How it works
The voltage divider is two resistors in series. The output is taken from the midpoint:
Vout = Vin × R2 / (R1 + R2)Current flows from Vin through R1 and R2 to ground continuously — this is the quiescent current. For battery-powered designs, minimise it by choosing high resistor values (100 kΩ–1 MΩ range).
Choosing resistor values
The ratio matters more than the absolute values. R1 = 10 kΩ, R2 = 10 kΩ gives the same output as R1 = 100 kΩ, R2 = 100 kΩ, but the second pair draws 10× less current.
The tradeoff: high resistance increases output impedance, which causes voltage sag when a load is connected. An ADC input draws negligible current (typically < 1 µA), so a 100 kΩ divider is fine. A linear regulator feedback network usually needs 10–100 kΩ. A base bias network for a transistor needs much lower values.
Rule of thumb: The divider’s output impedance (R1 ∥ R2) should be at least 10× lower than the input impedance of the load.
E-series resistors
Resistors come in standard E-series values — E12, E24, E48, E96. E24 covers most needs with 5% tolerance resistors. E96 is for precision work with 1% parts. The calculator shows the nearest standard value for both R1 and R2 so you pick parts that are actually stocked.
Tolerance impact
With 1% resistors and a 3.3 V to 1.65 V divider (R1 = R2 = 10 kΩ):
Worst-case Vout_min = 3.3 × (9.9 kΩ) / (10.1 kΩ + 9.9 kΩ) = 1.634 V
Worst-case Vout_max = 3.3 × (10.1 kΩ) / (9.9 kΩ + 10.1 kΩ) = 1.666 VThat’s ±1.9% — just under the worst-case tolerance of the resistors. For ADC reference dividers where you need < 0.5% accuracy, use 0.1% resistors or a precision voltage reference instead.
Common use cases in embedded systems
Battery voltage monitor: Divide a 4.2 V LiPo down to 3.3 V ADC range. Use high resistor values (100 kΩ + 100 kΩ) so the divider draws only 21 µA continuously. Add an enable transistor to disconnect entirely in deep sleep.
Level translation (analogue): Scale a 5 V sensor output to 3.3 V ADC input. R1 = 10 kΩ, R2 = 20 kΩ gives exactly 3.33 V, which is within the ADC’s 3.3 V full-scale range (3.3 V × 20/(10+20) = 2.2 V — wait, check your maths). If the 5 V signal drives to 4.5 V (logic high swing), do the calculation for the worst case.
Feedback for LDO regulators: Regulators like TLV3103 or ADP3338 set their output voltage via a feedback resistor divider to their internal reference voltage. The datasheet specifies R_top and gives a formula. The calculator helps you find standard resistor pairs.
Common mistakes
Forgetting the load. A voltage divider is not a voltage source. Connect an MCU GPIO (input impedance 100 kΩ typical) to a 47 kΩ divider and the actual output voltage drops measurably. Always check R_load >> R1 ∥ R2.
Using a voltage divider where a regulator belongs. If you need to power a sensor from a higher supply, do not use a voltage divider — use an LDO or buck converter. The divider voltage sags under load and varies with current draw.
Not accounting for input impedance variation. ADC input impedance on STM32 varies with the sample-and-hold charging time and sample time setting. The STM32 reference manual specifies R_AIN_max (e.g. 50 kΩ for STM32G0). Keep your divider impedance well below this.