What does this calculator do?
A voltage divider uses two resistors in series to produce a fraction of the input voltage. It is used for references, biasing, reading sensors and shifting levels, as long as whatever is connected to the output has a high enough impedance.
Vout = Vin × R2 / (R1 + R2)
R1 goes between Vin and Vout, and R2 between Vout and ground. The output depends on the share of R2 in the total R1 + R2. A load connected to the output sits in parallel with R2 and changes the result.
Worked example
With Vin = 12 V, R1 = 1 kΩ and R2 = 2.2 kΩ, Vout ≈ 8.25 V.
Practical tips
- Consider the impedance of the load connected to Vout.
- Very low values waste current; very high values make the output more sensitive to noise.
- For accurate references or changing loads, add an op-amp buffer.
Common mistake to avoid
A divider’s output changes as soon as the load draws current. For significant loads, include their equivalent resistance.
Complete voltage divider formulas
With R1 between the input and the output and R2 between the output and ground, the same current flows through both resistors (when there is no load). Every design relation follows from that:
- Output voltage: Vout = Vin · R2 / (R1 + R2)
- Divider current: I = Vin / (R1 + R2)
- Solving for R2: R2 = R1 · Vout / (Vin − Vout)
- Solving for R1: R1 = R2 · (Vin − Vout) / Vout
- Input voltage from a measurement: Vin = Vout · (R1 + R2) / R2
- Output resistance (Thévenin): Rth = R1 · R2 / (R1 + R2)
- With a load RL: replace R2 with R2 ∥ RL = R2 · RL / (R2 + RL)
- Power in each resistor: P = I² · R
Typical values
| Vin | R1 | R2 | Vout | Current | Typical use |
|---|---|---|---|---|---|
| 5 V | 10 kΩ | 10 kΩ | 2.50 V | 0.25 mA | Mid-point, biasing |
| 5 V | 1 kΩ | 2 kΩ | 3.33 V | 1.67 mA | Drop a 5 V signal to 3.3 V logic |
| 9 V | 10 kΩ | 4.7 kΩ | 2.88 V | 0.61 mA | Reference from a 9 V battery |
| 12 V | 10 kΩ | 3.3 kΩ | 2.98 V | 0.90 mA | Measure 12 V with a 3.3 V ADC |
| 12 V | 1 kΩ | 2.2 kΩ | 8.25 V | 3.75 mA | Calculator example |
| 24 V | 100 kΩ | 10 kΩ | 2.18 V | 0.22 mA | Monitoring a 24 V supply |
Values assume an unloaded output. High resistances reduce the current drawn but make the output more sensitive to loading and noise.
Worked example: measuring a 12 V battery with a 3.3 V ADC
A 12 V lead-acid battery can reach about 14.4 V while charging, and the divider output must never exceed 3.3 V.
- Maximum ratio: 3.3 / 14.4 ≈ 0.229. Pick a slightly smaller ratio for margin.
- With R1 = 39 kΩ and R2 = 10 kΩ: ratio = 10 / 49 ≈ 0.204.
- At 14.4 V: Vout = 14.4 × 0.204 ≈ 2.94 V. At 12.6 V: ≈ 2.57 V.
- Current drawn: 14.4 / 49 kΩ ≈ 0.29 mA, acceptable for permanent monitoring.
- In firmware, recover the real voltage with Vin = Vout × 4.9.
The output resistance is 39k ∥ 10k ≈ 7.96 kΩ. If your microcontroller’s ADC recommends a lower source impedance, add a 100 nF capacitor from the output to ground. Convert the reading to volts with the ADC calculator.
Common mistakes
- Forgetting the load: with Vin = 12 V and R1 = R2 = 10 kΩ you expect 6 V, but a 10 kΩ load leaves R2 ∥ RL = 5 kΩ and the output drops to 4 V.
- Using it as a power supply: a divider does not regulate; if the load current changes, so does the voltage. Use a regulator to power circuits.
- Swapping R1 and R2: the output is taken across R2, the resistor connected to ground.
- Ignoring tolerance: with ±5 % resistors the ratio can shift by several percent; use ±1 % parts for measurements.
- Not checking power: with low values and high voltages, each resistor can dissipate more than 1/4 W.
Frequently asked questions
Which resistor values should I use?
For signals and measurements, a total of 1 kΩ to 100 kΩ is common: low values waste current and very high values make the output sensitive to loading and noise.
Can I use a divider to go from 5 V to 3.3 V?
Yes, for slow or moderate input signals (for example the TX line of a 5 V module going into a 3.3 V microcontroller). For fast or bidirectional buses, a level shifter is a better choice.
How do I calculate R2 if I already have R1?
Use R2 = R1 · Vout / (Vin − Vout), round to the nearest standard value and check again with this calculator.
Want to understand why the divider works and see applications with sensors and potentiometers? Read the voltage divider guide.
Educational tool. Check tolerances, datasheets, standards and real operating conditions before using the results in critical applications.