How to use the Ohm's law calculator
Type any two of volts, amps, ohms and watts and the page works out the other two. The two boxes you typed in have a cyan edge and the two it answered have an amber edge. To look at a different pair, type in an amber box: it becomes one of yours and the oldest of yours turns into an answer. Tap a letter in the triangle to cover it and see the formula for that quantity, with your numbers in it. Put k, M, m, u or n after a number, like 4.7k or 10 mA, or pick a prefix from the menu beside the box. Below the answer you get all six formulas worked out, a what-if table, and a small circuit that shows the current as moving dots and the power as a glowing resistor. Nothing you type leaves your device unless you make a share link.
The formulas
P = V × I P = I² × R P = V² ÷ R
Ohm's law says that the current through a part is proportional to the voltage across it, and the constant between them is the resistance. V is volts, I is amps, R is ohms and P is watts. The triangle is only a memory aid: cover the one you want and read what is left. Power is volts times amps; put Ohm's law into that and you get the other two power formulas. Together there are twelve ways to find the four quantities, and the wheel tab lists them all with your numbers.
A worked example
A 12 V battery is connected across a 24 Ω resistor. The current is 12 V ÷ 24 Ω = 0.5 A, which is 500 mA. The power is 12 V × 0.5 A = 6 W. You can check it two other ways: (0.5 A)² × 24 Ω = 6 W and (12 V)² ÷ 24 Ω = 6 W. That is what the calculator shows when it opens. Now double the voltage to 24 V. The current doubles to 1 A, but the power goes up four times, to 24 W, because power depends on the square of the voltage or the current. The What if table does this for you.
A second example: a heater
A 1,500 W space heater plugged into 120 V draws 1,500 W ÷ 120 V = 12.5 A, and its element is 120 V² ÷ 1,500 W = 9.6 Ω. Real outlets are not exactly 120 volts, so type the voltage you measure. To turn the watts into kilowatt-hours and a bill, use the electricity cost calculator.
Milli, kilo and the other prefixes
Electronics uses prefixes so the numbers stay short. A milliamp (mA) is a thousandth of an amp, so 500 mA is 0.5 A. A kilohm (kΩ) is a thousand ohms and a megohm (MΩ) is a million. The page reads them as you type and shows answers with the prefix that keeps the number between 1 and 1,000. Capitals matter: m is milli and M is mega, a difference of a billion. The page also reads u or µ for micro (a millionth), n for nano and G for giga.
| Prefix | Symbol | Times |
|---|---|---|
| nano | n | 0.000000001 |
| micro | µ | 0.000001 |
| milli | m | 0.001 |
| kilo | k | 1,000 |
| mega | M | 1,000,000 |
| giga | G | 1,000,000,000 |
Resistors in series and in parallel
In series the same current goes through every resistor and the resistances add: 100 Ω and 200 Ω make 300 Ω. In parallel every resistor has the same voltage and the currents add. The total is one over the sum of the reciprocals, and for two resistors it is their product divided by their sum: 100 Ω and 200 Ω in parallel make 66.6667 Ω. A parallel total is always smaller than the smallest resistor. Type a supply voltage on the Resistors tab to see each one's voltage, current and watts, and the page checks that the drops add up to the supply.
Sizing a resistor for an LED
An LED drops a roughly fixed voltage, its forward voltage, and the resistor takes whatever the supply has left. So R = (supply − number of LEDs × forward voltage) ÷ current. As an example only, a 5 V supply, one LED that drops 2 V and a target of 16 mA gives (5 V − 2 V) ÷ 16 mA = 187.5 Ω. Resistors come in standard values, so the page picks the next one up from the E12 or E24 series: 220 Ω. That lowers the current a little, to 13.6364 mA, and the resistor turns 40.9091 mW into heat. Read your LED's datasheet for its real forward voltage and current, and choose a resistor rated for more than the watts shown.
What this calculator does not cover
It treats every part as a plain resistor. Ohm's law as used here holds for DC and for AC through purely resistive parts. It does not cover AC power factor, coils and capacitors, motors, transformers or three-phase power, and parts like diodes and LEDs do not have one fixed resistance. It does not size wire, breakers or fuses, and it is not a substitute for an electrician: wiring rules are set by your local electrical code, and the edition in force varies by place.
Where the numbers come from
The three Ohm's law forms and the three power forms come from the Electronics Tutorials page on Ohm's law and power. The NIST pages on the SI units give the volt as a watt per amp and the ohm as a volt per amp, and their prefix table gives the prefixes above. The Wikipedia pages on Ohm's law and on series and parallel circuits give the limits and the resistor rules. The E12 and E24 values are from the E series page, which names IEC 60063, and the LED method follows the SparkFun LED tutorial. All of it is rewritten here in our own words.
What people use it for
- Finding the current a heater, lamp or motor draws from its watts and volts.
- Choosing a resistor for an LED, a sensor or a voltage divider.
- Checking homework and lab work, with every step shown.
- Working out how much heat a resistor must shed.
Common mistakes
- Mixing prefixes. 10 mA is 0.01 A, not 10 A. Let the menu or the typed letter do it.
- Forgetting that power grows with the square. Twice the current is four times the watts.
- Using the wrong formula for resistors. Resistances add in series, but in parallel you add the reciprocals.
- Treating an LED like a resistor. It needs a resistor in series; the LED alone has no safe voltage-to-current rule.
- Using this for mains wiring decisions. It shows the arithmetic, not what is safe or legal.
Questions
What is Ohm's law?
Voltage equals current times resistance: V = I × R. With any two of the three you can find the third.
How do I find watts from volts and amps?
Multiply them: P = V × I. For example, 12 V at 0.5 A is 6 W.
How do I find amps from watts and volts?
Divide: I = P ÷ V. A 1,500 W heater on 120 V draws 12.5 A.
Does it work for AC?
For plain resistive loads such as a heater element, yes, with RMS volts and amps. It does not handle power factor, coils, capacitors or three-phase power.
Can I share my result?
Yes. Share makes a link with a picture for Facebook, X and Pinterest, a link you can copy, and a short video for TikTok made on your device. The link keeps only the two numbers you typed, for 90 days.