Voltage Divider Calculator
Loaded and unloaded divider output voltage
Required Parameters
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Quick Answer
A voltage divider uses two resistors in series to step down an input voltage to a lower output voltage (Vout = Vin × R2 / (R1 + R2)).
Voltage Divider Calculator — Engineering Guide
Use this interactive voltage divider calculator to determine the output voltage (Vout) of a resistive divider network. The voltage divider is one of the most frequently used circuits in electronics, found in sensor interfaces, level shifting, biasing networks, and feedback loops.
Voltage Divider Formula
The fundamental voltage divider equation is:
Vout = Vin × R2 / (R1 + R2)
Where:
- Vin = Input (source) voltage
- R1 = Top resistor (connected to Vin)
- R2 = Bottom resistor (connected to ground)
- Vout = Output voltage (measured across R2)
How It Works
A voltage divider works by distributing the input voltage across two series resistors in proportion to their resistance values. The output voltage is taken from the junction between R1 and R2.
The key insight is that current flows equally through both resistors (since they are in series), so the voltage drop across each resistor is proportional to its resistance.
Practical Design Examples
5V to 3.3V Level Shifting
To interface a 5V microcontroller output with a 3.3V input:
- Target ratio: Vout/Vin = 3.3/5 = 0.66
- Choose R2 = 10 kΩ
- Calculate R1: R1 = R2 × (Vin/Vout − 1) = 10k × (5/3.3 − 1) = 5.15 kΩ
- Nearest standard value: 5.1 kΩ (E24 series)
- Actual Vout = 5 × 10k / (5.1k + 10k) = 3.31V ✓
ADC Reference Voltage
Creating a 2.5V reference from a 5V supply for an ADC:
- Use two equal resistors: R1 = R2 = 10 kΩ
- Vout = 5 × 10k / (10k + 10k) = 2.5V
- Add a 100 nF bypass capacitor at the output for noise filtering
Sensor Biasing
A thermistor (NTC) voltage divider for temperature sensing:
- Place the thermistor as R2 (variable element)
- Choose R1 equal to the thermistor's nominal resistance at 25°C
- This provides maximum sensitivity around the nominal temperature
Loading Effects
Critical Warning: A voltage divider's output voltage changes when a load is connected. The load resistance appears in parallel with R2, reducing the effective R2 value and lowering Vout.
For accurate results, ensure the load impedance is at least 10× greater than R2. For example, if R2 = 10 kΩ, the load should be ≥ 100 kΩ.
Loaded output formula:
Vout(loaded) = Vin × (R2 ∥ RL) / (R1 + R2 ∥ RL)
Where R2 ∥ RL = (R2 × RL) / (R2 + RL)
Design Guidelines
- Keep total resistance reasonable — Too low wastes power (I = Vin/(R1+R2)); too high makes the circuit sensitive to noise and loading.
- Typical range: 1 kΩ to 100 kΩ for most applications.
- Use 1% tolerant resistors for precision applications — standard 5% resistors can cause significant output error.
- Add a buffer amplifier (voltage follower/op-amp) if the load impedance is comparable to R2.
- Consider temperature coefficients — matched TCR resistors maintain ratio accuracy over temperature.
Common Applications
- Microcontroller ADC inputs — Scale sensor voltages to the ADC range
- Feedback networks — Set gain in op-amp and regulator circuits
- Biasing circuits — Set operating points for transistors and comparators
- Level translation — Interface between different voltage logic levels
- Potentiometers — A pot is essentially an adjustable voltage divider
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Design Notes
When designing voltage dividers for ADC inputs or logic-level translation, always consider the 'Load'. If you pull current from the output node (a loaded divider), the effective bottom resistance decreases, dropping the output voltage. A rule of thumb is to ensure the standing current through the divider is at least 10 times the expected load current to maintain voltage stability. If you need low power and high stability, buffer the divider with a voltage follower (Op-Amp).
Common Mistakes
- 1
Swapping R1 and R2 in the equation, leading to an inverted ratio.
- 2
Ignoring the load resistance, causing the actual voltage to drop significantly when connected to a circuit.
- 3
Using excessively high resistor values (e.g., 10MΩ+) feeding into an ADC, which causes slow settling times and impedance mismatches.
Engineering Handbox
1. Calculate parallel resistance of R2 and Load (RL): Req = (R2 × RL) / (R2 + RL) = (20k × 100k) / 120k = 16.67kΩ 2. Apply divider formula: Vout = Vin × [ Req / (R1 + Req) ] 3. Vout = 5V × [ 16.67k / (10k + 16.67k) ] = 3.125V
Knowledge Base
What is the voltage divider formula?
Vout = Vin × R2 / (R1 + R2). R1 is the top resistor (connected to Vin) and R2 is the bottom resistor (connected to ground). The output voltage is always a fraction of the input voltage.
What happens if I add a load to the divider?
Adding a load resistor (RL) in parallel with R2 reduces the effective bottom-leg resistance and lowers the output voltage. The loaded formula becomes: Vout = Vin × (R2∥RL) / (R1 + R2∥RL), where R2∥RL = (R2×RL)/(R2+RL). Use our 'Loaded' mode to account for this.
Can I use a voltage divider as a power supply?
No. Voltage dividers are signal-level circuits, not power supplies. The output voltage will collapse under load because the divider cannot maintain voltage regulation. Use an LDO voltage regulator (like AMS1117) or a buck converter for powering devices.
Can I use a potentiometer as a voltage divider?
Yes. A potentiometer is essentially a variable voltage divider. Connect the input voltage to one outer pin, ground to the other, and take the output from the wiper (center pin). Rotating the wiper adjusts the R1/R2 ratio continuously.
How do I convert 5V to 3.3V with a voltage divider?
Use R1 = 10 kΩ and R2 = 20 kΩ. Calculation: Vout = 5 × 20k / (10k + 20k) = 3.33V. This works for slow, low-current signals (like ADC inputs) but NOT for powering 3.3V devices — use a voltage regulator for that.
Which resistor is R1 and which is R2?
By convention, R1 is the 'top' resistor connected between Vin and the output node. R2 is the 'bottom' resistor connected between the output node and ground. Swapping them in the formula gives the inverted (wrong) voltage — this is the most common voltage divider mistake.
How do I choose resistor values for a voltage divider?
The ratio R2/(R1+R2) determines Vout. For absolute values: use 1kΩ-100kΩ range. Lower values waste power but are more immune to loading. Higher values save power but are noise-sensitive. Rule of thumb: divider current should be ≥10× the expected load current.
How much current flows through a voltage divider?
The standing current is I = Vin / (R1 + R2). For example, with Vin = 12V, R1 = 10kΩ, R2 = 10kΩ: I = 12/20000 = 0.6 mA. This current flows continuously, even with no load — a key consideration for battery-powered circuits.
Does a voltage divider work with AC signals?
Yes, for purely resistive dividers with low-frequency AC. At high frequencies, parasitic capacitance across R2 creates a low-pass filter effect. For precision AC division, use a compensated probe-style divider (capacitor in parallel with R1) to maintain flat frequency response.
Why is my voltage divider output different from the calculated value?
Common causes: (1) Load impedance pulling the voltage down, (2) Resistor tolerance (±5% resistors can shift the ratio significantly), (3) Multimeter input impedance (~10MΩ) loading high-impedance dividers, (4) Temperature-induced resistance change (TCR). Use our Loaded mode and 1% tolerance resistors for accuracy.
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