Ohms Law & Circuit
Voltage Divider Calculator
Find the output of a two-resistor divider, the current it draws, and how much a real load pulls that output down.
Voltage Divider
Set Vin, R1 and R2. Add a load to see the real output.
How a Voltage Divider Works
A voltage divider is two resistors in series across a source, with the output taken from the junction between them. Because the same current flows through both, the supply voltage splits in proportion to the two resistances.
Only the ratio of the resistors sets the output. A 10 kΩ and 10 kΩ pair and a 100 Ω and 100 Ω pair both halve the input, but the second draws a hundred times more current and wastes a hundred times more power.
Connecting a load changes the picture, because the load sits in parallel with R2 and lowers the effective lower-leg resistance.
The calculator substitutes that combined value back into the divider formula to give the loaded output voltage, and reports the difference between the loaded and unloaded figures.
How to Use the Voltage Divider Calculator
Enter the input voltage and both resistor values, choosing units from the selectors beside each field. The load resistance field is optional: leave it blank for the ideal, unloaded case, or fill it in with the input resistance of whatever the divider feeds.
Select Calculate. The results panel gives the unloaded output voltage, the current circulating through the divider, and the power dissipated in each resistor. When a load is present, four more tiles appear: the loaded output, the effective R2 in parallel with the load, the total current drawn from the supply, and the current flowing into the load itself.
The small circuit diagram beneath the fields updates with your values so the arrangement stays clear, and a note beneath the results tells you how far the load has shifted the output as a percentage.
Worked Example: A 12 V Two-Resistor Divider
Take a 12 V supply with R1 = 1 kΩ and R2 = 1 kΩ.
The current is 12 V ÷ 2 kΩ = 6 mA, and each resistor dissipates 6 mA × 6 V = 36 mW, comfortably inside a quarter-watt part.
Now connect a 1 kΩ load. The lower leg becomes 1 kΩ in parallel with 1 kΩ, which is 500 Ω, and the output falls to 12 V × 500 Ω ÷ 1.5 kΩ = 4 V. That is a drop of a third from the ideal figure, and it illustrates the central weakness of dividers: they only hold their calculated output when almost nothing is drawn from them.
If instead the load were 100 kΩ, the lower leg would become 990 Ω and the output 5.97 V, an error under half a per cent. The useful rule of thumb is to keep the load resistance at least ten times, and preferably a hundred times, the value of R2.
Reading Your Divider Results
The unloaded output is what a perfect voltmeter with infinite input resistance would read. The loaded output is what the divider actually delivers to the circuit you attach. Where those two numbers differ noticeably, the divider is not a good voltage source for that load.
Circuit current tells you the standing consumption of the divider, which matters in battery-powered designs where a permanently connected divider drains the cell around the clock. The two power figures tell you what each resistor must dissipate; both need to sit well within the part's rating.
The effective lower-leg resistance is the value R2 takes on once the load is in parallel with it, and it is the number that explains why the output moved. A warning appears when the load pulls the output down by more than five per cent.
Where Voltage Dividers Are Used
The most common use is scaling a signal down so it fits within the range of something else. A microcontroller analogue input limited to 3.3 V can read a 12 V battery through a divider that scales the voltage by roughly a quarter. Because the analogue input draws almost no current, the divider holds its calculated ratio well.
Dividers also set reference voltages for comparators, provide bias for transistor stages, and form the fixed half of a resistive sensor bridge, where a thermistor or light-dependent resistor takes the place of one leg and the output voltage moves as the sensor's resistance changes.
What a divider cannot do is power a load. Using one to drop a supply voltage for anything that draws real current wastes energy as heat and produces an output that moves whenever the load changes. A regulator or a buck converter is the right component for that job.
Limits of the Divider Model
- The output impedance of a divider is R1 in parallel with R2, and every load interacts with it. The calculator shows that interaction, but the underlying weakness cannot be designed away.
- Resistor tolerance shifts the ratio. Two 5% resistors can combine to move the output by several per cent even before a load is attached; 1% parts are usual where the ratio matters.
- Very high resistor values reduce standing current but make the node sensitive to circuit board leakage, humidity and the input bias current of whatever it feeds.
- The model is purely resistive. In AC or high-frequency use, stray capacitance across R2 turns the divider into a frequency-dependent network.
Treat the output as an estimate for design work, and verify a built divider with a meter before relying on it.
Questions
Voltage Divider FAQs
Why is my measured output lower than the calculated value?
Almost always because of loading. Whatever you connected sits in parallel with R2 and lowers the effective lower-leg resistance. Enter that load in the optional field to see the predicted figure.
What resistor values should I choose?
Pick the ratio for the voltage you need, then pick the scale for the current you can afford to waste. Values in the 10 kΩ to 100 kΩ range suit most low-power signal work.
Can a voltage divider power a small load?
Not reliably. Any load that draws meaningful current will pull the output down and the divider will waste power as heat. Use a voltage regulator instead.
How do I keep the output stable?
Buffer it with an op-amp voltage follower. The follower presents a very high input resistance to the divider and a very low output resistance to the load, so the divider stays essentially unloaded.
Solve the Rest With the Ohms Law Wheel
Open the full Ohms Law Wheel calculator to solve voltage, current, resistance and power from any two known values.