📏 Enter known values
Formula Reference
Worked example
If we use the result to calculate Current, we should get the value you entered:
Ohm's law: V = I × R
Ohm's law relates the three basic quantities of a circuit: the voltage across a component equals the current through it times its resistance. Solving for each value gives the three forms of the law:
\[ V = I \times R \]
\[ I = \frac{V}{R} \]
\[ R = \frac{V}{I} \]
- Voltage (V), in volts.
- Current (I), in amperes. 1 mA = 0.001 A.
- Resistance (R), in ohms (Ω). 1 kΩ = 1,000 Ω.
How to use the calculator
- Enter two of the three values: voltage, current and resistance.
- Leave empty the one you want to find.
- Press Calculate. You can type units such as 500 mA or 4.7 kΩ and the calculator converts them.
Worked examples
1. Current through a 6 Ω resistor at 12 V
\[ I = \frac{12}{6} = 2\ \text{A} \]
2. Resistor for an LED
An LED needs 20 mA and drops 2 V. With a 9 V battery, the resistor has to take the other 7 V: \( R = \frac{7}{0.02} = 350\ \Omega \). The next standard value above is 390 Ω, which keeps the current slightly below 20 mA.
3. Voltage across a resistor
A current of 0.5 A flows through a 220 Ω resistor: \( V = 0.5 \times 220 = 110\ \text{V} \).
4. Resistance of a heater
A 230 V heater draws 8.7 A. Its resistance is \( R = 230 \div 8.7 \approx 26.4\ \Omega \).
Ohm's law and power
Combining V = I × R with P = V × I gives the power a resistor turns into heat:
\[ P = I^2 \times R = \frac{V^2}{R} \]
Example: 2 A through 6 Ω dissipate \( 2^2 \times 6 = 24 \) W. To calculate watts directly, use the watts, amps and volts calculator.
Current and power at 12 V
| Resistance | Current | Power |
|---|---|---|
| 1 Ω | 12 A | 144 W |
| 2 Ω | 6 A | 72 W |
| 4 Ω | 3 A | 36 W |
| 6 Ω | 2 A | 24 W |
| 10 Ω | 1.2 A | 14.4 W |
| 12 Ω | 1 A | 12 W |
| 24 Ω | 0.5 A | 6 W |
| 47 Ω | 0.255 A | 3.06 W |
| 100 Ω | 0.12 A | 1.44 W |
| 1,000 Ω | 0.012 A | 0.14 W |
When Do You Need Ohm's Law?
LEDs burn out without a resistor that limits their current. Ohm's law gives its value from the supply voltage, the LED voltage and the current it needs.
The most common use in electronics projectsKnowing the current a device draws (I = V ÷ R) tells you whether the cable and the fuse can handle it.
Prevents overheated cables and blown fusesMeasuring voltage and current gives the resistance of a component; if it does not match the expected value, the part may be damaged.
Find faulty components quicklyToasters, kettles and heaters are resistors. Their resistance and the mains voltage give the current and, with P = V² ÷ R, the power.
Understand what appliances consumeIn 12 V systems, Ohm's law sizes resistors, relays and wiring for lights and accessories.
Safe wiring in 12 V and 24 V systemsOhm's law is the first circuit law students learn, and series and parallel circuits build on it.
The base of every circuit exerciseCommon Mistakes
⚠️ Mixing Units
⚠️ Inverting the Division
⚠️ Using the Total Voltage on One Resistor
⚠️ Applying It to Non-Linear Parts
⚠️ Ignoring the Power Rating
⚠️ Measuring Resistance in a Live Circuit
Applications by Industry
Electronics
- LEDs: Current-limiting resistors
- Sensors: Voltage dividers to read sensors
- Circuit design: Pull-up and pull-down resistors
Electrical Installations
- Wiring: Current in cables to choose the cross-section
- Voltage drop: Voltage lost along long cable runs (V = I × R)
- Earthing: Resistance of ground connections
Automotive
- Lighting: Resistors for LED bulbs in 12 V systems
- Diagnostics: Resistance of sensors and injectors
- Accessories: Current drawn from the battery
Industry
- Heating: Resistance of heating elements
- Motors: Winding resistance checks
- Instrumentation: 4–20 mA current loops and their measuring resistors
Science & Labs
- Materials: Measuring the resistance of conductors
- Sensors: Thermistors and strain gauges, whose resistance changes
- Batteries: Internal resistance of cells
Education
- Physics: Introduction to electric circuits
- Exams: Problems with series and parallel resistors
- Labs: Checking the linear relation between V and I