Electrical
Ohm's Law Calculator
Check the relationship between voltage, current and resistance in a simple resistive circuit before interpreting a power reading.
Result
Electrical work is life-safety work governed by the NEC or your local code, and must be designed and installed by a licensed electrician. Conductor and overcurrent sizing depends on ambient temperature, conductor bundling, termination ratings, continuous-load rules and more. This is an educational estimate and must never be used as the basis for an installation. Ohm's law applies to purely resistive DC circuits. AC circuits with motors or electronics have reactance and a power factor that this does not model.
Get a Free Business AuditWhat moves the number
- The quantity selected under Solve for determines which pair of inputs is used.
- Resistance must describe the same operating condition as the voltage and current.
- Power describes the rate of energy use, not accumulated energy.
Where this one goes wrong
A cold resistance measurement need not describe a component at operating temperature. A result that disagrees with a nameplate may reflect the measurement conditions rather than a failed part. This arithmetic does not diagnose insulation condition or establish that a circuit is safe.
Questions
About this calculator
Which two readings should I enter?
Choose the result first, then use the two quantities required by that equation. Unused fields are not extra constraints, so conflicting entries do not produce an independent cross-check.
Can this model a motor?
Only as an oversimplified resistive load. Reactance and power factor require an AC model; winding resistance alone does not determine running current.
Does the answer select a protective device?
No. Any conductor or protection decision is a separate code-governed planning exercise, subject to equipment instructions and local requirements.
The formula
V = I × R; P = V × I
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How to use this calculator
- Solve for. Choose the unknown quantity. Only the inputs used by that calculation are shown.
- Voltage (V). Enter your own figure; the page starts at 120.
- Current (A). Enter your own figure; the page starts at 5.
The result updates as you type, so you can try a figure, see what it does to result, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Solve for | Resistance (Ω) |
| Voltage (V) | 120 |
| Current (A) | 5 |
What comes out
| Result | Value |
|---|---|
| Result main result | 24 Ω |
| Voltage | 120 |
| Current | 5 |
| Power | 600 |
With 120 V and 5 A, the circuit has 24 Ω of resistance and dissipates 600 watts.
How sensitive the answer is
Holding everything else steady and changing only voltage (v), this is what happens to result. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Voltage (V) | Result |
|---|---|
| 60 | 12 Ω |
| 90 | 18 Ω |
| 120 example above | 24 Ω |
| 180 | 36 Ω |
| 240 | 48 Ω |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Solve for
- Choose the unknown quantity. Only the inputs used by that calculation are shown.
- Voltage (V)
- A number, between 0 and 1000000, starting at 120.
- Current (A)
- A number, between 0 and 100000, starting at 5.
What the results tell you
| Result | What it is |
|---|---|
| Result | The headline result. A category or planning selection, rather than a measured quantity. |
| Voltage | A number. |
| Current | A number. |
| Power | A number. |
Before you rely on this number
Conductor sizing, voltage drop and load figures are governed by code, and the code takes precedence over any calculator. Ambient temperature, conduit fill, continuous load factors and the specific installation all change the answer. Treat this as a first pass, not a design.
Keep going
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