Voltage Divider Calculator
Calculate the output voltage of a two-resistor voltage divider, with the current drawn and the power in each resistor.
Two resistors turn one voltage into a smaller one — this is the number.
How the voltage divider calculator works
Two resistors in series across a supply split the voltage in proportion to their resistances. The voltage across the lower resistor is Vout = Vin × R₂ ÷ (R₁ + R₂). It is the standard way to scale a signal for a microcontroller input or set a reference voltage.
A divider only holds its ratio if the load draws much less current than flows through the divider itself — pick resistances about ten times smaller than the load, and check the power each one dissipates.
Formula: Vout = Vin × R₂ ÷ (R₁ + R₂)
Worked examples
| Inputs | Output voltage | Note |
|---|---|---|
| 12 V, 10 kΩ and 4.7 kΩ | 3.837 V | 3.84 V |
| 5 V, two 10 kΩ | 2.5 V | equal resistors halve it: 2.5 V |
| 24 V, 22 kΩ and 3.3 kΩ | 3.13 V | 3.13 V — into a 3.3 V input |
FAQFrequently asked questions
Why not use a divider to power something?
Because the output sags as soon as the load draws current, and the divider wastes power all the time. Use a regulator for power; dividers are for signals and references.
How do I choose R₁ and R₂?
Fix the ratio for the voltage you want, then scale both so the divider current is 10–100 times the load current — usually a few kΩ to a few hundred kΩ.
What tolerance does the output have?
Roughly the sum of the resistor tolerances: two 5% resistors can shift the output by up to about 5% of the ratio. Use 1% parts for references.
Can I add a third resistor?
Yes — the output across any resistor is Vin × that resistance ÷ the total. The same formula generalises.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.