Ohm's Law Calculator
Enter any two known values — voltage (V), current (I), resistance (R), or power (P) — and calculate all four instantly. Leave unknown fields at 0.
Enter any 2 known values. Set unknown fields to 0.
Voltage
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VCurrent
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A
Resistance
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Ω
Power
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W
Current (mA)
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Power (mW)
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Circuit Diagram
Ohm's Law Formulas
Find Voltage
V = I × R
V = P ÷ I
Find Current
I = V ÷ R
I = P ÷ V
Find Resistance
R = V ÷ I
R = V² ÷ P
Find Power
P = V × I
P = I² × R
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How to use this calculator
Four input fields. Enter exactly 2 values. Leave the others at 0.
Voltage (V) in volts. The electrical potential difference across the component or circuit.
Current (I) in amperes (A). The flow of charge through the circuit.
Resistance (R) in ohms (Ω). The opposition to current flow.
Power (P) in watts (W). The rate of energy dissipation or consumption.
Enter any 2 known values, leave the other 2 at 0, and click Calculate. The blue results panel shows all 4 values: voltage, current, resistance, and power, plus current in milliamps (mA) and power in milliwatts (mW) for low-power circuit work.
The Circuit Diagram below the results panel shows a simple schematic visualising the relationship between your values.
Example: 12V supply, 0.5A current
Voltage: 12 / Current: 0.5 / Resistance: 0 / Power: 0
Results:
- Voltage: 12 V
- Current: 0.5 A (500 mA)
- Resistance: 24 Ω (V / I = 12 / 0.5)
- Power: 6 W (V × I = 12 × 0.5)
A 12V circuit drawing 0.5A has a 24Ω load and dissipates 6 watts as heat or useful work.
Enter exactly 2 values and set the others to 0. If you enter 3 or 4 values, the calculator may pick a pair and ignore the rest, or flag an inconsistency if your values don’t satisfy Ohm’s Law. V = I × R must always be true for a valid circuit.
What each quantity actually means
Voltage (V) is electrical pressure. Think of water flowing through a pipe: voltage is the pressure pushing the water. Higher voltage pushes more current through the same resistance. Measured in volts (V). Named after Alessandro Volta.
Current (I) is the flow of electrons. Same water analogy: current is the flow rate. More pressure (voltage) or less resistance means more flow (current). Measured in amperes (A), named after André-Marie Ampère. The symbol is I from the French “intensité de courant.”
Resistance (R) is opposition to current flow. A narrow pipe resists water flow; a resistor resists current. Measured in ohms (Ω), named after Georg Simon Ohm. Every material has some resistance; conductors have very low resistance, insulators have very high resistance.
Power (P) is the rate at which energy is converted. In a resistor, electrical energy becomes heat. In an LED, it becomes light. In a motor, it becomes mechanical work. Measured in watts (W). Power is the product of voltage and current: push harder (more voltage) or flow more (more current) and you convert energy faster.
Voltage is the cause, current is the effect, resistance is what stands between them. Power is what gets done as a result.
The formulas: all 12 combinations
Ohm’s Law gives you 3 base relationships. Add power and you get 12 formulas covering every combination of 2 known quantities.
From Voltage and Current:
From Voltage and Resistance:
From Voltage and Power:
From Current and Resistance:
From Current and Power:
From Resistance and Power:
The calculator picks the right pair of formulas based on which 2 fields you fill in. You never need to choose the formula manually.
P = I² × R is why thin wires overheat under high current. Double the current through the same resistance and power dissipation quadruples (2² = 4). This is the physics behind wire gauge ratings, fuse sizing, and why extension cords have maximum current ratings.
The Ohm’s Law wheel: all relationships at a glance
A quick visual reference for which formula to use depending on what you know and what you need.
Find your row (which 2 values you know), then read across to the column for what you need to find. The dashes mean you already know that value.
Common voltage and current combinations: reference table
| Voltage (V) | Current (A) | Resistance (Ω) | Power (W) | Context |
|---|---|---|---|---|
| 1.5 | 0.1 | 15 | 0.15 | AA battery, small LED |
| 3.3 | 0.5 | 6.6 | 1.65 | Microcontroller (3.3V logic) |
| 5 | 0.5 | 10 | 2.5 | USB device (500mA) |
| 5 | 1.0 | 5 | 5.0 | USB fast charge (5W) |
| 5 | 2.0 | 2.5 | 10 | USB-C charging (10W) |
| 9 | 0.5 | 18 | 4.5 | 9V battery circuit |
| 12 | 0.5 | 24 | 6 | 12V DC, 0.5A load |
| 12 | 1.0 | 12 | 12 | 12V car circuit, 1A draw |
| 12 | 5.0 | 2.4 | 60 | 12V LED strip (60W/5m) |
| 24 | 2.0 | 12 | 48 | 24V industrial supply |
| 48 | 1.0 | 48 | 48 | 48V PoE (Power over Ethernet) |
| 120 | 10 | 12 | 1,200 | US mains, 10A circuit |
| 230 | 10 | 23 | 2,300 | European mains, 10A circuit |
| 240 | 13 | 18.5 | 3,120 | UK mains, 13A socket |
Resistor values: common combinations
| Resistance (Ω) | Voltage (V) | Current (mA) | Power (mW) | Use case |
|---|---|---|---|---|
| 220 | 5 | 22.7 | 113.6 | LED current limiting (5V) |
| 330 | 5 | 15.2 | 75.8 | LED current limiting (low brightness) |
| 470 | 5 | 10.6 | 53.2 | LED, microcontroller GPIO |
| 1,000 | 5 | 5.0 | 25 | Pull-up/pull-down resistor |
| 1,000 | 12 | 12 | 144 | Signal divider, low power |
| 4,700 | 5 | 1.06 | 5.32 | I²C pull-up |
| 10,000 | 5 | 0.5 | 2.5 | Standard pull-up resistor |
| 10,000 | 12 | 1.2 | 14.4 | High-impedance signal |
| 100,000 | 5 | 0.05 | 0.25 | Very high impedance |
Real-world examples
LED current limiting resistor
You’re running a red LED from a 5V supply. The LED has a forward voltage of 2.0V and wants 20mA (0.02A) of current. What resistor do you need?
LED resistor calculation
Voltage across resistor = supply voltage - LED forward voltage = 5 - 2.0 = 3.0V
Enter in calculator: V = 3.0, I = 0.02 (20mA), R = 0, P = 0
Result: R = V / I = 3.0 / 0.02 = 150 Ω
Power dissipated in resistor: P = V × I = 3.0 × 0.02 = 0.06W = 60mW
Use a standard 150Ω resistor (or nearest: 150Ω is a standard E24 value). A 1/4W (250mW) resistor handles it easily.
Checking wire gauge for a circuit
A 12V circuit carries 8A to a motor. You want to check whether 1.5mm² wire (resistance ≈ 12.1 mΩ/m) over a 3m run is adequate.
Wire resistance and voltage drop
Wire resistance = 12.1 mΩ/m × 3m × 2 (there and back) = 72.6 mΩ = 0.0726 Ω
Enter: I = 8, R = 0.0726, V = 0, P = 0
Voltage drop = I × R = 8 × 0.0726 = 0.581V
Power dissipated in wire = I² × R = 64 × 0.0726 = 4.65W
A 0.58V drop on a 12V circuit is 4.8% loss, acceptable. The wire dissipates 4.65W, well within the 15A continuous rating of 1.5mm² wire. Fine to proceed.
Finding current from power rating
A 60W incandescent bulb on a 120V US circuit. How much current does it draw?
Bulb current from power and voltage
Enter: V = 120, P = 60, I = 0, R = 0
I = P / V = 60 / 120 = 0.5A R = V² / P = 14,400 / 60 = 240 Ω (hot filament resistance)
The bulb draws 0.5A. A 15A household circuit handles 30 such bulbs before tripping the breaker (15 / 0.5 = 30).
Battery life estimation
A 9V battery (capacity: ~500mAh) powers a circuit drawing 25mA. How long will it last? What’s the resistance of the circuit?
Battery life and circuit resistance
Enter: V = 9, I = 0.025 (25mA), R = 0, P = 0
R = V / I = 9 / 0.025 = 360 Ω P = V × I = 9 × 0.025 = 0.225W = 225mW
Battery life = capacity / current = 500mAh / 25mA = 20 hours
In practice, battery life will be shorter because terminal voltage drops as the battery discharges, which reduces current and extends life somewhat, but also because the circuit may cut out before the battery is fully depleted.
Fuse sizing for a power supply
A 24V power supply powers equipment drawing a maximum of 3A. What fuse should you use?
Fuse sizing
Enter: V = 24, I = 3, R = 0, P = 0
P = V × I = 24 × 3 = 72W R = V / I = 24 / 3 = 8 Ω (effective load resistance at full draw)
Fuse rule: size at 125% of maximum continuous current = 3 × 1.25 = 3.75A
Use a 4A fuse (nearest standard value above 3.75A). A 3A fuse would blow at or near the rated load. A 5A or 10A fuse provides insufficient protection.
Common mistakes people make
Entering 3 or 4 values that contradict each other. V = I × R must always hold. If you enter V = 12, I = 2, R = 10, the calculator will flag an inconsistency because 2 × 10 = 20, not 12. Real circuits always satisfy Ohm’s Law. If your measurements don’t, one of the readings is wrong, the circuit is non-ohmic (like a diode or motor), or there’s a measurement error.
Ignoring forward voltage in LED circuits. LEDs are not ohmic. They have a forward voltage drop (typically 1.8-3.5V depending on colour) before they conduct. Calculating an LED resistor using the full supply voltage instead of (supply - Vf) gives too little resistance, too much current, and a burned-out LED.
Confusing AC and DC power calculations. Ohm’s Law in its simple form (V = I × R) applies to DC circuits and to purely resistive AC circuits. For AC circuits with capacitors or inductors (reactive loads), impedance (Z) replaces resistance and the power calculation involves a power factor. The calculator uses the pure Ohm’s Law relationships, which are accurate for DC and resistive AC loads.
Misreading milliamps as amps. A component rated at 20mA needs the current entered as 0.02A, not 20. Entering 20 instead of 0.02 gives a resistance 1,000× smaller and a power 1,000× larger than the actual values. Always convert mA to A (divide by 1,000) before entering.
Using nominal voltage instead of actual voltage. A “12V” battery may actually measure 12.6V when fully charged and 11.0V when nearly depleted. A “5V” USB supply varies between 4.75V and 5.25V. For precise resistor calculations (especially for LEDs), use the actual measured voltage, not the nominal value.
Ohm’s Law applies to resistors and conductors, not to all components. Diodes, transistors, LEDs, and motors are non-linear: their resistance changes with voltage and current. Using Ohm’s Law directly for these components gives wrong answers. For those components, you need the component’s datasheet IV curve, not a simple resistance value.
Ohm’s Law in series and parallel circuits
The calculator solves for a single component or a single equivalent resistance. For multi-component circuits, you need to find the equivalent resistance first.
Series circuits: resistances add directly.
Current is the same through every component. Voltage divides proportionally across each resistor.
Parallel circuits: reciprocals add.
For 2 resistors in parallel, a simpler form:
Voltage is the same across every branch. Current divides in inverse proportion to resistance.
Calculate the equivalent resistance first, then enter it into the Ohm’s Law calculator alongside your voltage or current to find the total circuit behaviour.
Quick reference: standard voltages and their typical currents
| System | Voltage | Typical current range | Max power (single circuit) |
|---|---|---|---|
| AA/AAA battery | 1.5V | 1mA-1A | 1.5W |
| Coin cell (CR2032) | 3V | 0.001-10mA | 30mW |
| Li-ion cell | 3.7V (nom) | 10mA-10A | 37W |
| USB 2.0 | 5V | up to 500mA | 2.5W |
| USB 3.0 | 5V | up to 900mA | 4.5W |
| USB-C (standard) | 5V | up to 3A | 15W |
| USB-C PD (max) | up to 48V | up to 5A | 240W |
| 9V battery | 9V | 10-500mA | 4.5W |
| 12V DC (automotive) | 12-14.4V | 0.1-30A | 360W+ |
| 24V DC (industrial) | 24V | 0.1-20A | 480W |
| 48V PoE | 48V | up to 960mA | 46W |
| 120V AC (US) | 120V | 15A max (standard circuit) | 1,800W |
| 230V AC (EU/UK) | 230V | 16A max (standard circuit) | 3,680W |
| 240V AC (UK) | 240V | 13A max (standard socket) | 3,120W |
The bottom line
Ohm’s Law is four quantities and one relationship. Every electronics calculation routes through it eventually, whether you’re sizing a resistor, checking wire ratings, calculating battery life, or specifying a fuse.
Enter any 2 values, set the others to 0, and click Calculate. The calculator picks the right formula pair automatically and shows all 4 values at once so you don’t need to run it twice for different unknowns.
The formula table above has all 12 combinations if you want to work through the arithmetic yourself. But that’s what the calculator is for.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that voltage equals current times resistance: V = I × R. If you know any two values, you can calculate the third. It applies to resistive circuits and is the foundation of circuit analysis.
How do I calculate power from voltage and current?
Power (watts) = Voltage × Current: P = V × I. Example: a 12 V device drawing 2 A consumes 24 W. You can also calculate P = I² × R or P = V² / R if you know resistance instead.
How do I find resistance if I know voltage and current?
R = V ÷ I. Example: if a circuit operates at 9 V and draws 30 mA (0.03 A), the resistance is 9 ÷ 0.03 = 300 Ω.
What current does a 60 W light bulb draw at 120 V?
I = P ÷ V = 60 ÷ 120 = 0.5 A (500 mA). Its resistance at operating temperature: R = V ÷ I = 120 ÷ 0.5 = 240 Ω.
When does Ohm's Law not apply?
Ohm's Law applies to linear (ohmic) resistors. It does not strictly apply to semiconductors (diodes, transistors), which have non-linear V-I relationships, or to components whose resistance changes with temperature or light.
How do I limit current to an LED?
Choose a resistor so I = (Vsupply − Vforward) / R, where Vforward ≈ 2 V for red/yellow LEDs and ≈ 3–3.5 V for blue/white. For 5 V supply with a 2 V red LED at 20 mA: R = (5 − 2) / 0.02 = 150 Ω.