When you connect a resistor to a battery, why does current flow? And why does a lower resistance allow more current?
Ohm's Law answers these questions with three quantities: voltage, current and resistance. If you know two, you can calculate the third—for a circuit whose resistance stays constant. In the calculator, all three fields remain visible: you can even edit the automatically calculated field to use it as a new input.
The big picture
The battery creates a voltage difference. The resistor opposes the flow of charge. Together, they determine how much current flows. In a closed circuit with a fixed resistor, increasing the voltage increases the current; increasing resistance reduces the current.
Voltage
Volts (V)Electrical potential difference between two points.
Current
Amperes (A)How much electric charge passes per second.
Resistance
Ohms (Ω)How strongly a material or component opposes current.
What is voltage?
Voltage (V) is the electrical potential difference between two points. Think of it as the amount of energy available per unit of charge. A 12 V battery provides 12 joules of energy per coulomb of charge moved between its terminals.
Voltage alone doesn't tell us how much current flows. That depends on the resistance of the connected circuit.
What is electric current?
Current (I) is the rate at which electric charge flows. One ampere means one coulomb of charge passes a point each second.
If 2 A flows through the resistor in our illustration, two coulombs of charge pass a point each second. In a single unbranched circuit, the current is the same at every point around the loop.
What is resistance?
Resistance (R) measures opposition to current flow. For the same applied voltage, a larger resistance produces less current. Resistance is measured in ohms (Ω).
Resistors are designed to have predictable resistance, but physical conductors and loads also have resistance. Temperature matters: a heating element and a cold tungsten lamp filament do not necessarily have constant resistance as they warm up.
One law, three useful arrangements
Depending on what you know, rearrange the same relationship:
Unit check: one ohm is one volt per ampere. For example, a 12 V supply across a 6 Ω resistor gives 2 A of current.
What do watts tell us?
Electrical power (P) is the rate at which energy is transferred. It's measured in watts (W). In a resistor, electric energy is converted primarily to heat.
Using Ohm's Law, we can replace voltage or current to obtain two more equivalent expressions for resistive loads.
In our example, 12 V and 2 A give 24 W. A resistor must be rated to safely handle its actual power dissipation, with appropriate thermal margins.
Why the voltage-current graph is a straight line
If resistance stays constant, voltage is proportional to current. A 6 Ω resistor needs 6 V to pass 1 A, 12 V to pass 2 A and 18 V to pass 3 A. Doubling voltage doubles current in this ideal model.
Two worked examples
12 V supply with a 6 Ω resistor
Known: voltage and resistance. Find current and power.
Result: current is 2 A and the resistor dissipates 24 W.
48 V DC heater drawing 8 A
Known: voltage and current. Find the equivalent operating resistance and power.
Result: the heater operates at an equivalent 6 Ω resistance and consumes 384 W under these conditions.
When does Ohm's Law apply?
This calculator assumes positive quantities and constant, linear resistance. It is designed for DC resistive circuits and can also be used with RMS voltage and current for purely resistive AC loads, where voltage and current are in phase.
For motors, transformers, capacitors and inductors on AC, resistance alone is not enough. Their current depends on impedance, which includes reactance and a phase angle. A diode, lamp filament or temperature-dependent resistor may not obey a constant-resistance model across all operating points.
Common mistakes to avoid
1 kΩ means 1,000 Ω. Convert units before entering values.
If current is zero, V/I does not produce a finite resistance. Open circuits need a different interpretation.
Real resistances may vary with operating temperature; do not assume a fixed resistance where it changes significantly.
For reactive AC loads, consider impedance and phase angle rather than using resistance alone.
Use Ohm's Law yourself
Choose any two known electrical quantities and see all the results with worked formulas.
Open Ohm's Law Calculator →Educational examples assume ideal components. Always follow appropriate electrical safety practices for real equipment and installations.