that point resistance becomes impedance, and the ratio of real to apparent power becomes the power factor. Ohm’s law still holds — it just needs a complex number to hold it in.
Frequently asked questions
What are the twelve Ohm's law formulas?
They are six pairs of rearrangements: V = IR, V = P/I and V = the square root of PR; I = V/R, I = P/V and I = the square root of P/R; R = V/I, R = V²/P and R = P/I²; P = VI, P = I²R and P = V²/R. Every one is derived from V = IR and P = VI, which is why the wheel is a memory aid rather than twelve things to learn.
What is the Ohm's law wheel or triangle?
The triangle covers V, I and R: cover the one you want and the remaining two show the operation. The wheel extends it to four quadrants including watts, with three formulas in each. Both are the same algebra drawn to be readable at a glance, which is why they end up on workshop walls.
Does Ohm's law work on AC circuits?
For resistive loads, exactly. For anything with inductance or capacitance you replace resistance with impedance, and voltage and current stop peaking together — which is where power factor comes from. On a heater, Ohm’s law is precise; on a motor, it needs impedance and a power factor to be honest.
Why does power rise with the square of current?
Because substituting V = IR into P = VI gives P = I²R. Double the current in a fixed resistance and you get four times the heat. This is the single most useful consequence of Ohm’s law in electrical work: it explains why a loose connection burns, why undersized wire fails suddenly rather than gradually, and why fuses are rated the way they are.
How do I measure resistance without an ohmmeter?
By measuring voltage across a load and current through it, then dividing — which is what this calculator does with the numbers. In practice you use an ohmmeter on a de-energised circuit, because putting a meter in series with a live one to read current is a job for a clamp meter and someone trained to use it.
How we calculate this
V = I × R, and the power law is P = V × I. Given any two of the four quantities, the other
two follow by substitution: from volts and amps, R = V ÷ I and P = V × I; from watts and ohms,
I = √(P ÷ R) and V = I × R; and so on for each pair. All arithmetic is on real-valued resistance,
which is exact for direct current and for purely resistive alternating-current loads. Circuits with
inductance or capacitance require impedance in place of resistance and introduce a phase angle between
voltage and current, which is what the power factor in the
conversion calculators accounts for.Worked example
With What you know Volts and amps, Volts 120, Amps 2, this page works out 60.00 ohms. V = I × R and P = V × I, so two of the four always give the other two
Sources:
The three things it explains
Why a loose connection burns. A loose terminal is a small resistance carrying the full circuit current. P = I²R at 20 A through even a tenth of an ohm is 40 watts in a space the size of a screw head.
Why voltage drop wastes power. The volts lost in a conductor are I × R, and the watts lost are I²R — turned into heat along the run and paid for on the bill.
Why an overload is not linear. A circuit at 125% of its rating carries 25% more current and generates 56% more heat. Nothing about overheating scales gently.