Ohm’s law calculator: voltage, current, resistance and power
Solve Ohm’s law and electrical power relationships from any two known quantities. The calculator supports V, A, Ω and W with common SI prefixes, returns the two missing values and shows the equations used.
Enter exactly two known quantities among voltage, current, resistance and power. Leave the other two fields empty and they will be calculated automatically.
Model used: an ohmic resistive component. Values are converted to coherent SI units before calculation and displayed with a readable prefix.
Results
Calculation details
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How does this tool work?
This Ohm’s law calculator connects four common electrical quantities: voltage V, current I, resistance R and power P. Enter any two known values and it calculates the other two while showing the equations used. Common prefixes such as millivolts, milliamps, kilo-ohms, milliwatts and kilowatts are handled directly, avoiding the factor-of-1,000 mistakes that often occur when units are converted by hand.
Ohm’s law is written V = R × I. For an ohmic conductor under comparable conditions, the voltage across it is proportional to the current through it, with resistance as the proportionality constant. The same equation can be rearranged as I = V/R or R = V/I. With coherent SI units, one volt divided by one ampere equals one ohm.
Electrical power adds a second relationship: P = V × I. Combining it with Ohm’s law gives P = I²R and P = V²/R. These equivalent forms allow the calculator to work even when power is one of the two known values, for example when a 100 Ω resistor is known to dissipate 1 W.
Every entered value is converted to the corresponding SI base unit before calculation: volts, amperes, ohms and watts. Therefore 12 V and 500 mA are treated as 12 V and 0.5 A, producing R = 24 Ω and P = 6 W. Results are then displayed with a practical SI prefix when that improves readability without changing the physical value.
Ohm’s law is not a universal description of every electrical component. An ideal resistor, or a conductor whose resistance stays effectively constant, is well represented by V = RI. A diode, LED, transistor or filament lamp whose temperature changes can have a nonlinear current-voltage characteristic. For such devices, this calculator only represents an operating point if using an equivalent resistance is actually meaningful.
How to use this tool
Choose two known quantities
Fill exactly two fields among V, I, R and P. Keep the other two empty so the problem has one unambiguous solution.
Select the units
Use volts or millivolts, amperes or milliamps, ohms or kilo-ohms, watts or milliwatts to match the values in your exercise or component data.
Read all four quantities
The two values you entered and the two calculated values are summarized with convenient SI prefixes.
Check the working
The equations and numerical substitutions appear below the results so you can verify or reproduce the method.
Ohm’s law and electrical power formulas
For an ohmic component, voltage, current and resistance are related by Ohm’s law. Electrical power also connects voltage and current.
Ohm’s law
V = R × I
Voltage V equals resistance R multiplied by current I.
Current
I = V ÷ R
Solve for I when voltage and resistance are known.
Resistance
R = V ÷ I
Solve for R when voltage and current are known.
Electrical power
P = V × I
Electrical power received or dissipated is voltage multiplied by current.
Power from I and R
P = I² × R
This form follows from P = VI and V = RI.
Power from V and R
P = V² ÷ R
This form follows from P = VI and I = V/R.
- V
- voltage, in volts (V)
- I
- electric current, in amperes (A)
- R
- electrical resistance, in ohms (Ω)
- P
- electrical power, in watts (W)
The calculator assumes ohmic behavior: R is treated as constant for the conditions being studied. A diode, a heating filament lamp and many electronic components can have a nonlinear voltage-current relationship.
Use cases
Physics and electricity exercises
Check an Ohm’s law calculation and follow the transition from symbolic formula to numerical result.
Choose a resistor or check a simple circuit
Estimate current, voltage drop or dissipated power in a resistive circuit before comparing the result with real component specifications.
Check power dissipation
Estimate whether the electrical power in a resistor is approaching or exceeding the component’s intended power rating.
Work across SI prefixes
Move between mA, A, mV, V, Ω, kΩ, mW and W without manual scaling.
Examples
Resistor connected to 12 V
I = 12/4 = 3 A, then P = 12 × 3 = 36 W.
Input
V = 12 V; R = 4 ΩResult
I = 3 A; P = 36 WSmall electronic circuit
20 mA = 0.020 A. R = 5/0.020 = 250 Ω and P = 5 × 0.020 = 0.1 W.
Input
V = 5 V; I = 20 mAResult
R = 250 Ω; P = 100 mWResistor specified by power
I = √(P/R) = √(1/100) = 0.1 A and V = √(PR) = √100 = 10 V.
Input
R = 100 Ω; P = 1 WResult
I = 100 mA; V = 10 V60 W load on 12 V
I = P/V = 60/12 = 5 A; the equivalent resistance at that operating point is V²/P = 2.4 Ω.
Input
V = 12 V; P = 60 WResult
I = 5 A; R = 2.4 ΩTips and best practices
- Do not confuse milliamps with amps: 20 mA is 0.020 A. The unit selector performs this conversion automatically.
- A resistor’s power rating should include suitable margin above the expected dissipation; the electrical calculation does not replace the component datasheet.
- For a network of several resistors, first find the equivalent resistance if you want to apply this simple calculation to the whole network.
- If three measured quantities are already known and do not agree exactly, do not force them into an ideal model: tolerances, heating and measurement uncertainty can explain differences.
- In AC circuits containing capacitors or inductors, resistance alone may not be sufficient; impedance and phase can become relevant.
Sources and references
- OpenStax, 9.4 Ohm's Law (opens in a new tab), University Physics Volume 2Page in English
- OpenStax, 19.4 Electric Power (opens in a new tab), PhysicsPage in English
- BIPM, Electrical units (opens in a new tab), SI Brochure — Annex 1Page in English
Frequently asked questions
What is the formula for Ohm’s law?
The main form is V = R × I. Rearrange it to I = V/R for current or R = V/I for resistance.
How do I calculate electrical power with Ohm’s law?
Electrical power is P = V × I. Substituting V = RI gives P = I²R, while substituting I = V/R gives P = V²/R.
Why must I enter exactly two values?
Two independent quantities are enough to determine the remaining two in this model. One value leaves infinitely many possibilities, while three entered values may conflict because of an error or real-world measurement variation.
Can I use milliamps and kilo-ohms?
Yes. Supported units include µA, mA, A, mV, V, kV, mΩ, Ω, kΩ, MΩ, µW, mW, W and kW. Values are converted to coherent SI units before the equations are solved.
Does Ohm’s law work for an LED or diode?
Not as a single constant resistance over their full range. Diodes have nonlinear current-voltage characteristics, so their characteristic curve or a suitable operating-point model should be used.
What does a resistance of 1 Ω mean?
In the ohmic model, a 1 Ω component carrying 1 A has 1 V across it. The relation V/A corresponds to the ohm.
Can these results be used to design a real circuit?
They provide a foundation for simple resistive calculations. Real design must also consider tolerances, power ratings, temperature, supply behavior, wiring, protection and component specifications.
Are my values sent to Bethemesh?
No. The calculation runs locally in your browser and does not require the entered values to be sent to a server.
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