Ohm’s Law Calculator
Use Ohm’s Law to calculate the voltage, current, resistance, or power in an electrical circuit. Enter any two values to find the other two.
For example, enter voltage and wattage to find the current and resistance.
Calculation Formulas:
 
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How to Use Ohm’s Law
Ohm’s Law defines the relationship between electric current, resistance, and voltage. More specifically, it states that the current through a circuit element is directly proportional to the potential difference applied to it and inversely proportional to the resistance.[1]
Ohm’s Law allows you to calculate voltage, current, power, and resistance of an electrical circuit if you know at least two of the other values.
For example, if you know the voltage and resistance, use the calculator above to find power and current using Ohm’s Law. Alternatively, you can use the calculator to solve power and resistance if you know the voltage and current.
Ohm’s Law Formula
The Ohm’s Law formula is I = V/R, where I is the current through a conductor measured in amperes, V is the potential difference across the conductor measured in volts, and R is the resistance of a conductor measured in ohms.[2]
I = V / R
The formula states that the current I is equal to the voltage V divided by the resistance R.
Components of Ohm’s Law
Quantity | Unit | Symbol |
---|---|---|
Voltage (V) | volt | V |
Current (I) | ampere | A |
Resistance (R) | ohm | Ω |
Power (P) | watt | W |
Ohm’s Law Triangle
The Ohm’s Law triangle illustrates how to calculate voltage, current, or resistance. To use it, cover the unit you want to calculate to reveal the formula to solve it.
![Ohm's Law triangle with the formulas to calculate voltage, current, and resistance](../wp-content/uploads/2022/09/ohms-law-triangle.png)
For instance, to solve for volts, cover the V with your thumb, which reveals that the voltage is equal to I × R.
Watt’s Law Power Formula
Watt’s Law defines the power formula, which states that electric power measured in watts is equal to the circuit’s current times the voltage.[3] This formula is very similar to Ohm’s Law and can help solve for power or wattage.
We often use the power formula in conjunction with Ohm’s Law to solve electrical properties when the power of the circuit is known.
P = I × V
Thus, the power formula states that power P is equal to the current I times the voltage V.[4]
Power Formula Triangle
Similar to the Ohm’s Law triangle, the power formula triangle illustrates the formula to find watts, volts, or amps. Just like before, cover the unit you want to solve to reveal the formula to solve it.
For instance, to solve for amps, cover the I with your thumb, which reveals that the current is equal to P ÷ V.
![Power formula triangle with the formulas to calculate power, voltage, and current](../wp-content/uploads/2022/09/power-formulas.png)
Our watts to amps calculator uses this formula to convert between power and current in electrical circuits, for example.
Ohm’s Law Wheel
We can use Ohm’s Law to calculate the volts, watts, amps, or ohms, given that at least two measurements are known. The formula allows us to derive the equations for calculating any measurement given two other known values.
The Ohm’s Law wheel shows all the formulas you can use to find volts, watts, amps, or ohms. Note that all of the formulas in the wheel assume a power factor of 1.[5]
![Ohm's Law wheel showing all the formulas to find the voltage in volts, power in watts, current in amps, and resistance in ohms](../wp-content/uploads/2022/09/ohms-law-wheel.png)
Ohm’s Law Wheel Formulas
The Ohm’s Law wheel illustrates the following formulas:
Voltage Formulas
- Voltage = Current × Resistance
- Voltage = Power ÷ Current
- Voltage = Power × Resistance
Power Formulas
- Power = Voltage × Current
- Power = Voltage2 ÷ Resistance
- Power = Current2 × Resistance
Current Formulas
- Current = Voltage ÷ Resistance
- Current = Power ÷ Voltage
- Current = Power ÷ Resistance
Resistance Formulas
- Resistance = Voltage ÷ Current
- Resistance = Voltage2 ÷ Power
- Resistance = Power ÷ Current2
We use Ohm’s Law for many things, such as determining the maximum microwave size or the maximum number of light fixtures a circuit can safely handle without creating a fire hazard.
Formula Examples
We can use the following examples to learn how certain electrical circuit parameters can be calculated:
Example #1 – Let’s calculate the voltage when a current of 10 amps flows through a resistance of 2 ohms.
Voltage = Current × Resistance
Voltage = 10 A x 2 Ω
Voltage = 20 V
Example #2 – Let’s find the power consumed by a device supplied with 5 volts and a current of 2 amps.
Power = Voltage × Current
Power = 5 V × 2 A
Power = 10 W
Example #3 – Let’s find the resistance of a circuit if it consumes 20 watts of power when supplied with 5 volts.
Resistance = Voltage ÷ Current
Resistance = 52 V ÷ 20 W
Resistance = 25 V ÷ 20 W
Resistance = 1.25 Ω
Example #4 – Let’s find the current flowing through a circuit if it consumes 40 watts with a 10 resistance of Ω.
Current = Power ÷ Resistance
Current = 40 W ÷ 10 Ω
Current = 4
Current = 2 A
Our lighting cost calculator can help find lighting energy usage, and our electricity cost calculator can help find the costs of powering electric devices.
Use Ohm’s Law to help size an electrical circuit or find out how large of a heater can be safely used on a regular outlet. You might also find our voltage drop calculator to determine voltage drop, the minimum wire size needed, and the maximum wire length for your next electrical project.
Frequently Asked Questions
What do the letters in the Ohm's Law formula mean?
In the Ohm’s Law formula, V represents voltage, I represents intensity, or current, and R represents resistance.
Georg Simon Ohm created Ohm’s Law in a paper published in 1827,[6] well before voltage, current, and resistance units were defined.
It wasn’t until 1881 that the Volt, Ampere, and Ohm were defined, over 50 years after Ohm’s Law was published. This explains why the letters are not related to modern-day units used in the formula.
What are the Ohm's Law formulas for three-phase circuits?
The Ohm’s Law formulas for 3-phase AC power are a little different since the phase constant (1.732) and power factor need to be considered. The Ohm’s Law formula for three-phase circuits is:
current = power ÷ (power factor × voltage × 1.732)
Does Ohm's Law apply to AC or DC circuits?
The Ohm’s Law formulas do apply to both AC and DC circuits. For AC circuits, the impedance needs to be considered, thus the Ohm’s Law formula for AC circuits becomes:
Z = V/I
Thus, the impedance Z is equal to the voltage V divided by the current I.
How is Ohm's Law used in real life?
The Ohm’s Law formulas form the foundation for nearly all electrical calculations that are used to design circuits for everything from electrical power plants to the circuits used in electronics and computers.
References
- Brandon Mitchell, Robert Ekey, Roy McCullough, and William Reitz, A Fan-tastic Quantitative Exploration of Ohm’s Law, https://aapt.scitation.org/doi/full/10.1119/1.5021431
- National Institute of Standards and Technology, Ampere: The Present, https://www.nist.gov/si-redefinition/ampere-present
- Honore, P., Basic Electronic Circuits Part-1, Analog Circuits, https://www.google.com/books/edition/Basic_Electronic_Circuits_Part_1_Analog/-J6rAgAAQBAJ
- Mike Holt, Electrician’s Math and Basic Electrical Formulas, https://www.mikeholt.com/instructor2/img/product/pdf/1302643872-sample.pdf
- Miller, C., NFPA's Electrical References, National Fire Protection Association, 2004, Jones & Bartlett Learning, 54. https://www.google.com/books/edition/NFPA_s_Electrical_References/raUyIi7i-asC
- Encyclopeadia Britannica, Ohm's Law, https://www.britannica.com/science/Ohms-law