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Use this free ohms law calculator to find voltage, current, resistance, or power. Enter any two known values and the calculator solves for the rest using V=IR and power dissipated formulas.
Ohm's Law is the most fundamental rule in electrical engineering. Discovered by German physicist Georg Simon Ohm in 1827, it describes the relationship between voltage, current, and resistance in an electrical circuit. The law is expressed as a simple formula:
V = I × R
Where V is voltage (in volts), I is current (in amps), and R is resistance (in ohms). This equation tells us that voltage equals current multiplied by resistance.
The easiest way to understand Ohm's Law is to think of electricity like water flowing through a pipe:
If you increase the water pressure (voltage) while keeping the pipe the same size (resistance), more water flows (current increases). If you make the pipe narrower (increase resistance) while keeping the pressure the same, less water flows (current decreases). That is Ohm's Law in a nutshell.
Click one of the four buttons — Voltage, Current, Resistance, or Power — to choose the value you want to find. The selected field becomes the output.
Type in two of the remaining three fields. For example, enter voltage and resistance to find current. The calculator works with any combination of two inputs.
Always use base units: volts, amps, ohms, and watts. Convert milliamps to amps (divide by 1,000) or kilohms to ohms (multiply by 1,000) before entering values.
The calculator updates in real time as you type. The result appears in the highlighted output field. Use the Clear All button to start a new calculation.
The core Ohm's Law equation (V = I × R) can be rearranged to solve for any unknown. Combined with the power equation (P = V × I), you get 12 useful formulas. A helpful trick is the Ohm's Law triangle: write V at the top, I on the bottom left, and R on the bottom right. Cover the letter you want to find, and the remaining two show you the formula.
V = I × R
V = P ÷ I
V = √(P × R)
I = V ÷ R
I = P ÷ V
I = √(P ÷ R)
R = V ÷ I
R = V² ÷ P
R = P ÷ I²
P = V × I
P = I² × R
P = V² ÷ R
A 12V battery is connected to a 240-ohm resistor. What is the current?
Solution: I = V ÷ R = 12 ÷ 240 = 0.05 A (50 mA)
A 120V household outlet drives 0.5 A through a light bulb. What is the bulb's resistance?
Solution: R = V ÷ I = 120 ÷ 0.5 = 240 ohms
A heating element with 20 ohms of resistance draws 6 amps. What voltage is applied?
Solution: V = I × R = 6 × 20 = 120 volts
When current flows through a resistor, electrical energy is converted into heat. This is called power dissipated, measured in watts (W). Every resistor, wire, and component in a circuit dissipates some power as heat. Understanding power dissipation is critical for choosing the right components and preventing overheating.
By combining the power equation (P = V × I) with Ohm's Law (V = I × R), we get three equivalent ways to calculate power dissipated:
P = I × V
Use when you know current and voltage
P = I² × R
Use when you know current and resistance
P = V² ÷ R
Use when you know voltage and resistance
Every resistor has a maximum power rating (typically 1/4 W, 1/2 W, 1 W, 2 W, 5 W, or 10 W). If the power dissipated exceeds the resistor's rating, it will overheat, change value, smoke, or even catch fire. This is why engineers always calculate power dissipation when designing circuits.
Heat generation in resistors also affects nearby components. In tightly packed circuit boards, excessive heat from one resistor can damage solder joints, degrade capacitors, and reduce the lifespan of semiconductors. Good thermal management starts with accurate power dissipation calculations.
Problem: A 470-ohm resistor is connected to a 9V battery. How much power is dissipated? What power rating should the resistor have?
Step 1: Use P = V² ÷ R = 9² ÷ 470 = 81 ÷ 470 = 0.172 watts (172 milliwatts)
Step 2: The resistor dissipates 0.172 W. A standard 1/4 watt (0.25 W) resistor provides enough headroom and is the correct choice. A 1/8 watt resistor would be too small and could overheat.
| Voltage (V) | Resistance (Ω) | Current (A) | Power Dissipated (W) | Min. Resistor Rating |
|---|---|---|---|---|
| 3.3 V | 100 Ω | 0.033 A | 0.109 W | 1/4 W |
| 5 V | 220 Ω | 0.023 A | 0.114 W | 1/4 W |
| 9 V | 470 Ω | 0.019 A | 0.172 W | 1/4 W |
| 12 V | 100 Ω | 0.12 A | 1.44 W | 2 W |
| 24 V | 47 Ω | 0.51 A | 12.26 W | 15 W or heatsink |
| 120 V | 1,440 Ω | 0.083 A | 10 W | 10 W wirewound |
This table shows common voltage, current, and resistance combinations you will encounter in everyday electronics and household wiring. Use it as a quick reference for common ohms law calculations.
| Voltage (V) | Current (A) | Resistance (Ω) | Power (W) | Typical Use Case |
|---|---|---|---|---|
| 3.3 V | 0.02 A | 165 Ω | 0.066 W | LED circuit (microcontroller) |
| 5 V | 0.5 A | 10 Ω | 2.5 W | USB device charging |
| 9 V | 0.1 A | 90 Ω | 0.9 W | Battery-powered sensor |
| 12 V | 1.0 A | 12 Ω | 12 W | Automotive accessory |
| 24 V | 2.0 A | 12 Ω | 48 W | Industrial control relay |
| 120 V | 1.0 A | 120 Ω | 120 W | Household light bulb (US) |
| 120 V | 10.0 A | 12 Ω | 1,200 W | Space heater (US) |
| 120 V | 15.0 A | 8 Ω | 1,800 W | Hair dryer (US 15A circuit) |
| 230 V | 5.0 A | 46 Ω | 1,150 W | Electric kettle (EU/UK) |
| 240 V | 20.0 A | 12 Ω | 4,800 W | Electric oven element (US) |
| 480 V | 10.0 A | 48 Ω | 4,800 W | Industrial 3-phase motor |
Ohm's Law applies to both series and parallel circuits, but the way you calculate total resistance differs. Understanding this difference is key to analyzing real-world circuits.
In a series circuit, components are connected end-to-end. The same current flows through every component.
Example: Three 100Ω resistors in series give R_total = 300Ω. With 12V applied: I = 12/300 = 0.04A, and each resistor drops 4V.
In a parallel circuit, components are connected across the same two points. Each component sees the full supply voltage.
Example: Three 100Ω resistors in parallel give R_total = 33.3Ω. With 12V applied: I_total = 12/33.3 = 0.36A, each branch carries 0.12A.
Every wire has resistance. Using Ohm's Law, you can calculate voltage drop across long wire runs. If a 100-foot 14 AWG wire carries 15A, the voltage drop is V = I × R = 15 × 0.253 = 3.8V. If this exceeds 3% of the supply voltage, you need thicker wire.
To power an LED, you need a current-limiting resistor. The formula is R = (V_supply - V_LED) / I_LED. For a red LED (2V, 20mA) on a 5V supply: R = (5 - 2) / 0.02 = 150Ω. Power dissipated in the resistor: P = 0.02 × 3 = 0.06W.
Fuses protect circuits from overcurrent. Use I = P / V to find the expected current draw of your device. A 1,500W space heater on a 120V circuit draws 12.5A, so a 15A fuse provides protection with some headroom. A 10A fuse would blow during normal operation.
If you know a battery's capacity (e.g., 2,000 mAh) and your circuit's current draw, you can estimate run time. A circuit drawing 250 mA from a 2,000 mAh battery lasts about 2,000 / 250 = 8 hours. Use Ohm's Law to find the current draw from voltage and resistance.
The most common mistake. If a datasheet says 20 mA, you must use 0.02 A in the formula. Using 20 instead of 0.02 gives a result that is off by a factor of 1,000. Always convert: divide mA by 1,000 to get amps.
Calculating resistance correctly but ignoring power dissipated is dangerous. A 100-ohm resistor at 12V dissipates 1.44W. A standard 1/4W resistor will overheat and potentially catch fire. Always verify that your resistor's power rating exceeds the calculated power dissipated.
A 4.7kΩ resistor is 4,700 ohms, not 4.7 ohms. Using the wrong unit changes the result by a factor of 1,000. Always convert: 1 kΩ = 1,000 Ω, 1 MΩ = 1,000,000 Ω.
LEDs, diodes, and transistors are non-ohmic — their resistance changes with voltage and current. You cannot simply divide voltage by current to get a constant resistance. These devices require specialized analysis using their datasheets and characteristic curves.
Over long distances, wire resistance adds up. A 200-foot run of 14 AWG copper wire has about 0.506 ohms of resistance. At 15A, that creates a 7.6V drop — enough to cause equipment malfunction. Always account for wire resistance in long cable runs.
In AC circuits, use RMS (root mean square) voltage, not peak voltage. US household voltage is 120V RMS, but the peak is about 170V. Using the wrong value gives incorrect current and power calculations. Multimeters display RMS by default.
Ohm's Law states that the current flowing through a conductor is equal to the voltage across it divided by the resistance. The formula is V = I x R, where V is voltage in volts, I is current in amps, and R is resistance in ohms. Think of it like water flowing through a pipe: voltage is the water pressure, current is how much water flows, and resistance is how narrow the pipe is.
To calculate voltage, multiply the current in amps by the resistance in ohms. The formula is V = I x R. For example, if 2 amps flow through a 10-ohm resistor, the voltage across it is 2 x 10 = 20 volts.
To calculate current, divide the voltage by the resistance. The formula is I = V / R. For example, if 12 volts are applied across a 4-ohm resistor, the current is 12 / 4 = 3 amps.
To calculate resistance, divide the voltage by the current. The formula is R = V / I. For example, if a 9-volt battery drives 0.5 amps through a circuit, the resistance is 9 / 0.5 = 18 ohms.
Power dissipated is the amount of electrical energy converted to heat in a resistor. It is calculated using P = I x V, P = I squared x R, or P = V squared / R. A resistor must have a power rating equal to or greater than the power dissipated to avoid overheating and failure.
If resistance is zero and voltage is applied, the formula predicts infinite current, which represents a short circuit. In practice, every conductor has some resistance, but very low resistance paths can cause dangerously high currents and are a major safety concern.
Ohm's Law applies to AC circuits when you replace resistance with impedance (Z), which accounts for the effects of capacitors and inductors. The modified formula is V = I x Z. For purely resistive AC circuits, the standard V = I x R formula works the same as in DC circuits.
In a series circuit, total resistance is the sum of all individual resistances (R_total = R1 + R2 + R3). In a parallel circuit, the reciprocal of total resistance equals the sum of reciprocals of each resistance (1/R_total = 1/R1 + 1/R2 + 1/R3). Parallel resistance is always less than the smallest individual resistor.
Ohm's Law applies perfectly to ohmic materials like most metals at constant temperature. Non-ohmic materials such as diodes, transistors, and thermistors do not follow a linear voltage-current relationship and require more advanced analysis.
Ohm's Law helps determine safe operating limits for circuits. It allows engineers to calculate maximum current flow, select appropriate wire gauges, size circuit breakers correctly, and prevent overheating or fire hazards in electrical installations.
Divide milliamps by 1,000 to get amps. For example, 250 mA equals 0.25 A. You must use amps (not milliamps) in Ohm's Law formulas to get correct results in volts, ohms, and watts. Similarly, convert kilohms to ohms by multiplying by 1,000.
Use R = (V_supply - V_LED) / I_LED. For example, to run a red LED (2V forward voltage, 20mA current) from a 5V supply: R = (5 - 2) / 0.02 = 150 ohms. Then calculate power dissipated: P = 0.02 x 3 = 0.06W, so a standard 1/4 watt resistor works.
This calculator is provided for educational and reference purposes only. Always verify critical electrical calculations with a qualified electrician or engineer. UnitTables is not responsible for errors or any consequences arising from the use of this tool.