Electrical
Voltage Calculator
Reconcile a resistive circuit's voltage using power and current, or current and resistance, without mistaking a nearby standard voltage for a measurement.
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.
Get a Free Business AuditWhat moves the number
- The selected calculation method determines whether watts or resistance is relevant.
- Readings taken under different loads may not describe one electrical state.
- Rounding small current values can substantially change a divided result.
Where this one goes wrong
Nearest standard system is a numerical comparison, not identification of a building's supply. Similar calculated values cannot establish phase arrangement, earthing, neutral availability or equipment compatibility.
Questions
About this calculator
Why do the two methods disagree?
Check that the inputs represent the same operating point and that the load fits the resistive model. AC power factor or temperature-dependent resistance can invalidate a simple comparison.
Does zero current mean zero supply voltage?
No. Division by zero cannot determine voltage, and an open circuit can still be energized. A displayed fallback is not evidence of a safe circuit.
Can I use the nearest-system result to connect equipment?
Use it only for planning. Supply verification, equipment instructions and the applicable electrical code govern any connection.
The formula
V = P ÷ I, or V = I × R
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How to use this calculator
- Calculate from. Choose the unknown quantity. Only the inputs used by that calculation are shown.
- Power (watts). Enter your own figure; the page starts at 1,800.
- Current (A). Enter your own figure; the page starts at 15.
The result updates as you type, so you can try a figure, see what it does to voltage, 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 |
|---|---|
| Calculate from | Power and current |
| Power (watts) | 1,800 |
| Current (A) | 15 |
What comes out
| Result | Value |
|---|---|
| Voltage main result | 120.0 |
| Nearest standard system | 120 V |
| Power at that voltage | 1,800 |
| Circuit resistance | 8 |
That works out to 120.0 volts. Closest standard system voltage is 120 V.
How sensitive the answer is
Holding everything else steady and changing only power (watts), this is what happens to voltage. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Power (watts) | Voltage |
|---|---|
| 900 | 60.0 |
| 1,350 | 90.0 |
| 1,800 example above | 120.0 |
| 2,700 | 180.0 |
| 3,600 | 240.0 |
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
- Calculate from
- Choose the unknown quantity. Only the inputs used by that calculation are shown.
- Power (watts)
- A number, between 0 and 10000000, starting at 1,800.
- Current (A)
- A number, between 0 and 100000, starting at 15.
What the results tell you
| Result | What it is |
|---|---|
| Voltage | The headline result. A decimal figure. |
| Nearest standard system | A category or planning selection, rather than a measured quantity. |
| Power at that voltage | A number. |
| Circuit resistance | 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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