Watch: Voltage Dividers Explained in 4 Minutes
Numbers on a page only tell part of the story. A short video showing the circuit on a breadboard, a multimeter on Vout, and what happens the moment a load is clipped in makes the loading effect click in a way the formula alone doesn't.
Suggested video topics to film or source for this page:
- Building an unloaded divider and measuring Vout against the calculator above
- Clipping in loads of different sizes and watching Vout sag in real time
- A 5V-to-3.3V logic level shifter test between an Arduino and an ESP32
- Reading an LDR or thermistor through a divider on an oscilloscope
What is a voltage divider?
A voltage divider is a passive circuit that turns an input voltage into a smaller output voltage. It's built from two resistors in series, with the output taken from the point between them.
It's one of the first circuits most people build, and one of the most common. Biasing a transistor, reading a sensor with an Arduino, or setting a reference level into an op-amp all lean on the same divider rule: two resistors, one output node, no active components required.
The voltage divider formula
The formula comes straight from Ohm's law. In a series circuit the current is the same through both resistors, so the voltage across each one is proportional to its resistance.
Unloaded divider
With nothing connected to the output, the math is simple:
Vout = Vin × [ R2 / (R1 + R2) ]
Where:
- Vin = source voltage
- R1 = the upper resistor, connected to Vin
- R2 = the lower resistor, connected to ground
Loaded divider
Connect a load (RL) across R2, such as a sensor or an input pin with some leakage, and it sits electrically in parallel with R2. That lowers the effective resistance of the bottom half of the divider, and Vout drops below what the unloaded formula predicts. This is the "loading effect," and it catches a lot of people the first time they measure a divider under real conditions instead of on paper.
Req = (R2 × RL) / (R2 + RL)
Vout(loaded) = Vin × [ Req / (R1 + Req) ]
A rule of thumb: keep RL at least 10 times larger than R2 if you want the loaded and unloaded results to stay close.
Common application: logic level shifting
A lot of 3.3V microcontrollers, including the ESP32 and Raspberry Pi, share a board with 5V sensors or older logic. Wiring a 5V output straight into a 3.3V input can damage the pin, so the signal needs to come down first.
A simple resistive divider handles this. A 1kΩ upper resistor and a 2kΩ lower resistor bring 5V down to about 3.33V, which is close enough for most digital inputs. It's not a substitute for a proper level shifter IC in every case (fast signals and bidirectional lines need more care), but for a one-way, low-speed line it's often all you need.
Practical applications
- Sensor reading. LDRs and thermistors change resistance with light or temperature. Put one in a divider and a microcontroller can read the shift as a changing voltage.
- Level shifting. Stepping a 5V signal down to 3.3V so it doesn't damage a lower-voltage input pin.
- Potentiometers. A volume knob is a variable voltage divider, adjusting how much signal reaches the amplifier.
- Reference levels. Setting a bias or midpoint voltage for an op-amp input.
Frequently asked questions
How do I calculate voltage drop across a resistor?
Use Vdrop = Vtotal × (R / Rtotal) for a series circuit. If you already know the current, you can also subtract Vout from Vin to find the drop across R1 directly.
Does the load affect Vout?
Yes, and often more than people expect. The load sits in parallel with R2, which pulls down the resistance on the bottom half of the divider and drags Vout down with it. Keeping the load resistance well above R2, at least 10 times higher, keeps the sag small.
Can I use a voltage divider as a power supply?
Not a good idea. The resistors burn power as heat (I²R losses), and Vout will sag whenever the load current changes. For actual power delivery, use a linear regulator like the LM317 or a buck converter instead. A divider is fine for a reference or a signal, not for running a load that needs real current.
Why is my measured Vout lower than the calculator shows?
Almost always the loading effect. If a multimeter, a microcontroller pin, or any other device is connected to the output while you measure, it's acting as RL and pulling the reading down. Try the loaded formula above instead of the unloaded one to check your math.
What resistor values should I pick?
Lower resistor values give a stiffer divider (less sag under load) but burn more constant current and more power. Higher values save power but are more sensitive to loading and to any leakage current at the output pin. 1kΩ to 10kΩ is a common working range for signal-level dividers; go lower if you need to drive a real load, higher if power consumption matters more than accuracy.