Free tool
Ohm's Law Calculator
Any two of volts, amps, ohms and watts give the other two. What Ohm's law does not give you is a motor's running current from an ohmmeter reading.
V = I × R, and P = V × I
V = I × R: voltage is current times resistance. With power added, P = V × I = I² × R = V² ÷ R. A 9.6 Ω element on 120 V draws 12.5 A and makes 1,500 W. Those formulas hold for DC and for resistive AC loads. For motors and power supplies, current depends on impedance and power factor instead — use the watts to amps calculator with a power factor.
What you know
The voltage across the part, not the supply if other parts share it.
At operating temperature — a cold element or filament reads lower.
The answer
- Voltage (given)
- 120 V
- Current
- 12.5 A
- Resistance (given)
- 9.6 Ω
- Power
- 1,500 W
I = V ÷ R
P = V² ÷ R
What this does not tell you
A wire size or a breaker size — conductors and overcurrent protection are chosen from the NEC tables and the equipment listing, not from this arithmetic. Whether a part is rated for the result — a resistor dissipating 1,500 W needs a power rating above that. Starting or inrush current, which for a cold filament or a motor can be several times the running figure.
Informational use only, please verify before you rely on it
This reproduces Ohm's law (V = I × R) and the power relationship (P = V × I) for DC and resistive loads. It is a preparation aid, not a determination. It does not size conductors or overcurrent protection, does not account for impedance, power factor, temperature or inrush, and does not decide whether equipment is suitable for a location. Confirm against the equipment nameplates and applicable codes.
This tool is provided for general informational and educational purposes only. Its output is an illustrative estimate generated from the values you enter and from general assumptions that will not match every deal, market, lender, or homeowner. It is not tax, legal, accounting, financial, or professional advice, and it is not a quote, an offer, a credit decision, or a guarantee of pricing, approval, timing, savings, or eligibility.
You are solely responsible for independently confirming all information presented here including any figures, rates, fees, margins, timelines, tax treatment, and federal, state, local, or utility incentives, with the applicable lender, authority having jurisdiction, and your own qualified tax, legal, and financial advisors before acting on it, relying on it, or presenting it to a homeowner or any third party. Incentive programs, lender terms, and permitting requirements change frequently and vary by jurisdiction.
Seamless Home is not a tax advisor, law firm, lender, or licensed installing contractor, and makes no representation or warranty as to the accuracy, completeness, or currency of the information produced by this tool. To the fullest extent permitted by law, Seamless Home accepts no liability for any decision made or action taken in reliance on it.
All twelve formulas
| To find | Use |
|---|---|
| Voltage (V) | I × R · P ÷ I · √(P × R) |
| Current (I) | V ÷ R · P ÷ V · √(P ÷ R) |
| Resistance (R) | V ÷ I · V² ÷ P · P ÷ I² |
| Power (P) | V × I · I² × R · V² ÷ R |
Each row is one relationship rearranged. Whichever pair you start from, the four answers agree — which makes a quick checksum: if a heater's label says 1,500 W at 120 V, its element should measure about 9.6 Ω hot, and 12.5 A should read on a clamp meter. The space heater post walks through what that 12.5 A does to a household circuit.
Where Ohm's law stops
Resistance is not impedance. On AC, a motor winding, a transformer or a switching power supply opposes current with impedance, and the current it draws runs out of step with the voltage. Real power becomes V × I × power factor, which is why a nameplate can list volt-amps and watts as two different numbers — what is power factor explains the gap, and the VA to amps calculator works from the volt-amp rating.
Resistance moves with temperature. Metal conductors and filaments rise in resistance as they heat, so a cold reading understates the operating figure and a filament draws a surge the instant it is switched on.
Two values must describe the same part. The supply voltage and the current of a whole circuit give that circuit's total, not the resistance of one device on it. When several loads share a circuit, work from each one's own ratings — the volts to watts calculator covers how a resistive rating scales when the supply voltage changes, such as a 240 V element on a 208 V service.
Arithmetic is not a wire size. Knowing a load draws 12.5 A does not choose the conductor or the breaker; the NEC does, with continuous-load and ampacity rules this page does not apply. Small currents measured in milliamps convert with the milliamps to amps calculator.
Frequently Asked Questions
What is Ohm's law?+
Voltage equals current times resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I. Adding power, P = V × I, gives the full set — any two of volts, amps, ohms and watts fix the other two.
What are the Ohm's law formulas for power?+
P = V × I, P = I² × R and P = V² ÷ R. They are the same relationship written from different starting points. A 9.6 Ω heating element on 120 V draws 12.5 A and dissipates 1,500 W, whichever formula you use.
Does Ohm's law work for AC?+
For purely resistive AC loads — heating elements, incandescent filaments — yes, using RMS voltage and current. For motors, transformers and electronic power supplies it does not in this form: they have impedance rather than plain resistance, and real power is V × I × power factor.
Can I work out a motor's current from its measured resistance?+
No. The resistance an ohmmeter reads across a winding is a small fraction of the winding's impedance when it is running, so V ÷ R would predict a current many times too high. Use the nameplate full-load amps, or the watts and power factor, instead.
Why does a cold element read a different resistance?+
Because resistance changes with temperature. A tungsten filament's cold resistance is a fraction of its hot resistance, which is why bulbs draw a surge at switch-on. Enter the resistance at operating temperature, or start from the rated volts and watts.
How do I convert milliamps and kilohms?+
Divide milliamps by 1,000 for amps and multiply kilohms by 1,000 for ohms. The calculator accepts either unit and shows small currents in both milliamps and amps.
Does this size a wire or a breaker?+
No. It reproduces the arithmetic of Ohm's law as a preparation aid. Conductor size and overcurrent protection come from the NEC tables, the equipment listing and the continuous-load rules, and should be confirmed by the licensed professional doing the work.
Related resources
Watts to Amps Calculator
Current from watts with power factor applied, carried to a breaker size.
Learn more →Volts to Watts Calculator
Watts from volts plus current or resistance, and how ratings scale with voltage.
Learn more →Milliamps to Amps Calculator
mA to A, with watts at a voltage and the GFCI trip range for scale.
Learn more →What Is Power Factor?
Why volt-amps and watts differ on AC equipment.
Learn more →Electrical figures read the way the nameplate wrote them
Seamless Home runs the design, permit and interconnection layer for the partners it works with — which is where a load's volts, amps and watts get checked against the equipment actually specified. Coverage is confirmed per service area rather than promised as blanket availability.
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