Voltage divider calculations are easy to explore with this interactive tool. Change the input voltage Vin, upper resistor R1, or lower resistor R2 to update the output voltage Vout, voltage ratio, currents, and circuit diagrams instantly.
You can also connect a load resistor RL to see why a circuit that gives 2.5 V on paper may deliver a lower voltage in practice. If you only need a result, enter your values in the calculator below.
Voltage divider calculator
The starting values are Vin = 5 V, R1 = 10 kΩ, and R2 = 10 kΩ. Without a load, Vout is 2.5 V. Enable the load resistor RL to calculate the voltage drop caused by a connected circuit.
The diagram shows the ideal DC source, R1, R2, Vout, and GND and their connections. Enabling the load adds RL between Vout and GND. Currents appear below as values and equations rather than arrows.
Inputs
Results and linked diagrams
How to use the calculator
- Set the input voltage Vin.
- Set the upper resistor R1 and lower resistor R2 in kΩ.
- Watch Vout, the voltage ratio, currents, and diagrams change.
- Enable load resistor RL to model a connected circuit.
Try the presets for a half-voltage divider, approximately 3.3 V or 1.8 V from 5 V, and the effect of a load.
Voltage divider formula and calculation
Connect R1 and R2 in series across Vin and take Vout from their junction. With no load connected, the output voltage is:
Vout = Vin × R2 / (R1 + R2)
The series current is I = Vin / (R1 + R2). Vout is the voltage across R2, so Ohm’s law gives Vout = I × R2, yielding the divider formula above.
When R1 and R2 are equal
If R1 = R2, Vout is half of Vin. For example, Vin = 5 V with R1 = R2 = 10 kΩ gives Vout = 2.5 V. This result assumes no significant current is drawn from the output.
Why a load resistor lowers Vout
When you connect another circuit to Vout, its input resistance RL is in parallel with R2. The effective lower resistance Req is:
Req = R2 ∥ RL = (R2 × RL) / (R2 + RL)
With a load, Vout = Vin × Req / (R1 + Req). Because Req is smaller than R2, Vout falls below its no-load value.
For Vin = 5 V and R1 = R2 = RL = 10 kΩ, Req is 5 kΩ and Vout is about 1.67 V rather than 2.5 V. Use the “Show load effect” preset to check this.
What is the Thevenin resistance Rth?
If the source is set to 0 V, the resistance seen looking into Vout is Rth = R1 ∥ R2. When Rth is not small relative to RL, loading cannot be ignored. The calculator displays Rth alongside Vout.
Examples: deriving about 3.3 V and 1.8 V from 5 V
Example: about 3.3 V from 5 V
With R1 = 5.1 kΩ and R2 = 10 kΩ, Vout is about 3.31 V. Actual values depend on resistor tolerances and the input resistance of the connected circuit; check the allowed voltage range.
Example: about 1.8 V from 5 V
With R1 = 18 kΩ and R2 = 10 kΩ, Vout is about 1.79 V. If you need an accurate 1.8 V, account for resistor tolerances when choosing standard values.
Frequently asked questions
If R1 equals R2, is Vout always half of Vin?
Only with no load or a sufficiently high load resistance. A lower RL reduces the effective resistance of R2 and pulls Vout below half of Vin.
Can a voltage divider produce a voltage higher than Vin?
No. A passive resistor divider produces a voltage between 0 V and Vin. To increase voltage, use a boost converter or another suitable circuit.
Are larger resistor values always better for saving power?
They reduce steady current but increase output resistance. Loading, leakage current, noise, and ADC acquisition time then matter more. Choose absolute values based on the circuit you are driving, not just the ratio.
Why do measured and calculated voltages differ?
Common causes include resistor tolerance, meter or ADC input resistance, input leakage, wiring resistance, supply variation, and temperature. Connecting a meter can itself add a load.
Summary
- Without a load, use Vout = Vin × R2 / (R1 + R2).
- With RL connected, replace R2 by R2 ∥ RL; Vout decreases.
- For ADC inputs, check Rth and acquisition time as well as the voltage ratio.
- Do not use a resistor divider as a power supply for a varying load.
Use the calculator above to explore how R1, R2, and RL affect Vout. For a real circuit, also consult the data sheets of the components and connected devices.
Disclaimer
This article and calculator use ideal DC assumptions and are intended for learning and rough estimates. They do not fully account for resistor tolerance, temperature behavior, parasitic effects, supply changes, input leakage, transients, or component ratings. For products, equipment, or safety-critical circuits, consult the relevant data sheets and verify the design thoroughly. This site accepts no liability for losses arising from use of this article or tool.
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