Example 1
A desk lamp is rated 60 W, and a series ammeter reads 0.5 A. The quotient is 120 V, the usual mains voltage in the United States.
U = 120 V
Voltage from Power and Current Calculator
Enter power P in watts and current I in amperes. The calculator divides power by current using U = P/I and reports the voltage in volts. For 60 W and 0.5 A the result is 120 V, and for a 2300 W kettle at 10 A it is 230 V. The model does not use cos φ or Ohm’s law U = I·R; zero current is not used in the division.
Result
Enter power in watts and current in amperes. Voltage, conversions, and the chart will appear here.
For power from voltage and current, use the P = U·I calculator; for current from power and voltage, use the I = P/U calculator.
How it works
P60 W
÷ I0.5 A
U120 V
Voltage in a DC circuit follows from the definition of electric power. Power is the product P = U·I, so the unknown voltage is the quotient U = P/I. In the SI, one volt is one watt per ampere: 1 V = 1 W/A. This is the textbook rearrangement of electric power (Halliday, Resnick, and Walker; Young and Freedman, University Physics), not Ohm’s law.
The calculator uses the two numbers you enter: power P in watts and current I in amperes. The result is their quotient. For 60 W and 0.5 A you get 60 / 0.5 = 120 V. For a 2300 W kettle at 10 A the result is 230 V. The same 60 W at 2 A gives 30 V, because the current in the denominator is four times larger.
What the result panel shows. Besides voltage in volts, the calculator reports the same value in millivolts and kilovolts, repeats the power you entered (with kilowatts when |P| ≥ 1000 W) and the current you entered (with milliamperes when |I| < 1 A), and the multiples of 120 V and of 230 V. The chart under the result holds the entered current and places power at 0, 1/4, 1/2, 3/4, 1, 3/2, and 2 times the absolute value of the entered power. When power is zero, the power axis runs from 0 W to 2 W.
Limits of the model. The calculator does not apply Ohm’s law U = I·R, does not solve I = P/U, and does not know the power factor cos φ. On alternating current, real power is P = U·I·cos φ. The quotient P/I is then the RMS voltage only for a resistive load, when cos φ = 1. The result does not judge insulation, wire size, or breaker selection.
A comma and a period in the fields are the same decimal separator. The power field expects watts: enter 2.3 kW as 2300. Current must not be zero, because division by zero is undefined. Zero watts at a nonzero current gives U = 0 V. Negative values are allowed and give a voltage with the sign of the quotient.
Examples on this page: 60 W and 0.5 A → 120 V; 100 W and 2 A → 50 V; 2300 W and 10 A → 230 V; 6 W and 0.5 A → 12 V; 0.1 W and 0.02 A → 5 V. The button “Fill this example” enters those same numbers and refreshes the result panel and the chart. The fields start empty.
Formula
U = P / I
[U] = W/A = V; 1 kW = 1000 W; 1 mV = 0.001 V; 1 kV = 1000 V
Model: DC electric power P = U·I (the SI watt, BIPM), rearranged for voltage. James Prescott Joule related conductor heat to I²R, and under Ohm’s law I²R = U·I. This calculator solves for U only. Order of computation: U = P / I when I ≠ 0, then conversions to mV and kV and the multiples of 120 V and 230 V.
Theory, derivation, and scope
Definition of electric power and of U = P/I
DC electric power is the product of voltage and current: P = U·I. When power and current are known, and current is not zero, voltage is the quotient U = P/I. In the SI, 1 V = 1 W/A = 1 J/C. That is a unit definition, not Ohm’s law and not an insulation test.
When voltage and current change with time, the instantaneous product p(t) = u(t)·i(t) is instantaneous power. This calculator takes one number P and one number I, so it describes the steady case, or one pair of values at a power factor of one.
History: Volta, Ampère, Ohm, and Joule
Alessandro Volta presented the electric pile in 1800. André-Marie Ampère described the force between current-carrying conductors in the 1820s. Georg Simon Ohm, in Die galvanische Kette, mathematisch bearbeitet (1827), stated the relation of voltage, current, and resistance.
In 1841, in the Philosophical Magazine, James Prescott Joule showed that heat released in a metallic conductor is proportional to the square of the current, to the resistance, and to time. Ohm’s law then gives P = I²R = U·I, so Joule heating and electric power are the same fact in two writings. The unit names commemorate Volta, Ampère, and James Watt. The BIPM SI Brochure defines the watt, the volt, and the ampere.
Assumptions: the product U·I, and current not zero
The formula U = P/I is valid when the power you enter is the product of that voltage and that current. For direct current this is an arrow convention: the sign of power says whether the element absorbs power or delivers it. Current must not be zero.
The calculator does not assume a linear element. Resistance may depend on temperature, and the result is still the quotient of the two numbers you enter. The calculator does not check whether that pair appears on a device nameplate.
Derivation from P = U·I and Joule heating
Step 1. Definition: P = U·I. Step 2. When the current is not zero, dividing both sides by the current leaves U = P/I. Step 3. Units: a watt divided by an ampere is a volt. Step 4. For 60 W and 0.5 A the voltage is 60 / 0.5 = 120 V. For 2300 W and 10 A the voltage is 230 V. For the same 60 W at 2 A the voltage is 30 V.
Step 5. If the element obeys Ohm’s law, U = I·R, so P = I²R. Resistance could then be recovered as R = U/I, but this calculator does not solve for it. A diode, a motor, and a switch-mode supply are not a resistor, so attaching ohms to the result would overclaim the formula.
Alternating current, RMS value, and power factor
For sinusoidal voltage and current, real power is P = U_rms · I_rms · cos φ, where φ is the phase shift and U_rms and I_rms are root-mean-square values. That equation gives P / I_rms = U_rms · cos φ.
The quotient from this calculator is the RMS voltage only when cos φ = 1, that is for a resistive load such as a heater or a classic incandescent lamp. A motor often has a power factor below one, and the same quotient is then not the supply voltage. The calculator has no phase-angle field and no reactive-power field.
Units: volt, watt, ampere, and the kilowatt trap
1 kW = 1000 W, 1 mV = 0.001 V, 1 kV = 1000 V, and 1 mA = 0.001 A. The power field expects watts. Entering 2.3 instead of 2300 at a current of 10 A gives 0.23 V instead of 230 V, a result a thousand times too small. A comma and a period are the same separator.
The panel also reports the multiple of 120 V and the multiple of 230 V. For a 120 V result the multiple of 120 V is 1, and the multiple of 230 V is 120/230. For a 230 V result the multiple of 230 V is 1. These are the voltage divided by those two reference values, not a verdict that the installation is correct.
Sources
The definition, the units, and the derivation come from general physics textbooks and metrology documents: D. Halliday, R. Resnick, and J. Walker, Fundamentals of Physics (current, resistance, and power); H. D. Young and R. A. Freedman, University Physics; J. P. Joule, On the Heat evolved by Metallic Conductors of Electricity, and in the Cells of a Battery during Electrolysis, Philosophical Magazine, 1841; G. S. Ohm, Die galvanische Kette, mathematisch bearbeitet, 1827; BIPM, The International System of Units (SI), 9th edition; NIST Special Publication 330. This page is for educational purposes.
How to use
- Enter power P in watts, for example 60. Enter 2.3 kW as 2300.
- Enter current I in amperes, for example 0.5 or 10. The calculator rejects zero amperes.
- Choose Calculate, or wait for the update after both numbers are entered. 60 W and 0.5 A give 120 V. 2300 W and 10 A give 230 V.
- Read the voltage, millivolts, kilovolts, the the values you entered, and the multiples of 120 V and 230 V. The chart shows U at constant I.
- When you need watts, open the P = U·I page. When you need amperes, open the I = P/U page. When you know resistance, open Ohm’s law.
Symbol glossary
The list below covers the symbols in U = P/I. For 60 W and 0.5 A the result is 120 V; for 2300 W and 10 A it is 230 V. The current must not be zero. Electric potential V = W/q is a different quantity with its own calculator.
- U
- The unknown voltage in volts. 60 / 0.5 = 120 V. This is not the potential V = W/q.
- P
- Power in watts. 2300 W at 10 A gives 230 V. Convert kilowatts to watts before you enter them.
- I
- Current in amperes. I = 0 cannot stand in the denominator. At 60 W, a current of 0.5 A gives 120 V.
Real-life examples
Example 2
A bench supply delivers 100 W at 2 A. The terminal voltage is 50 V.
U = 50 V
Example 3
A kettle nameplate says 2300 W and the current is 10 A. The voltage that matches that pair is 230 V.
U = 230 V
Example 4
A cabin lamp draws 6 W at 0.5 A. That is a 12 V automotive circuit, not a 5 V USB port.
U = 12 V
Example 5
An indicator LED draws 0.1 W at 0.02 A. The voltage across the LED is 5 V.
U = 5 V
Example 6
A shop heater rated 500 W draws 5 A. It runs at 100 V.
U = 100 V
Example 7
A fan on a vehicle circuit: 120 W at 10 A. The voltage is 12 V.
U = 12 V
Example 8
A 1000 W load at 4 A needs 250 V, a little above a 230 V European supply.
U = 250 V
Example 9
A lithium-ion cell: 3.7 W at 1 A. The cell voltage is 3.7 V.
U = 3.7 V
Example 10
A telecom rack bus: 960 W at 20 A. The bus voltage is 48 V.
U = 48 V
Ways to use this calculator
- Check whether a 2300 W nameplate at a measured current of 10 A matches 230 V.
- Read a supply voltage when you know the load power and the ammeter current, for example 60 W at 0.5 A.
- Compare the result with a 120 V or 230 V supply through the multiple, without mental arithmetic.
Common questions
What does this voltage calculator compute?
In this model voltage is power divided by current: U = P/I, because DC electric power is P = U·I. The calculator divides the power you enter in watts by the current you enter in amperes and reports the result in volts, millivolts, and kilovolts, and as a multiple of 120 V and of 230 V.
Which units should I enter for power and current?
Enter power in watts [W] and current in amperes [A]. The result is in volts [V]. A kilowatt is not the unit of the power field: enter 2.3 kW as 2300 W. A comma and a period are the same decimal separator, so 0.5 and 0,5 are the same current.
Why does zero current produce no result?
Division by zero is undefined. At I = 0 the formula U = P/I has no value, whatever the power is. The calculator shows a message and does not draw the chart. Zero watts at a nonzero current is valid and gives 0 V.
How is this different from the power page P = U·I?
Here the unknown is voltage, and the inputs are power and current. In the power calculator the unknown is power in watts, and the inputs are voltage and current. It is the same trio P, U, and I and the same equation, with a different unknown. Current from power and voltage, I = P/U, is the third rearrangement and has its own calculator.
How is this different from Ohm’s law?
Ohm’s law ties voltage to current and resistance: U = I·R. This calculator does not know resistance and does not solve for it. Voltage comes only from power and current. When current and resistance are known, use the Ohm’s-law page.
Does the formula apply to alternating current?
For direct current, U = P/I is the voltage across the element when the power you enter is the product U·I. For a sine wave and a resistive load the same quotient is the RMS voltage, because the power factor is 1. When cos φ is below 1, real power divided by RMS current equals U·cos φ, not the RMS voltage itself. The calculator has no phase-angle field.
What do a negative power or a negative current mean?
The calculator keeps the sign of the quotient. Two negative signs give a positive voltage, and exactly one negative sign gives a negative voltage. The sign follows the arrow convention you chose. Nameplate values are usually entered as absolute numbers.
Will the calculator size a wire or a breaker?
No. The result is a voltage from two numbers. It does not account for conductor area, length, temperature, insulation, or the trip curve of a breaker. Wire resistance is a separate page, from resistivity, length, and cross-sectional area.
What does the chart under the result show?
The chart holds the current you entered and places power at 0, 1/4, 1/2, 3/4, 1, 3/2, and 2 times the absolute value of the power you entered. The vertical axis is voltage U = P/I at that current. When the entered power is zero, the power axis runs from 0 W to 2 W so the slope 1/I is visible.
Where does the formula come from?
The equation P = U·I is the definition of electric power: 1 W = 1 V·A. James Prescott Joule related heat in a conductor to I²R, and Ohm’s law makes I²R equal to U·I. The volt is named for Alessandro Volta. The browser does the arithmetic, and Hubert Ligocki reviewed the page.
How do I calculate voltage with U = P/I?
Divide power in watts by current in amperes. The result is in volts. For 60 W and 0.5 A: U = 60/0.5 = 120 V.
What is U at 60 W and 0.5 A?
120 V. The same power at 2 A gives 30 V, because the current in the denominator is four times larger.
What is U for a 2300 W kettle at 10 A?
230 V. That is a typical power–current pair at a 230 V mains voltage for a resistive load.
How do I enter 2.3 kW in the power field?
Convert to watts: 2.3 kW = 2300 W. The field expects watts, not kilowatts.
Does U = P/I include the power factor cos φ?
No. On AC, real power is P = U·I·cos φ, so P/I equals U only when cos φ = 1. A motor with cos φ < 1 needs a separate calculation.
Knowledge sources
The definition, the unit, the history, and the derivation come from physics textbooks and metrology documents.
- Wikipedia — Electric power
- BIPM — The International System of Units (SI), 9th edition
- NIST SP 330 — The International System of Units
- NIST SP 811 — Guide for the Use of the International System of Units
- Halliday, D.; Resnick, R.; Walker, J., Fundamentals of Physics — current, resistance, and power.
- Young, H. D.; Freedman, R. A., University Physics — electric power.
- Joule, J. P., Philosophical Magazine, 1841 — heat in a conductor.
- Ohm, G. S., Die galvanische Kette, mathematisch bearbeitet, 1827.
Page updated November 3, 2026.