How to use Ohm's law, voltage dividers and LED resistors in one calculator
This article covers the three modes of the Ohm's law calculator: working out voltage, current, resistance and power from any two values you know; sizing a voltage divider; and finding the right series resistor for one or more LEDs. Each section walks through the controls, the logic behind the numbers, and the real-world pitfalls that catch beginners and experienced builders alike.
Three tools behind one dropdown
The calculator opens with a single dropdown labelled What are you working out. It offers three choices: Ohm's law, Voltage divider, and LED series resistor. Picking one changes which input fields appear in the panel on the right. The result area on the left updates as soon as you type a number, so there is no button to press and no delay to wait for.
This layout matters because the three tasks share a common thread. A voltage divider is just two resistors in series with a tap between them, and the maths behind it is Ohm's law applied twice. An LED circuit is a resistor sized to carry a set current at a set voltage drop, which is Ohm's law applied once. Putting them together saves you from opening three separate pages and re-entering the supply voltage each time.
Ohm's law mode: any two give you everything
When the mode is set to Ohm's law, four fields appear: Voltage in volts, Current in amps, Resistance in ohms, and Power in watts. Fill in any two and the other two are filled in for you. If you know voltage and resistance, it gives you current and power. If you know current and power, it gives you voltage and resistance. Every pairing works.
This is more useful than a simple V equals I times R formula, because it rolls the power equations in as well. Power is what tells you whether a resistor will survive a given circuit. A quarter-watt resistor carrying a current that puts half a watt through it will overheat and eventually fail. Seeing the power figure next to the other values keeps that check in front of you.
Standard resistor values and the E12 series
Resistors are not made in every possible value. They come in fixed steps called preferred values. The most common set is E12, which gives twelve values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, and 82. The same pattern repeats at every power of ten, so you can buy 220 ohms or 2,200 ohms but not 200 ohms.
The calculator shows the exact value the maths produces and then the nearest E12 value beside it. In practice you almost always use the E12 value, because that is what you can buy from any parts shop. The gap between the exact and E12 figures is rarely large enough to matter. A 5 per cent tolerance resistor can vary more from its own label than the gap between two E12 steps.
Voltage divider mode: R1, R2 and the loading trap
Switch the dropdown to Voltage divider and the panel shows Supply voltage, R1 -- top, and R2 -- bottom. The output voltage is the supply multiplied by R2 divided by the sum of R1 and R2. With both resistors at 10,000 ohms and a 5 V supply, the output is 2.5 V.
That number is only true when nothing draws current from the output. The moment you connect a load, it sits in parallel with R2, lowers the effective bottom resistance, and pulls the output down. A 10k/10k divider feeding a 10k load gives 1.67 V, not 2.5 V. This is why dividers are used for sensing and reference levels, not for powering things. The calculator warns about loading because it is the single most common reason a divider reads the wrong voltage on the bench.
LED series resistor mode
The third mode adds fields for Supply voltage, LED forward voltage, LED current in milliamps, and LEDs in series. An LED is not like a regular load. It has a fixed forward voltage drop that depends on its colour and chemistry. A red LED drops about 2 V. A blue or white one drops about 3.2 V. The resistor has to handle only the leftover voltage, not the full supply.
On a 5 V supply with a single red LED at 2 V, the resistor sees 3 V. At the standard 20 mA, that means 150 ohms. The nearest E12 value is also 150 ohms, so it works out neatly. But if you sized the resistor from the full 5 V, you would get 250 ohms, and the LED would be noticeably dimmer because less current flows. Getting this subtraction wrong is the most common LED circuit mistake.
The LEDs in series field handles chains. Two red LEDs in series drop 4 V total, leaving only 1 V for the resistor on a 5 V supply. Three would need more than 5 V and the calculator would flag it as impossible. Each LED in a series string adds its forward voltage, and the supply must exceed the total or the circuit will not light up.
Why parallel LEDs need separate resistors
A common temptation is to put several LEDs side by side and share one resistor among them. This fails because no two LEDs have exactly the same forward voltage. The one with the lowest drop steals most of the current, runs hot, and burns out first. Once it fails, the next lowest takes over, and the chain reaction kills them all.
In a series chain this problem does not exist because every LED carries the same current. Current has no choice but to flow through each one in turn. So the rule is simple: series chains share one resistor, but parallel branches each need their own. The calculator handles the series case directly. For parallel LEDs, run it once and then use that same resistor value for each branch.
Derating: why the calculated power is not enough
The power figure the calculator shows is the minimum the resistor must handle. In a real circuit you should use a resistor rated for at least double that number. A resistor running at its full rated power gets hot enough to discolour a board and can fail after hours or weeks of continuous use.
If the calculated power is 0.1 watts, a standard quarter-watt resistor is fine because 0.25 is more than double 0.1. If it is 0.2 watts, a quarter-watt part is right on the edge and a half-watt part is the safer choice. Carbon film resistors in through-hole packages are cheap enough that going one size up costs almost nothing and saves a lot of debugging later.
Putting it into practice
A typical session might start in Ohm's law mode to check whether a resistor you already have will work for a new purpose. You enter the resistance and the voltage across it, and the calculator tells you the current and the power. If the current is too high, try a larger resistance. If the power exceeds your resistor's rating, you need a beefier part.
Then you switch to LED mode because the same project has an indicator light. You enter the supply, the LED colour's forward voltage, and the current you want. The calculator gives you the exact resistance and the nearest value you can buy. Finally, if the circuit needs a stable reference at half the supply, you switch to divider mode, pick two equal resistors, and check whether the load you plan to attach will pull the output off target.
All three calculations share the same supply voltage field. Changing it once updates whichever mode you are in. And every result updates live as you type, so you can explore what-if scenarios by nudging a single value and watching the rest follow.
Privacy and where the maths runs
Every calculation happens inside your browser on your own device. No values are sent to any server, and there is no account or login. You can disconnect from the internet after loading the page and it will keep working. The Reset button in the panel header clears all fields back to their starting values if you want a fresh start.
For a tool that handles nothing more sensitive than resistor values, this might sound like an obvious choice. But the principle matters: your numbers stay on your machine, and the page works offline. That is true for every tool on the site, and it is worth knowing before you type anything into any calculator anywhere.