Enter voltage U in volts and current I in amperes. The calculator divides voltage by current using R = U/I and reports resistance in ohms, plus conductance and power from the same pair. For 12 V and 0.5 A the result is 24 Ω, and for 230 V and 10 A it is 23 Ω. The model does not combine series or parallel networks and does not compute impedance; zero current has no quotient.
Enter voltage and current. Resistance, conductance, and power will appear here.
To solve for any one of U, I, and R, use the Ohm’s law calculator. For power from voltage and current, use the P = U·I calculator.
Electrical resistance in Ohm's law is the ratio of voltage to current: R = U/I. The SI unit is the ohm: 1 Ω = 1 V/A. Georg Simon Ohm wrote down that proportion in 1827. The calculator applies it to the two numbers you enter. It does not solve the full U, I, R triangle.
The calculator uses voltage U in volts and current I in amperes. For 12 V and 0.5 A you get 12 / 0.5 = 24 Ω. For 230 V and 10 A the result is 23 Ω. A 5 V drop at 0.02 A gives 250 Ω. Twelve volts and 24 A give 0.5 Ω.
What the result panel shows. Besides resistance in ohms, the calculator shows conductance G = I/U and the power at the terminals P = U·I. At 12 V and 0.5 A the power is exactly 6 W, and the conductance is 1/24 siemens. When the resistance is below 1 Ω, the panel also shows milliohms; from 1,000 Ω it shows kilohms, and from 1,000,000 Ω megohms. The chart under the result draws R at the fixed current while voltage moves around the value you entered.
Limits of the model. The calculator does not combine resistors in series or in parallel, and it does not compute wire resistance from resistivity, length, and area. It does not apply a temperature coefficient, it does not compute impedance on alternating current, and it does not separate load resistance from the internal resistance of a source. The result is the ratio of the voltage drop you enter to the current you enter.
A current of zero has no quotient: the calculator then shows a message and does not draw a result. Zero volts at a nonzero current gives R = 0, and conductance is out of range. A negative result means the voltage and the current have opposite signs, not a passive resistor. A comma and a period in the fields are the same decimal separator. Enter milliamperes as amperes: 20 mA is 0.02.
Examples on the page: 12 V and 0.5 A → 24 Ω; 230 V and 10 A → 23 Ω; 5 V and 0.02 A → 250 Ω; 460 V and 10 A → 46 Ω; 12 V and 150 A → 0.08 Ω. The button “Fill this example” enters those same numbers and refreshes the result panel and the chart. The fields start empty.
R = U / I
[R] = V/A = Ω; 1 kΩ = 1,000 Ω, 1 MΩ = 1,000,000 Ω, 1 mΩ = 0.001 Ω, 1 S = 1/Ω
Model: Ohm's law (Georg Simon Ohm, 1827), solved for resistance. The rearrangements U = I·R and I = U/R follow from the same formula, but this calculator finds R only. From the same inputs it also shows G = I/U and P = U·I. Order of computation: R = U / I when the current is not zero.
Ohm's law says that in many conductors the current is proportional to the voltage, and the factor in that proportion is the resistance: R = U/I. It is an empirical law. It holds on the range where voltage versus current is a straight line through the origin.
The ohm is a derived SI unit: 1 Ω = 1 V/A = 1 kg·m²·s⁻³·A⁻². The definition this calculator uses is the elementary one: the resistance between two points is one ohm when a constant potential difference of one volt produces a current of one ampere. The practical realization of the ohm in metrology uses the quantum Hall effect and the von Klitzing constant. This calculator does not perform that calibration. It divides the two numbers you enter.
Georg Simon Ohm (1789–1854) published Die galvanische Kette, mathematisch bearbeitet in Berlin in 1827. He studied galvanic circuits and showed that the current in a given conductor is proportional to the exciting force, which we now write as voltage. Contemporaries received the book coldly. Recognition came later, and Ohm's name became the name of the unit.
In the school form U = I·R the three quantities are peers. This calculator solves the formula for R only. A separate Ohm's law page accepts any two of the three and fills in the third. The split is deliberate: this form has two fields and one unknown, so resistance is not entered as an input.
The model assumes direct current, a temperature that stays fixed during the measurement, and an ohmic element. A diode, a filament that is still heating up, and an electrolyte do not lie on that straight line. Metals increase their resistance with temperature, approximately R = R₀ (1 + α·ΔT). This calculator does not ask for α or for a reference temperature. The result is the resistance at the moment U and I were measured, not a cold catalog value.
On alternating current, impedance takes the place of resistance, and the reactance of a coil or a capacitor depends on frequency. Separate pages compute inductive and capacitive reactance. Here the current is a real number in amperes, and the result is a real number in ohms.
Step 1. Measure the voltage drop U across the element and the current I through that element, with consistent arrows. Step 2. Divide: R = U/I, provided the current is not zero. For 12 V and 0.5 A: 12 / 0.5 = 24. The units: a volt divided by an ampere is an ohm. Step 3. Conductance is the reciprocal, G = 1/R = I/U, in siemens. At 12 V and 0.5 A that is exactly 1/24 siemens.
Step 4. The power dissipated in the element at those same values is P = U·I, which is 6 W. The forms P = I²·R and P = U²/R give the same number only when the resistance comes from that same voltage and current. The panel computes power directly as the product, without a second measurement.
If the element is a uniform wire, resistance is tied to geometry: R = ρ·l/S, where ρ is resistivity, l is length, and S is cross-sectional area. The wire-resistance calculator handles that relation. Resistivity, length, and area do not appear here.
Two resistors in series give a sum of resistances, and two in parallel give a sum of reciprocals. Those combinations live on separate pages. This calculator returns one equivalent resistance for a voltage–current pair, whatever sits between the terminals you measured.
1 kΩ = 1,000 Ω, 1 MΩ = 1,000,000 Ω, and 1 mΩ = 0.001 Ω. The panel also shows kilohms from 1,000 Ω, megohms from 1,000,000 Ω, and milliohms below 1 Ω. A comma and a period in a field are the same separator. Enter a milliampere as a fraction of an ampere. The unit switch in the header does not change the formula: volts, amperes, and ohms stay.
A short in this model is zero volts at a nonzero current, so R = 0 and conductance is not a finite number. An open circuit, current equal to zero, has no quotient. A negative resistance means the voltage arrow and the current arrow oppose each other. Check the signs on the schematic before you treat the element as a source. The calculator does not reject negative numbers, because the division is then defined.
G. S. Ohm, Die galvanische Kette, mathematisch bearbeitet, Berlin, 1827. D. Halliday, R. Resnick, J. Walker, Fundamentals of Physics, the chapter on current and resistance. H. D. Young and R. A. Freedman, University Physics, the chapter on current and resistance. BIPM, The International System of Units (SI), 9th edition, definition of the ohm. NIST Special Publication 330 and SP 811. This page is for educational purposes.
R denotes resistance. U denotes voltage, and I denotes current. For 12 V and 0.5 A the result is 24 Ω.
A 12 V automotive bulb draws 0.5 A. The equivalent resistance of the filament at that moment is 24 Ω, and the power at the terminals is 6 W. A cold filament has a different resistance; the calculator uses only the numbers you enter.
R = 24 Ω
A heater on a 230 V supply draws 10 A. The ratio is 23 Ω, and the power is 2,300 W. That is the resistance range of a heater, not of a small 12 V circuit-board resistor.
R = 23 Ω
A series resistor with an LED has 5 V across it at 20 mA, which is 0.02 A. That resistor is 250 Ω. This is not the resistance of the diode itself.
R = 250 Ω
A 9 V battery feeds a load that draws 0.1 A. The equivalent resistance of the load is 90 Ω, and the power is 0.9 W.
R = 90 Ω
A heating element is measured at 460 V while it draws 10 A. The resistance is 46 Ω, and the power is 4,600 W. The same 460 V at 20 A would be 23 Ω.
R = 46 Ω
A starter on 12 V draws 150 A. The equivalent resistance of the circuit, including brushes and cables, is 0.08 Ω. That is not the copper resistance of the winding alone. Power at the terminals is 1,800 W.
R = 0.08 Ω
A USB-port load runs at 5 V and draws 2 A. The equivalent resistance is 2.5 Ω, and the power at the terminals is 10 W.
R = 2.5 Ω
A 24 V circuit, for example a truck electrical system, draws 4 A. The equivalent resistance is 6 Ω, and the power is 96 W.
R = 6 Ω
Electrical resistance in Ohm's law is the ratio of voltage to current: R = U/I. The calculator divides the voltage you enter in volts by the current you enter in amperes and shows the result in ohms. From the same numbers it also shows conductance G = I/U and power P = U·I.
Voltage in volts, current in amperes, and the result in ohms. Convert milliamperes before you enter them: 20 mA is 0.02 A. Resistance is the result, not an input, so a kilohm does not go in a field. A comma and a period are the same decimal separator.
Division by zero is not defined. The calculator does not show a resistance while the current is zero, and it leaves a message. An empty form computes nothing: both fields need a number. Zero volts at a nonzero current gives R = 0, and conductance is then out of range.
There you supply any two of the three — voltage, current, and resistance — and the calculator fills in the third from U = I·R. Here there are only two fields, voltage and current, and the only unknown is resistance. It is the same Ohm relation, solved for R alone.
Yes. Twelve volts and twenty-four amperes give 0.5 Ω. A negative ratio appears when the voltage and the current have opposite signs. That result does not describe a passive resistor. It describes arrow directions that you should check on the schematic.
No. The formula R = U/I is for an ohmic range at constant temperature and direct current. The calculator does not apply a temperature coefficient, it does not compute impedance or reactance, and it does not split load resistance from the internal resistance of a source. The voltage in the field should be the drop across the element whose resistance you want.
Resistance from resistivity, length, and area is covered by the wire-resistance calculator. Series adds resistances, and a parallel combination adds their reciprocals. This calculator does not use a material or a resistor count. It uses the measured voltage and current and returns one equivalent resistance.
Power P = U·I is the product of the same two numbers, not a separate measurement. At 12 V and 0.5 A that is 6 W. The separate power page is there when you want watts alone. Here the panel shows watts next to the ohms, so the same measurement has an energy reading.
No. Volts, amperes, and ohms stay on both sides of the switch. The calculator divides the numbers you enter by R = U/I whether the header says metric or US.
The chart draws resistance at the current from the current field, while voltage moves around the value you entered. It illustrates the proportion R = U/I. It is not a laboratory trace. The button “Fill this example” enters the example numbers and refreshes the panel and the chart.
Divide voltage in volts by current in amperes. The result is in ohms. For 12 V and 0.5 A: R = 12/0.5 = 24 Ω.
24 Ω. Conductance G = I/U is then 1/24 S, and power P = U·I is 6 W.
23 Ω. That is a typical pair for a heater or kettle on mains voltage: power is then 2300 W.
As 0.02 A. Convert milliamperes to amperes before dividing.
No. Here the result is resistance from the U–I pair. Impedance includes inductor and capacitor reactance and phase — those quantities are not on this form.
The definition, the unit, the history, and the derivation below come from standard physics and metrology sources.
Page updated November 3, 2026.