Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two values and get instant results for electrical circuits.
What Is Ohm's Law?
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. Expressed as the formula V = I × R, it is the most fundamental relationship in electrical engineering and the basis for nearly every electrical calculation.
Named after German physicist Georg Simon Ohm, who published the relationship in 1827, Ohm's Law connects three core electrical quantities: voltage (V) measured in volts, current (I) measured in amperes, and resistance (R) measured in ohms (Ω). When combined with the power equation P = V × I, these four values form the complete Ohm's Law wheel used by electricians worldwide.
Ohm's Law Formula Chart: All 12 Equations
The Ohm's Law wheel contains 12 formulas that let you calculate any electrical value when you know two others. This table is the complete reference for voltage, current, resistance, and power calculations.
| To Find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage (V) | V = I × R | V = P ÷ I | V = √(P × R) |
| Current (I) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance (R) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power (P) | P = V × I | P = I² × R | P = V² ÷ R |
Tip: Memorize V = I × R and P = V × I. Every other formula is derived from these two equations. Enter any two known values into the calculator above to solve for the rest instantly.
How to Calculate Voltage, Current, Resistance, and Power: Step-by-Step
Ohm's Law lets you find any electrical value when you know two others. Whether you are troubleshooting a circuit or sizing components, these steps will guide you through the process.
Step 1: Identify Two Known Values
Start by determining which two electrical values you already know. These could be voltage (V), current (I), resistance (R), or power (P). You might read them from a multimeter, a device nameplate, or circuit specifications.
Step 2: Select the Correct Formula
Pick the formula that uses your two known values to find the unknown. For example, if you know voltage and resistance, use I = V / R to find current. If you know current and resistance, use V = I x R to find voltage.
Step 3: Plug in Your Values and Solve
Substitute your known values into the formula and calculate. Make sure your units are consistent: volts for voltage, amps for current, ohms for resistance, and watts for power.
Step 4: Verify Your Result
Cross-check by plugging all values back into an alternate formula. If the numbers agree, your calculation is correct. For instance, confirm that P = V x I gives the same power as P = I² x R.
Formula
V = I x R | P = V x I | P = I² x R | P = V² / R
Where: V = Voltage (Volts), I = Current (Amps), R = Resistance (Ohms), P = Power (Watts)
Worked Example
Scenario: A 12V battery is connected to a 6-ohm resistor. Find the current and power.
- Step 1: Known values: V = 12V, R = 6 ohms
- Step 2: Use I = V / R for current, then P = V x I for power
- Step 3: I = 12 / 6 = 2A
- Step 4: P = 12 x 2 = 24W. Verify: P = 2² x 6 = 24W (matches)
Result: The circuit draws 2 amps and consumes 24 watts.
How This Calculator Works
This calculator uses Ohm's Law and power formulas to find any missing electrical value when you know at least two others. Just enter the values you know, and it automatically calculates what you're missing.
For example, if you know a circuit has 12 volts and draws 2 amps, the calculator will instantly tell you the resistance is 6 ohms and the power consumption is 24 watts. It's the same math you'd do by hand, just faster and without the risk of calculation errors.
The tool handles all the unit conversions too. Whether you're working with milliamps or kilovolts, just enter your numbers and the calculator does the rest.
Ohm's Law Formulas
Basic Ohm's Law
- V = I × R (Voltage = Current × Resistance)
- I = V ÷ R (Current = Voltage ÷ Resistance)
- R = V ÷ I (Resistance = Voltage ÷ Current)
Power Formulas
- P = V × I (Power = Voltage × Current)
- P = I² × R (Power = Current² × Resistance)
- P = V² ÷ R (Power = Voltage² ÷ Resistance)
Where: V = Voltage (Volts), I = Current (Amps), R = Resistance (Ohms), P = Power (Watts)
Frequently Asked Questions
What is Ohm's Law and why is it important?
Ohm's Law is the foundation of electrical work. It shows the relationship between voltage, current, and resistance in any circuit. Every electrician uses it daily to size wires, calculate loads, and troubleshoot problems.
Can I use this for AC circuits or just DC?
Ohm's Law works for both AC and DC circuits, but for AC you need to use RMS values (the standard voltage and current readings on most meters). For complex AC analysis with reactive components, you'd need more advanced calculations.
Why do I only need to enter two values?
Because the four electrical quantities (voltage, current, resistance, power) are all mathematically related. Once you know any two, you can calculate the other two using Ohm's Law and power formulas.
What if my calculated current seems too high?
High current usually means the resistance is too low for the voltage applied. This could indicate a short circuit or undersized wire. Always double-check your measurements and make sure the resistance value is correct.
How accurate are these calculations for real-world applications?
Very accurate for resistive loads like heaters and incandescent lights. Motors, transformers, and electronic devices are more complex, but Ohm's Law still gives you good ballpark numbers for troubleshooting and basic calculations.
Can I use this to calculate wire size?
This calculator shows you the current in a circuit, which is the starting point for wire sizing. But you'll also need to consider temperature, conduit fill, and voltage drop - our wire sizing calculator handles all those factors.
What's the difference between watts and volt-amps?
For resistive loads, they're the same. But for reactive loads like motors, volt-amps (apparent power) is higher than watts (real power) due to power factor. This calculator shows watts, which is what you pay for on your electric bill.
Why does my multimeter reading not match the calculator?
Could be several reasons: the load isn't purely resistive, there's voltage drop in the wiring, or the resistance changes with temperature. LED lights and electronic devices especially don't follow simple Ohm's Law behavior.
Is this calculator good for automotive electrical work?
Absolutely. Most automotive electrical systems are 12V DC, which is perfect for Ohm's Law calculations. It's great for figuring out if a component is drawing too much current or if there's a resistance problem in a circuit.
How do I know if my power calculation is safe?
Compare the calculated power to the component's power rating. If your calculation shows 100 watts but the resistor is only rated for 25 watts, it'll overheat and fail. Always use components rated well above your calculated power.
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