Current (V/R)
Calculate electric current in amperes from voltage and resistance with this free Ohm\u2019s law calculator. The calculation applies the formula I = V / R, where I is current, V is voltage in volts, and R is resistance in ohms. The default example shows 10 V across 20 Ω, which produces 0.5 A.
Result
Formula & Conversion Factor
The calculator applies Ohm\u2019s law in its current form: I = V / R, where I is current in amperes, V is voltage in volts, and R is resistance in ohms. Dividing volts by ohms gives amperes directly, with no extra conversion factor needed. The same relationship can be rearranged to solve for voltage or resistance.
| Quantity | Formula | Example |
|---|---|---|
| Current (I) | I = V / R | 10 V ÷ 20 Ω = 0.5 A |
| Voltage (V) | V = I × R | 0.5 A × 20 Ω = 10 V |
| Resistance (R) | R = V / I | 10 V ÷ 0.5 A = 20 Ω |
Worked Example
Find the current through a 6 Ω resistor connected to a 12 V battery:
- Start with the voltage: 12 V.
- Divide by the resistance: 12 ÷ 6 = 2.
- The current is therefore 2 A.
A second check with the default values: 10 V ÷ 20 Ω = 0.5 A, which is the number shown in the result box above.
Current Reference Table
Common voltage and resistance combinations and the current they produce (I = V / R):
| Voltage (V) | Resistance (Ω) | Current (A) |
|---|---|---|
| 1.5 | 0.5 | 3 |
| 5 | 10 | 0.5 |
| 9 | 4.5 | 2 |
| 10 | 20 | 0.5 |
| 12 | 6 | 2 |
| 120 | 240 | 0.5 |
| 240 | 480 | 0.5 |
Related Tools
External Resources
Frequently Asked Questions
- How do I calculate current from voltage and resistance?
- Divide the voltage in volts by the resistance in ohms. For example, 10 V across 20 Ω gives 10 ÷ 20 = 0.5 A. Enter both values above and press Calculate to get the current in amps.
- What is the formula for current?
- The formula is I = V / R, where I is current in amperes, V is voltage in volts, and R is resistance in ohms. It is one rearrangement of Ohm’s law, V = I × R. The same law also gives V = I × R and R = V / I.
- What units are used in the current calculation?
- Voltage is entered in volts (V), resistance in ohms (Ω), and the result is current in amperes (A). One ampere equals 1000 milliamperes, so 0.5 A is the same as 500 mA. The unit relationship follows directly from volts divided by ohms.
- What is the current when 10 V is applied across 20 Ω?
- The current is 10 / 20 = 0.5 A, or 500 mA. This is the default example built into the calculator, so the result box shows it before you type anything. Change either field to see the current update when you press Calculate.
- What happens to the current if resistance increases?
- For a fixed voltage, current and resistance are inversely related: if resistance doubles, current halves. For example, 12 V across 6 Ω gives 2 A, but the same 12 V across 12 Ω gives only 1 A. That is why a larger resistor limits current in a circuit.
- Does Ohm’s law apply to AC circuits?
- For purely resistive AC loads, I = V / R still works with RMS voltage and current values. Circuits with inductors or capacitors also have reactance, so the general form uses impedance: I = V / Z, where Z includes both resistance and reactance.
- How do I convert amps to milliamps?
- Multiply the current in amperes by 1000 to get milliamperes. For example, 0.5 A = 500 mA and 2 A = 2000 mA. Going the other way, divide milliamps by 1000 to return to amps.
- What is Ohm’s law in full?
- Ohm’s law states that the current through a conductor is proportional to the voltage across it, with the constant of proportionality being the resistance: V = I × R. It holds accurately for ohmic materials at a constant temperature. The three forms V = I × R, I = V / R, and R = V / I let you solve for any one quantity.