How a voltage divider works
A voltage divider is two resistors in series across a voltage, with the output taken from the point between them. It is the simplest way to make a smaller voltage from a larger one: to read a 12 V battery with a 3.3 V microcontroller, to set a reference voltage, or to scale a sensor. This calculator gives the output voltage, the current and power in each resistor, how much a load pulls the output down, and the R2 you need for a particular output.
How to use the voltage divider calculator
- Enter the input voltage Vin, the voltage across both resistors together.
- Type R1 and R2. R1 is the top resistor (from Vin to the output) and R2 the bottom one (from the output to ground). Write values as 4700, 4.7k,
4k7,470Ror 2.2M. - Add a load if you have one: whatever the output feeds, such as an ADC input or a module. The drawing and the tiles then show the loaded output.
- Want a particular output? Type it and the calculator works out the R2 that gives it with your R1, plus the nearest standard value; one click puts it in.
- Read the results: Vout, the current, the power in R1 + R2 and the ratio, with the maths in the Calculation tab and everything in the Results table. Click a resistor in the drawing for its own calculation.
The voltage divider formula
R1 and R2 are in series, so the same current flows through both: I = Vin / (R1 + R2). The output is the voltage across R2, I × R2:
Vout = Vin × R2 / (R1 + R2)
Your divider Vout = 12 V × 4.7 kΩ / (10 kΩ + 4.7 kΩ) = 3.837 V.
Only the ratio matters for the voltage: 1 kΩ and 2 kΩ give the same output as 10 kΩ and 20 kΩ. The absolute values set the current, the power and how much a load disturbs the output. Turned round, the resistor for a wanted output is:
R2 = R1 × Vout / (Vin − Vout)
Choosing the resistor values
- Pick the ratio first from the output you need, then scale both resistors together.
- Larger values waste less current (10 kΩ + 4.7 kΩ across 12 V draw under 1 mA) but are disturbed more by a load and pick up more noise.
- Smaller values hold the output steadier under load but draw more current and heat up more; check the power column.
- Use standard values: the calculator suggests the nearest E24 (5%) value. Two 1% resistors keep the output within about 2%.
- A potentiometer is an adjustable divider: the wiper is the output tap between the two parts of the track.
What a load does to the output
Anything connected across the output sits in parallel with R2 and lowers it, so Vout falls. With a load RL the formula uses R2 ∥ RL = R2 RL / (R2 + RL) instead of R2. The usual rule of thumb: keep the load at least ten times R2 and the output stays within about 10%. A divider is for sensing and setting voltages, not for powering things: for power use a regulator.
Your divider, step by step
- Add the resistors: R1 + R2 = 10 kΩ + 4.7 kΩ = 14.7 kΩ.
- Find the ratio: R2 / (R1 + R2) = 4.7 kΩ / 14.7 kΩ = 0.3197, so R2 takes 32% of the input.
- Multiply by Vin: Vout = 12 V × 0.3197 = 3.837 V; R1 drops the other 8.163 V.
- Current and power: I = 12 V / 14.7 kΩ = 816.3 µA; P1 = 6.66 mW, P2 = 3.13 mW.
Worked example: 12 V with 10 kΩ and 4.7 kΩ
R1 + R2 = 14.7 kΩ. The ratio is 4.7 / 14.7 = 0.3197, so Vout = 12 V × 0.3197 = 3.837 V. The current is 12 V / 14.7 kΩ = 816.3 µA, R1 dissipates 6.66 mW and R2 3.13 mW: nothing a ¼ W resistor minds. Feed that output into a 100 kΩ ADC input and it becomes 3.718 V, a sag of 3.1%, because 100 kΩ is well over ten times 4.7 kΩ.
Questions
Can a voltage divider give more than the input?
No. The ratio R2 / (R1 + R2) is always below 1, so the output is always below Vin. The target helper says so if you ask for more.
Why does my output read lower than calculated?
Whatever you connected is loading the divider. Put its resistance in the load field to see the real output, or use smaller R1 and R2 (or a buffer op-amp) so the load matters less.
Can I power a 3.3 V board from 5 V with a divider?
Not well. The board's current changes, so the output would wander, and the resistors would waste power. Use a regulator; dividers are for signals and references.
Which resistor should be larger?
The output is across R2, so a larger R2 means a higher output. For an output of exactly half, make them equal; the series resistor calculator shows how any chain of resistors shares a voltage.