Source: https://mokxi.com/learn/ohms-law-calculator Updated: 2026-10-05 Learn # Ohm's law calculator, with a circuit to check it on Beginner Written by the Mokxi team, updated October 5, 2026 Start building free Open the editor Or see every lesson. - 261 parts on the bench - 31 boards running now - 1.00x real time, on every board Click to open it in the editor Three 10k resistors across 5 V and a multimeter on the lowest joint. It reads 1.666 V, as Ohm’s law says, less a hair for the meter. Ohm's law ties together three things in every circuit: voltage (V, in volts), current (I, in amps) and resistance (R, in ohms). Know any two and you can work out the third. Add power (P, in watts) and you can also check that a resistor will not get too hot. Use the calculator below. Pick the two values you know, type them in, and it fills in all four. Then scroll down for the formulas, three worked examples, and the mistakes that trip people up. The circuit above is where you can check the math. It is three 10k resistors in a row across 5 volts, with a multimeter on one of the joints. Open it in the editor, move the meter, change a resistor and see if the reading matches what the calculator says. Ohm's law calculator: V = I × R, P = V × I I know Voltage and resistanceVoltage and currentCurrent and resistancePower and voltagePower and currentPower and resistance Voltage (V) VmV Resistance (R) ΩkΩMΩ Voltage (V) 5.00 V Current (I) 22.7 mA Resistance (R) 220 Ω Power (P) 114 mW Under a quarter watt, so a common 1/4 W resistor is fine for this. ## The Ohm's law formulas There is really one formula, V = I × R, turned around three ways. Voltage is current times resistance. Current is voltage divided by resistance: I = V / R. Resistance is voltage divided by current: R = V / I. Power adds three more. Power is voltage times current: P = V × I. Put Ohm’s law into that and you also get P = I² × R and P = V² / R. The calculator uses whichever one fits the two values you gave it. Many people remember the first three with the Ohm’s law triangle. Draw a triangle with V at the top and I and R side by side at the bottom. Cover the one you want. If the other two sit side by side, multiply them. If one sits over the other, divide. Cover V and you see I beside R, so V = I × R. Cover I and you see V over R, so I = V / R. ## Example 1: the resistor for an LED This is the most common reason people look up Ohm’s law. You have a 5 V Arduino pin and a red LED, and you want about 15 mA through it. A red LED drops about 2 V on a typical datasheet, so the resistor sees what is left: 5 V minus 2 V is 3 V. Now use the calculator with voltage and current: 3 V and 15 mA. It gives 200 ohms. Resistors come in standard sizes, so round up to the next one, 220 ohms. That is why 220 ohms is the resistor in almost every Arduino LED circuit. The power is small too: 3 V times 15 mA is 45 mW, well under the quarter watt a common resistor can take. Our Ohm’s law page runs this exact loop in Mokxi, where the red LED drops about 1.7 V at this current. ## Example 2: the circuit above The circuit above has three 10k resistors in a row, from 5 V down to ground. They add up to 30k. So the current is 5 V divided by 30,000 ohms, which is about 167 microamps. Type 5 V and 30 kΩ into the calculator to check. The same current flows through all three resistors. So each one drops 167 µA times 10k, which is about 1.667 V. The meter’s red lead is on the lowest joint, one resistor above ground, so it should read 1.667 V. It actually reads 1.666 V. That is not a mistake. A real meter has a very large resistance of its own, 10 megohms here, and putting it across the circuit pulls the reading down by a tiny amount. Mokxi’s meter does the same thing. Move the red lead up to the middle joint and it reads about 3.33 V, two thirds of the supply. ## Example 3: will this resistor get too hot? Say you put a 100 ohm resistor across 12 V. The calculator, with voltage and resistance, gives 120 mA and 1.44 W. Most small hobby resistors are rated for a quarter watt, 0.25 W. This one would get very hot on a real bench and could burn. The fix is to use a bigger resistance, which lets less current through, or a resistor rated for more power, such as 2 W. The calculator warns you when the power goes over a quarter watt. One honest note: Mokxi’s resistors do not burn out. It will show you a current a real resistor could not survive, rather than pretend it caught fire. So check the watts yourself before you build. ## Units and prefixes Most mistakes with Ohm’s law are unit mistakes. Put everything in plain volts, amps and ohms before you do the math, or let the calculator’s unit menus do it for you. Milli (m) means one thousandth: 15 mA is 0.015 A. Micro (µ) means one millionth: 167 µA is 0.000167 A. Kilo (k) means one thousand: 10 kΩ is 10,000 ohms. Mega (M) means one million: 10 MΩ is 10,000,000 ohms. A quick check: 5 V across 1 kΩ is 5 mA. If you get 5,000 A, a kilo went the wrong way. ## Check it on a meter A calculator gives you the number the circuit should have. A meter tells you the number it does have. When the two disagree, something in the circuit is not what you think it is. In the circuit above, click a resistor and change its value in the properties panel, then read the meter again. Change the bottom one to 20k and the chain adds up to 40k. The calculator says the current drops to 125 µA and the bottom resistor now drops 2.5 V. The meter should agree. Our multimeter guide shows how to measure current and resistance too. ## Common mistakes Using the supply voltage when you need the voltage across the part. For an LED resistor, subtract the LED’s drop first. Mixing units, like volts with kilohms, and getting an answer a thousand times off. Forgetting power. The right resistance can still be the wrong resistor if it is rated for less power than it has to handle. Using Ohm’s law on parts that do not follow it. An LED, a diode or a motor does not have a fixed resistance. Use Ohm’s law on the resistor next to it instead. ## Questions What is the formula for Ohm's law? V = I × R: voltage equals current times resistance. It rearranges to I = V / R and R = V / I. For power, P = V × I, P = I² × R and P = V² / R. How do I calculate watts from volts and ohms? Square the voltage and divide by the resistance: P = V² / R. For example, 5 V across 100 ohms is 25 divided by 100, which is 0.25 W. What is the Ohm's law triangle? A memory aid: V on top, I and R side by side underneath. Cover the value you want. Side by side means multiply, one over the other means divide. Does this calculator work for AC? It works for DC circuits and for plain resistors on AC. Capacitors and coils on AC have impedance that changes with frequency, which this calculator does not handle. Mokxi’s circuit simulator can run those circuits. Related ## Keep going Ohm's Law, in a Circuit You Can Run Picking a Resistor for an LED Voltage Dividers and Why They Are Bad Power Supplies Series vs Parallel Circuits, Seen With LEDs How to Use a Multimeter: Practice on a Virtual One Resistor Color Code Calculator and Chart How the multimeter is modeled The online circuit simulator The project page, with the full sketch Beginner ## Build this for real Open the editor, change a value and watch the number move with it. Nothing to install, and no account needed. Start building free Open the editor