Reference · updated 10 November 2026
Voltage divider calculator: Vout, R1 and R2
A voltage divider is two resistors in series across a supply. The output is taken from the point between them, and it is a fixed fraction of the input: Vout = Vin × R2 ÷ (R1 + R2). Enter Vin, R1 and R2 and the calculator gives Vout, the current through the divider and the power in each resistor. Enter a wanted Vout instead of R2 and it solves R2.
iPhone and iPad (iOS 17 or later): free on the App Store. Android later.
Voltage divider calculator
This calculator needs JavaScript. Without it, use the formulas and the worked examples below.
R1 is the resistor from Vin to the output, R2 the resistor from the output to ground. Pick the unit of each value in its own box (mV, V, kV, Ω, kΩ or MΩ). A decimal point or a decimal comma both work. Fill in Vin and R1, then either R2 (to get Vout) or the wanted output voltage (to get the R2 that gives it); entering both is refused. The load box is optional: it puts a resistance in parallel with R2, see the section on loading below.
The formulas
For an unloaded divider, nothing connected to the output:
Vout = Vin × R2 ÷ (R1 + R2)
The same current flows through both resistors:
I = Vin ÷ (R1 + R2)
The power in each resistor is P = I² × R, so P1 = I² × R1 and P2 = I² × R2. To find R2 for a wanted output with a given R1:
R2 = R1 × Vout ÷ (Vin − Vout)
This only works when Vout is lower than Vin; a divider cannot raise a voltage.
What the formulas are not: they are for an unloaded divider. Anything connected to the output, such as a circuit input or a meter, is in parallel with R2 and lowers Vout. They also use nominal values; two 1 % resistors can move the ratio by about 1 %.
What a load does
With a load resistance RL across R2, the lower leg is no longer R2 but R2 and RL in parallel: R2' = R2 × RL ÷ (R2 + RL). Use R2' in place of R2 in the formula above. The output always drops, and the lighter the load (the larger RL), the smaller the drop. As a rule of thumb, make RL at least ten times R2 if the output must stay close to the unloaded value. In the calculator, enter the load in the last box; it then shows both the loaded output and the output without load.
Worked examples
Unloaded divider. Vin = 12 V, R1 = 10 kΩ, R2 = 5 kΩ. Vout = 12 × 5 ÷ 15 = 4 V. The current is 12 ÷ 15 000 = 0.8 mA, the power in R1 is 0.0008² × 10 000 = 6.4 mW and in R2 3.2 mW.
| Vin | R1 | R2 | Vout | Current | P in R1 | P in R2 |
|---|---|---|---|---|---|---|
| 12 V | 10 kΩ | 5 kΩ | 4 V | 0.8 mA | 6.4 mW | 3.2 mW |
Solving R2. A 3.3 V logic input from a 5 V supply, with R1 = 10 kΩ. R2 = 10 000 × 3.3 ÷ (5 − 3.3) = 19.41 kΩ. The nearest standard value is 18 kΩ or 20 kΩ, which gives 3.21 V or 3.33 V. The current is 5 ÷ 29 412 = 0.17 mA.
| Vin | R1 | Wanted Vout | R2 needed | Current |
|---|---|---|---|---|
| 5 V | 10 kΩ | 3.3 V | 19.41 kΩ | 0.17 mA |
The same divider with a load. Vin = 12 V and R1 = R2 = 10 kΩ give 6 V unloaded. With a 10 kΩ load across R2, the lower leg is 10 000 × 10 000 ÷ 20 000 = 5 kΩ, so Vout = 12 × 5 ÷ 15 = 4 V instead of 6 V. With a 100 kΩ load the output is 5.71 V.
| R1 | R2 | Load | Vout unloaded | Vout loaded |
|---|---|---|---|---|
| 10 kΩ | 10 kΩ | 10 kΩ | 6 V | 4 V |
| 10 kΩ | 10 kΩ | 100 kΩ | 6 V | 5.71 V |
Measure it live
To check a divider, connect a Bluetooth multimeter the app reads live, such as the OWON OW18B, and measure Vin across the pair and Vout across R2 with the circuit powered. The meter's own input resistance is in parallel with R2, so on a divider of high resistances check the meter's specified input impedance before trusting the reading. Switch to ohms with the power off to read R1 and R2 on their own.
The calculator in the app has preset formulas for Ohm's law and power, series and parallel, dividers, LED resistors, RC, dB and unit conversion, and a parallel operator for typing a sum directly. Cal Box fills it from your live reading and recent captures. For the current and power of any single resistor, use the Ohm's law calculator; for the lower leg of a loaded divider by hand, the parallel resistor calculator.
What the app does here, and what it doesn't
| What it does | Plan |
|---|---|
| A calculator with SI prefix keys, engineering input such as 4k7 and a parallel operator. | Free |
| Presets for Ohm's law and power, series and parallel, dividers, LED resistors, RC, dB and unit conversion. | Free |
| Cal Box fills the calculator from your live reading and recent captures, and suggests Ohm's law. | Free |
Questions
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), where R1 is the resistor between Vin and the output and R2 the resistor between the output and ground. 12 V across 10 kΩ and 5 kΩ gives 4 V.
How do I choose R1 and R2 for a wanted output voltage?
Pick R1 first, large enough to keep the current and the power low, then use R2 = R1 × Vout ÷ (Vin − Vout). For 3.3 V from 5 V with R1 = 10 kΩ, R2 is 19.41 kΩ; the nearest standard values give 3.21 V (18 kΩ) or 3.33 V (20 kΩ).
Why is my divider output lower than the calculated value?
The formula is for an unloaded divider. Whatever is connected to the output is in parallel with R2 and pulls Vout down. Enter the load resistance in the calculator to see the loaded value, or make R1 and R2 smaller so the load matters less (at the cost of more current).
Can I use a voltage divider as a power supply?
Not for a load that draws real current, because the output falls as the load current rises and the resistors waste power all the time. A divider suits a signal, a reference or a measurement input that draws almost no current.
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