Ohm's Law Calculator — V · I · R · P

Real-world use cases

• Size a current-limiting resistor for an LED

  You have a 5 V supply and a red LED that drops about 2 V at its rated 20 mA. The resistor must drop the remaining 3 V. Tick V and I, type 3 V and 20 mA, and read R = 150 Ω and P = 60 mW. Now you know to grab a 150 Ω resistor (or the nearest E12 value, 150 Ω exactly here) and that a standard 1/4 W part has plenty of headroom over the 60 mW it dissipates. No more guessing whether your LED will cook itself.

• Check whether a resistor will overheat

  A 220 Ω resistor sits across a 12 V rail in a prototype. Will it cope? Tick V and R, enter 12 V and 220 Ω: the tool returns I = 54.5 mA and P = 0.65 W. A common 1/4 W (0.25 W) resistor would run hot and drift — you need at least a 1 W part, or a higher resistance. Catching this on the calculator beats catching it by smell on the bench.

• Find the resistance of a mains appliance

  A label reads "230 V, 1500 W" on a space heater and you want its element resistance and current draw for a fuse calculation. Tick V and P, enter 230 V and 1500 W: the tool gives I = 6.52 A and R = 35.3 Ω. That tells you a 10 A circuit handles it but two of them on one 13 A ring would trip the breaker.

• Teach Ohm's law with the wheel

  For a classroom or self-study, change one input and watch which slice of the 12-segment wheel lights up. Solve the same circuit from {V,I}, then from {V,R}, then from {R,P}, and the highlighted formula moves around the wheel — making the relationship between the twelve forms concrete instead of a wall of equations to memorise.

• Convert a bench-supply reading into power

  Your lab supply shows 9.0 V and the meter reads 0.35 A into the board. How much power is the board drawing? Tick V and I, type 9 V and 350 mA, and read P = 3.15 W. Now you can check it against the supply's wattage limit and decide whether the board needs a heatsink before you leave it running overnight.

Common pitfalls

• Mixing units before you do the math. People multiply 5 V by "20" for an LED and get 100, forgetting the current is 20 mA = 0.02 A. Keep the prefix attached: 5 V × 0.02 A = 0.1 W. This tool's prefix dropdowns do the conversion for you, but if you compute by hand, write the unit next to every number.

• Sizing a resistor for resistance but forgetting power. R = V/I tells you the value, but P = I²R tells you the heat it must shed. A 150 Ω resistor that is "correct" for 0.5 A dissipates 37.5 W and will burn a 1/4 W part instantly. Always read the power figure too and pick a wattage with headroom.

• Applying V = IR to a reactive AC load. With capacitors or inductors the opposition is impedance, not resistance, current and voltage shift out of phase, and P = VI overstates the real power. Use these equations only for DC or purely resistive AC loads (heaters, incandescent bulbs).

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