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Voltage Divider Calculator

Calculate output voltage from R1 and R2, or find the right resistor pair for a desired output โ€” with schematic diagram and E24 suggestions

Mode:
Vin R1 Vout R2
V
Ω
Ω

Results

Enter values above to see results
Output Voltage
Vout = Vin × R2 / (R1 + R2)
Find R1
R1 = R2 × (Vin − Vout) / Vout
Current
I = Vin / (R1 + R2)

About This Tool

A voltage divider is two resistors in series that produce an output voltage that is a fraction of the input. It's one of the most fundamental circuits in electronics โ€” used for level shifting, sensor reading, biasing transistors, and creating reference voltages. This calculator works in two directions. In Forward mode, enter Vin, R1, and R2 to calculate Vout, current draw, and power dissipation. In Reverse mode, enter Vin and your desired Vout, and the calculator suggests standard E24 resistor pairs that achieve the closest match โ€” saving you from trial and error. The interactive schematic updates in real time so you can visualize the circuit as you adjust values. Whether you're designing a 3.3V level shifter from a 5V supply or reading a battery voltage with a microcontroller ADC, this tool gives you the resistor values and all the numbers you need.

How to Use

1. Choose Forward mode (known resistors) or Reverse mode (target voltage) 2. Forward: enter Vin, R1, and R2 โ€” see Vout, current, and power 3. Reverse: enter Vin and desired Vout โ€” see suggested E24 resistor pairs 4. Click a suggested pair to load it into the calculator 5. Use "Copy Link" to save or share the calculation

Formula

Vout = Vin ร— R2 / (R1 + R2) Current = Vin / (R1 + R2) Power(R1) = Iยฒ ร— R1 Power(R2) = Iยฒ ร— R2 Ratio = R2 / (R1 + R2)

Frequently Asked Questions

What is a voltage divider?
A voltage divider is a simple circuit with two resistors in series connected between a voltage source and ground. The output voltage is taken from the junction between the two resistors. The formula is Vout = Vin ร— R2 / (R1 + R2), where R1 is the top resistor (connected to Vin) and R2 is the bottom resistor (connected to ground).
How do I calculate voltage divider resistor values?
If you know Vin and your desired Vout, first calculate the ratio: R2/(R1+R2) = Vout/Vin. Then pick R2 (e.g., 10 kฮฉ) and calculate R1 = R2 ร— (Vin โˆ’ Vout) / Vout. For example, to get 3.3V from 5V: ratio = 0.66, pick R2 = 10 kฮฉ, then R1 = 10k ร— (5 โˆ’ 3.3) / 3.3 = 5.15 kฮฉ. Use the closest standard value: 5.1 kฮฉ.
How do I step down 5V to 3.3V with a voltage divider?
Use R1 = 5.1 kฮฉ and R2 = 10 kฮฉ. This gives Vout = 5 ร— 10,000 / (5,100 + 10,000) = 3.31V, very close to 3.3V. The current draw is about 0.33 mA, which is negligible. For a more precise output, use R1 = 1.7 kฮฉ and R2 = 3.3 kฮฉ (Vout = 3.30V exactly).
Does a voltage divider reduce current?
A voltage divider does not regulate current โ€” it reduces voltage. The output current capacity is very limited because any load connected to Vout changes the effective R2 (load in parallel with R2). For loads drawing significant current, use a voltage regulator instead. Voltage dividers are best for signal-level applications (ADC inputs, bias networks) where the load current is negligible.
What values should I use for R1 and R2?
Common values are in the 1 kฮฉ โ€“ 100 kฮฉ range. Too low (under 100 ฮฉ) wastes power and heats up. Too high (over 1 Mฮฉ) makes the circuit sensitive to noise and leakage. For microcontroller ADC inputs, 10 kฮฉ total is a good starting point. For audio circuits, 47 kฮฉ โ€“ 100 kฮฉ is typical.
Can I use a voltage divider with AC?
Yes, a resistive voltage divider works with AC signals. The output amplitude is reduced by the same ratio R2/(R1+R2). However, for AC applications, capacitive dividers or transformer-based circuits are often more efficient. Note that a resistive divider on AC will dissipate power continuously.
What is the voltage divider ratio?
The ratio is R2 / (R1 + R2), which equals Vout / Vin. A ratio of 0.5 means Vout is half of Vin (equal resistors). A ratio of 0.33 means Vout is one-third of Vin (R1 is twice R2). The ratio is always between 0 and 1 โ€” a passive voltage divider can only reduce voltage, never increase it.

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