Ohm's Law Calculator
Solve for voltage, current, or resistance. Power dissipation included.
Known Values
Results
V = I × R
V-I CHARACTERISTIC
V-I Characteristic (R = 100Ω)
Ohm's Law
When to use this: Use this calculator when you need to find the missing value in a simple resistive circuit — voltage, current, resistance, or power. Works for DC circuits directly, and for AC if you use impedance Z instead of resistance R.
Think of electricity like water flowing through a pipe. Voltage is the pressure pushing the water, current is how much water flows per second, and resistance is how narrow the pipe is. A wider pipe (lower resistance) lets more water through at the same pressure — same logic, different physics. This analogy isn't perfect, but it gives you an intuition for why the three quantities are linked.
Ohm's Law captures that relationship in one equation: V = I × R. Double the voltage across a fixed resistor and the current doubles. Halve the resistance and the current doubles again. It works in reverse too: if you measure 12V across a component drawing 0.5A, you know its resistance is 24Ω without touching it.
In practice you'll use all three forms. Sizing a current-limiting resistor? Use R = V / I. Need to know the current draw of a 1kΩ pull-down on 3.3V? Use I = V / R = 0.0033A = 3.3mA. The calculator above handles all three modes: pick what you're solving for, enter the two known values, and you get the answer plus power dissipation.
Power dissipation is the part beginners often skip — and it's how components get destroyed. P = V × I gives you the wattage in watts. A resistor rated 0.25W passing 0.3W will get hot and eventually fail. Standard rule: choose a resistor rated at least twice the calculated dissipation. If the formula says 0.1W, use a 0.25W or 0.5W part.
Ohm's Law only holds for resistors and other linear elements at constant temperature. Diodes, LEDs, and transistors are non-linear — their resistance changes with current and voltage. If you try to apply V = I × R to an LED, you'll get nonsense. For those components you need their datasheet curves, not Ohm's Law.
Ohm's Law
V = I × RPower (P = V × I)
P = V × I = I² × R = V² / RKey Points
- Valid for linear (ohmic) resistors at constant temperature
- Non-linear devices (diodes, transistors) do not follow Ohm's Law
- Power dissipated as heat: check maximum ratings
- Double the voltage → double the current (same R)
Common Uses
- Selecting current-limiting resistors
- Calculating voltage drops across components
- Estimating power dissipation and thermal design
- Verifying circuit measurements
Practical Examples
5V supply, 220Ω resistor — how much current flows through the LED?
I = 5V / 220Ω = 22.7 mA, P = 113 mW
12V source with a 1 kΩ resistor — what is the current draw?
I = 12V / 1kΩ = 12 mA, P = 144 mW
5V supply, want exactly 20 mA — what series resistor value is needed?
R = 5V / 0.02A = 250 Ω → use E24 270 Ω
Common Values Reference
| Circuit | Typical R | Supply V | Current | Power |
|---|---|---|---|---|
| LED indicator (5V) | 150–330 Ω | 5 V | 10–20 mA | 50–100 mW |
| Pull-up resistor | 4.7–10 kΩ | 3.3–5 V | 0.3–1 mA | 1–5 mW |
| Base bias (BJT) | 10–100 kΩ | 5–12 V | 50–500 µA | <1 mW |
| Power resistor (heater) | 10–100 Ω | 12 V | 120 mA–1.2 A | 1.4–14 W |
| Shunt (current sensing) | 0.01–1 Ω | — | 1–10 A | 10–100 mW |
When Ohm's Law does NOT apply
Ohm's Law only works for linear, ohmic components at constant temperature. These are the components where it breaks down:
| Component | Why it's non-linear | What to use instead |
|---|---|---|
| Diode / LED | Exponential I-V curve — current doubles for every ~18mV increase in Vf | Shockley equation or datasheet Vf table |
| Transistor (BJT) | Collector current = β × base current — a controlled source, not a resistor | Operating point analysis (Ic = β × Ib) |
| MOSFET | Drain current depends on gate voltage squared in saturation region | Square-law model or SPICE simulation |
| Incandescent bulb | Resistance increases 10× when hot — a 60W bulb has ~880Ω hot but ~8Ω cold | Use hot resistance (P = V²/R) at operating point |
| Thermistor (NTC) | Resistance drops exponentially with temperature | Steinhart-Hart equation or lookup table |
How to choose a resistor: the full process
Calculating the resistance value is just step one. These are the decisions you need to make for a complete resistor selection:
- Resistance value: Calculate with R = V / I. Round to the nearest E24 (or E96 for precision) standard value.
- Power rating: Calculate P = V × I. Choose a resistor rated at least 2× this value. Standard ratings: 1/8W, 1/4W, 1/2W, 1W, 2W, 5W.
- Tolerance: ±5% (gold, E24 series) for most circuits. ±1% (brown, E96 series) for filters, precision dividers, and anything needing better than 2% accuracy.
- Temperature coefficient: Matters for precision circuits — a ±100ppm/°C resistor changes 0.01% per °C. Metal film resistors are typically 50–100ppm/°C; carbon film can be 200–500ppm/°C.
- Package: Through-hole for prototyping, SMD 0805 or 1206 for production PCBs. For high power, use wirewound or power resistors with heatsinking.
Common mistakes when applying Ohm's Law
Entering 20 instead of 0.02 for 20mA. With V=5V and I=20: R = 5/20 = 0.25Ω instead of 250Ω. Always convert milliamps to amps before calculating.
Calculating resistance correctly but forgetting to check watts. A 100Ω resistor across 12V dissipates 1.44W — a standard 1/4W resistor will fail within minutes.
Trying to calculate LED current as I = V/R using the LED's "resistance". LEDs don't have a fixed resistance — use the V=Vf + I×R model with a series resistor.
Ohm's Law and power formulas (complete)
V = I × R I = V / R R = V / I
P = V × I P = I² × R P = V² / R
Combined: V = √(P × R) I = √(P / R) R = V² / PQuick-reference power examples
| Load | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| LED (red) | 2.0V | 20mA | 100Ω | 40mW |
| Arduino Uno | 5V | 50mA | 100Ω | 250mW |
| Raspberry Pi 4 | 5V | 600mA | 8.3Ω | 3W |
| 60W bulb equiv | 120V | 0.5A | 240Ω | 60W |
| EV charger | 240V | 32A | 7.5Ω | 7.68kW |
| Microwave | 120V | 10A | 12Ω | 1200W |
More worked examples
Supply 5V, red LED (Vf=2.0V, If=20mA): R = (5V – 2V) / 20mA = 150Ω → use 150Ω standard value (E12 series)
P_resistor = 0.02² × 150 = 60mW → ¼W resistor sufficient
Output: 5V, claims 2A. Load = phone (R≈2.5Ω): I = 5V/2.5Ω = 2A ✓
P = 5V×2A = 10W — verify with USB ammeter.
H-bridge Rds(on)=0.1Ω, motor current 3A: P_loss = I²×R = 9×0.1 = 0.9W per switch
4 switches = 3.6W total heat — heatsink required.
Design tip: Always calculate power dissipation before selecting components. Resistor derating: use resistor rated at 2× calculated power for reliability. At 3× rated power, most resistors fail within seconds — never skip the P = I²R check. For PCB resistors: 0402 = 62mW, 0603 = 100mW, 0805 = 125mW, 1206 = 250mW max.
Did you know? Georg Ohm published his law in 1827. His work was initially dismissed by the German scientific community — the Minister of Education even called it a "web of naked fancies." It took years before Ohm's Law gained the recognition it deserved.