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About voltage dividers
A voltage divider is two resistors in series. The voltage in the middle is a fixed part of the input voltage:
Vout = Vin × R2 ÷ (R1 + R2)
R1 goes from Vin to the middle, and R2 from the middle to ground. With 5 V in, R1 = 10 kΩ and R2 = 20 kΩ, Vout = 5 × 20 ÷ 30 = 3.333 V.
This calculator works both ways. Find Vout gives the output from Vin, R1 and R2 — with an optional load on the output. Find a resistor starts from the voltage you want and the one resistor you have: it gives the exact value of the other resistor, the nearest standard value, and the closest standard pairs.
Features
- Find Vout — from Vin, R1 and R2, with the current, the power in each resistor and the output resistance
- Find a resistor — the other resistor for the Vout you want, exact and rounded to E12, E24 or E96
- Closest standard pairs — the five best pairs you can buy, with their real Vout
- Load effect — add the resistance of what you connect and see how much Vout drops
- Logic level check — tells you when Vout fits a 3.3 V input
- Schematic — the divider drawn with your values
- Units — Ω, kΩ or MΩ in the list, or type 4k7 or 10k
5 V to 3.3 V
A common job: send a 5 V signal (the TX pin of an Arduino Uno, a 5 V sensor) into a 3.3 V input (ESP32, ESP8266, Raspberry Pi). A 3.3 V pin reads HIGH above about 2.3 V and should not see more than about 3.6 V.
| R1 | R2 | Vout from 5 V |
|---|---|---|
| 1 kΩ | 2 kΩ | 3.333 V |
| 10 kΩ | 20 kΩ | 3.333 V |
| 1.8 kΩ | 3.3 kΩ | 3.235 V |
| 4.7 kΩ | 10 kΩ | 3.401 V |
| 2.2 kΩ | 3.3 kΩ | 3.000 V |
| 1 kΩ | 1.5 kΩ | 3.000 V |
Small resistors (1 kΩ + 2 kΩ) keep fast signals clean but draw more current: 1.667 mA. Large ones (10 kΩ + 20 kΩ) draw only 0.1667 mA but react more slowly. For a serial line at 9600 or 115200 baud, both work.
Reading a battery or a 12 V supply
An analog input can only read up to its own supply voltage: 5 V on an Arduino Uno, about 3.3 V on an ESP32. To measure more, divide first and multiply back in your code.
| Measure | R1 | R2 | Vout at the top of the range |
|---|---|---|---|
| 9 V battery | 20 kΩ | 10 kΩ | 3.000 V |
| 12 V supply | 15 kΩ | 10 kΩ | 4.800 V |
| 24 V supply | 39 kΩ | 10 kΩ | 4.898 V |
| LiPo cell (4.2 V) | 100 kΩ | 100 kΩ | 2.100 V |
In your code: real voltage = measured voltage × (R1 + R2) ÷ R2. With 20 kΩ and 10 kΩ, that is × 3. Leave some room at the top: a "12 V" supply can give 12.6 V or more.
The load effect
Whatever you connect to Vout acts like a resistor in parallel with R2. It pulls Vout down. With 5 V, R1 = 10 kΩ and R2 = 20 kΩ:
| Load on Vout | Vout | Drop |
|---|---|---|
| none | 3.333 V | — |
| 1 MΩ | 3.311 V | 0.66 % |
| 100 kΩ | 3.125 V | 6.25 % |
| 10 kΩ | 2.000 V | 40 % |
Rule of thumb: the load should be at least 10 to 100 times bigger than R2. Microcontroller inputs are very high (many megaohms), so they are fine.
Signal yes, power no
A divider is fine for signals: logic levels, sensor readings and ADC inputs. It is not a power supply. Vout drops as soon as something draws current, and the resistors waste power all the time. To power a 3.3 V board or a motor from a higher voltage, use a voltage regulator or a buck converter.
Frequently asked questions
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2). R2 is the resistor between the output and ground.
How do I choose R1 and R2?
First the ratio: R2 ÷ (R1 + R2) = Vout ÷ Vin. Then the size: values from 1 kΩ to 100 kΩ suit most signals. Pick one resistor, and let Find a resistor work out the other.
Which resistors turn 5 V into 3.3 V?
1 kΩ and 2 kΩ, or 10 kΩ and 20 kΩ, give 3.333 V. With only E12 values, 1.8 kΩ and 3.3 kΩ give 3.235 V.
Can I power a 3.3 V board from a voltage divider?
No. The board's current makes Vout drop and change all the time. Use a 3.3 V regulator.
Does a divider work on a two-way line?
No. It only lowers a signal that goes one way, from a 5 V output into a 3.3 V input. For two-way lines like I2C, a divider is not enough.
Why is my measured Vout lower than the result?
Something loads the output, or the resistors are off by their tolerance (± 5 % each for E24 parts). Add the load under More options to see its effect.
Related tools
- Ohm's Law Calculator — volts, amps, ohms and watts
- Resistor Color Code Calculator — find R1 and R2 in your box by their colors
- LED Resistor Calculator — the resistor for an LED
- Battery Life Calculator — how long the battery you measure will last
- Ratio Calculator — the same idea with plain numbers
How to use it
- Pick Find Vout or Find a resistor
- Find Vout: type Vin, R1 and R2 (pick Ω, kΩ or MΩ, or type 4k7)
- Find a resistor: type Vin, the Vout you want and the resistor you have
- Add a load under More options to see how Vout drops
- Or tap an example like 5 V → 3.3 V
