Contents
How the voltage divider calculator works
Enter the source voltage and the values of the upper and lower resistors in kilohms. The calculator gives the unloaded output voltage, circuit current, voltage drop across R1, total dissipation, and division ratio.
The ratio set by R1 and R2
The output is measured across R2, from the junction of the resistors to ground. For an ideal source and no connected load, the relationship is:
The same current passes through both resistors. With resistance entered in kΩ, produces milliamps directly. Total resistor power in milliwatts is .
Three quick circuit checks
- 12 V, 10 kΩ, and 20 kΩ: the output is 8 V, current is 0.4 mA, and the pair dissipates 4.8 mW. R2 receives two thirds of the input.
- 5 V with two 10 kΩ resistors: equal values produce 2.5 V. Current is 0.25 mA and total power is 1.25 mW.
- 15 V, 40 kΩ, and 10 kΩ: the ratio is 0.2, so the unloaded output is 3 V. The circuit continuously draws 0.3 mA.
A connected load changes the divider
This result assumes infinite load resistance. A real input sits in parallel with R2, lowers the effective lower resistance, and usually pulls the output below the calculated value. The effect becomes important when the load is not much larger than the divider's output resistance.
A divider is useful for reference and sensing signals, but it is usually a poor power supply. Check resistor tolerance, individual power ratings, input leakage, and voltage limits before building the circuit.
Voltage-divider questions
The short answers below cover resistor order, scaling, loading, and power.
Which resistor is R1?
R1 connects the source to the output junction. R2 connects that junction to ground, and the output is measured across R2.
Can the output exceed the input?
Not with two positive resistors in this passive unloaded circuit. The ratio R2 divided by R1 plus R2 remains between zero and one.
What happens if I multiply both resistors by ten?
The unloaded output voltage stays the same because the ratio is unchanged. Current and total dissipation become ten times smaller.
Can I power an LED from a voltage divider?
Usually not. An LED is a nonlinear load and changes the divider voltage. Use a current-limiting resistor or a suitable driver instead.
How do I include a load resistance?
Combine the load in parallel with R2, then use that equivalent resistance as the lower branch. This basic calculator intentionally shows only the unloaded case.
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