Ohm’s law links three quantities: voltage (V), current (I), and resistance (R). If you know any two, you can find the third. The triangle is a memory aid that gives you the right formula every time without memorising three separate equations.
The triangle
Draw a triangle divided into three parts: V on top, I bottom-left, R bottom-right. To find a value, cover it with your finger. Cover V and you see I x R side by side, meaning multiply: V = I x R. Cover I and you see V over R, meaning divide: I = V / R. Cover R and you see V over I: R = V / I. That is the whole trick.
Units: V in volts, I in amps, R in ohms. If current is given in milliamps, convert first: 1 amp = 1,000 milliamps. A 250 milliamp current is 0.25 amps. Forgetting this conversion is the most common beginner error.
Worked example
A 12-volt battery drives current through a 4-ohm resistor. Cover I in the triangle: I = V / R = 12 / 4 = 3 amps. Now find the power too: power P = V x I = 12 x 3 = 36 watts, so the resistor must handle at least 36 watts or it will overheat.
Reverse it: you want 2 amps through a 10-ohm resistor. Cover V: V = I x R = 2 x 10 = 20 volts. And to find the resistor for a 9-volt supply at 0.5 amps: cover R: R = V / I = 9 / 0.5 = 18 ohms.
Series and parallel reminders
In a series circuit the same current flows everywhere, and resistances add: total R = R1 + R2. In a parallel circuit the voltage is the same across each branch, and currents add. Ohm’s law still applies to each part separately, which is how you solve mixed circuits one piece at a time.
A frequent mistake is applying Ohm’s law to the whole circuit when only part values are known. Always be clear which voltage and which resistance belong together: the voltage across a component with the current through that same component.
When the law does not apply
Ohm’s law describes ohmic conductors, mainly metals at steady temperature. It does not describe diodes, LEDs, or filament bulbs well, because their resistance changes with voltage or temperature. A bulb’s resistance when cold is far lower than when glowing, so a cold measurement predicts a much higher current than the real one. For those parts, use their datasheet curves instead of the triangle.
Power, heat, and resistor ratings
Current through a resistor makes heat, and every resistor has a power rating in watts that must not be exceeded. Our 12-volt, 4-ohm example dissipates 36 watts, so a small quarter-watt resistor would burn out instantly. It needs a resistor rated comfortably above 36 watts, typically 50 watts or more for margin.
The safety margin rule is simple: choose a rating at least 1.5 times the expected dissipation, more for enclosed spaces or hot environments. Undersized resistors fail by overheating, drifting in value, or in the worst case scorching a board. Power is not an afterthought in circuit design; it is what sets component sizes.
Try it faster with the tool
You can follow every step above by hand, but the free Online Ohm’s Law Calculator does the arithmetic for you. Enter your numbers and it gives the answer right away, so you can check your hand calculation or skip straight to the result.
Quick tips
- Cover the value you want; the rest shows the formula.
- Convert milliamps to amps before calculating.
- Voltage and resistance must belong to the same component.