Example 1
Reference for this page: 0.5 A through a 100 Ω resistor. A common school and bench case.
U = 50 V · P = 25 W
Ohm's Law Calculator
Enter any two of voltage U, current I, and resistance R. The calculator fills the third from U = I·R and shows power P = U·I. At 0.5 A and 100 Ω the result is 50 V and 25 W. The model is for an ohmic load; it does not include impedance Z or power factor cos φ.
Result
Enter any two of U, I, and R. Leave the third field blank. The result, power, and chart will show up here.
Resistance from voltage and current only: R = U/I. Power: P = U·I.
How it works
I0.5 A
R100 Ω
U50 V
Ohm’s law links voltage, current, and resistance linearly: U = I·R. Georg Simon Ohm published this relation in 1827 in Die galvanische Kette, mathematisch bearbeitet. For 0.5 A and 100 Ω the voltage is 50 V, because 0.5 × 100 = 50. The same pair gives power P = U·I = 25 W.
The calculator uses three fields: current I in amperes (prawo-ohma-a), resistance R in ohms (prawo-ohma-b), and voltage U in volts (prawo-ohma-c). Enter any two numbers and leave the third field blank. The missing quantity comes from U = I·R, I = U/R, or R = U/I. A comma and a period in the fields are equivalent. Enter 2.2 kΩ as 2200 and 20 mA as 0.02 A.
What the result panel shows. Besides the three Ohm quantities, the calculator reports power P = U·I and gently highlights the quantity it just solved. At 0.5 A and 100 Ω you see 50 V, 0.5 A, 100 Ω, and 25 W. The chart plots U against I at the fixed R from the result: a straight line through the origin.
Limits of the model. The calculator assumes an ohmic branch at steady temperature and direct current. It does not compute AC impedance, a temperature coefficient, or the split between source and load resistance. Series and parallel networks and wire resistance from geometry have their own pages. Zero current blocks solving for R, and zero resistance blocks solving for I.
Examples on the page include 12 V and 6 Ω → 2 A, a 5 V USB port with a 10 Ω load → 0.5 A, and a 230 V illustration from 10 A and 23 Ω with the caveat that it is not a wiring model. The “Fill this example” button enters those numbers and refreshes the panel and chart.
Formula
U = I · R
I = U / R; R = U / I; P = U · I
Named model: Ohm’s law (Georg Simon Ohm, Die galvanische Kette, mathematisch bearbeitet, Berlin 1827). The calculator fills the missing quantity from three fields. It does not compute AC impedance, wire resistance from geometry, or resistor networks.
Theory, derivation, and domain
Ohm's law and the ohm
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.
History: Georg Simon Ohm, 1827
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.
Model assumptions: linear range, constant temperature, direct current
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.
Derivation, conductance, and power from the same pair
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.
Equivalent resistance versus resistivity, series, and parallel
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.
Units, signs, and the limits of the school formula
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.
Sources
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.
How to use
- Enter current I in amperes, resistance R in ohms, or voltage U in volts — any two. Leave the third field blank.
- Convert prefixes first: 20 mA is 0.02 A, and 2.2 kΩ is 2200 in the resistance field.
- Click Calculate, or wait for the live result. For 0.5 A and 100 Ω the calculator shows 50 V, 25 W of power, and highlights voltage as the solved unknown.
- Read the panel: all three Ohm quantities, power P = U·I, and the U(I) chart at fixed R.
- Zero current blocks solving for R; zero resistance blocks solving for I. Need R = U/I only? Open the resistance calculator.
Symbol glossary
U is voltage, I is current, R is resistance, and P is power. For 0.5 A and 100 Ω the result is 50 V and 25 W.
- U
- Voltage [V], field prawo-ohma-c. U = I·R. At 0.5 A and 100 Ω it is 50 V.
- I
- Current [A], field prawo-ohma-a. I = U/R. Zero amperes blocks solving for R.
- R
- Resistance [Ω], field prawo-ohma-b. R = U/I. Zero ohms blocks solving for I.
- P
- Power [W] at the terminals, P = U·I. At 50 V and 0.5 A it is 25 W.
Worked examples
Example 2
A 12 V car battery feeds a 6 Ω load. The calculator finds the current; it does not model cranking.
I = 2 A · P = 24 W
Example 3
A 24 V system, for example on a truck: known voltage and current give an equivalent resistance.
R = 8 Ω · P = 72 W
Example 4
LED ballast resistor: 20 mA and 330 Ω give the drop across the resistor, not the diode alone.
U = 6.6 V · P = 0.132 W
Example 5
A 5 V USB port and a 10 Ω load: 0.5 A is a load model, not a port rating.
I = 0.5 A · P = 2.5 W
Example 6
Wall-socket order of magnitude: 10 A and 23 Ω give 230 V. This is not a home-wiring model.
U = 230 V · P = 2,300 W
Example 7
Heater element: 5 A through 44 Ω gives about 220 V in this numeric model.
U = 220 V · P = 1,100 W
Example 8
4–20 mA loop: 4 mA through 2.5 kΩ gives 10 V across the sense resistor.
U = 10 V · P = 0.04 W
Example 9
A 2 A load at 2.5 Ω: 5 V, as in the USB load example.
U = 5 V · P = 10 W
Example 10
Amplifier / sensor front end: 1 mA through 10 kΩ gives 10 V on a high-impedance node.
U = 10 V · P = 0.01 W
Common questions
What does this calculator compute, and from which formulas?
The calculator fills the missing quantity from Ohm’s law: U = I·R, I = U/R, and R = U/I. Enter any two of the three fields and leave the third blank. At 0.5 A and 100 Ω the voltage is 50 V, and the power P = U·I is 25 W. The panel shows all three quantities and the watts.
Which units should I enter for U, I, and R?
Voltage in volts, current in amperes, resistance in ohms. Enter 20 mA as 0.02 A and 2.2 kΩ as 2200 in the resistance field, because the field does not read the kilo prefix. A comma and a period are the same separator. The header unit switch does not rescale V, A, or Ω.
How do I leave a field blank for the third unknown?
Fill exactly two fields with numbers and leave the third empty. The calculator detects what is missing and applies the right rearrangement. If you fill all three or only one, the panel asks for exactly two values.
Why does zero current block resistance, and zero resistance block current?
For R = U/I, division by zero is undefined, so I = 0 cannot yield R. For I = U/R, zero resistance implies an unbounded current in the model, which the calculator rejects. That is a limit of the formula, not a mistaken entry.
Does the calculator cover AC, temperature, and non-ohmic parts?
No. U = I·R applies to a linear ohmic branch at steady temperature and direct current. A diode, a warming filament, and AC impedance lie outside this model. The calculator does not apply a temperature coefficient or reactance.
Where does the power P = U·I in the panel come from?
Once U, I, and R are known, the power at the terminals is the product of voltage and current. At 50 V and 0.5 A that is 25 W. It is the same product as on the dedicated power page, shown here beside the Ohm result.
How is this page different from the R = U/I resistance calculator?
That page has only voltage and current, and resistance is the only unknown. Here the form has three fields and any one may be blank. Same Georg Simon Ohm (1827), different field layout.
What about series, parallel, and wire resistance?
The calculator uses one resistance R between the terminals where you measure U and I. Series sums, parallel reciprocals, and R = ρ·l/S have their own pages. Here you either enter an equivalent R or compute it from the other two fields.
Are negative voltage or negative current allowed?
Yes, as numbers in the fields. The sign describes arrow directions on the schematic. Negative U and negative I can still give a positive resistance if the quotient is positive. The calculator does not verify your arrow convention.
What does the U versus I chart show?
At the fixed resistance from the result it draws the straight line U(I) = R·I. It illustrates the linearity of Ohm’s law, not an oscilloscope trace. The “Fill this example” button enters the example numbers and refreshes the panel and chart.
Are a comma and a period equivalent?
Yes. 0,5 A and 0.5 A are the same value. The fields start empty; placeholder text is not an entered number.
Does the 10 A and 23 Ω example describe home wiring?
It illustrates the 230 V order of magnitude from multiplying two numbers, not a breaker, conductor, or wiring-code model. The calculator does not check protection or wire size.
How do I calculate voltage with U = I·R?
Multiply current in amperes by resistance in ohms. For 0.5 A and 100 Ω: 0.5·100 = 50 V.
What are U and P at 0.5 A and 100 Ω?
U = 50 V and P = U·I = 25 W. The same numbers also satisfy P = I²·R = 25 W.
How do I find current when I know U and R?
Divide voltage by resistance: I = U/R. At 12 V and 24 Ω the result is 0.5 A.
How is this calculator different from R = U/I?
Here you can leave any one of U, I, or R blank. The dedicated resistance page only computes R = U/I with U and I filled.
Does the result include cos φ or AC impedance?
No. The relation is U = I·R for an ohmic load. Real power U·I·cos φ and Z are not fields on this page.
Sources
The definition, history, and derivation below come from standard physics and metrology sources. No invented DOIs.
- e-rara.ch — Ohm, Die galvanische Kette (1827)
- Wikipedia — Ohm's law
- BIPM — The International System of Units (SI), 9th edition
- NIST SP 330 — The International System of Units
- Halliday, D.; Resnick, R.; Walker, J., Fundamentals of Physics — electric current and resistance.
- Young, H. D.; Freedman, R. A., University Physics — current and resistance.
Page updated November 3, 2026.