Free calculator
Voltage divider calculator
Two resistors in series split a voltage. The output, taken across the bottom resistor, is a fixed fraction of the input set by their ratio. Find Vout from your resistors, or pick a Vout and get the resistor that gives it.
Across both resistors.
4.7k = 4,700 Ω.
Vout is the voltage across this one.
Whatever you connect to Vout. Leave empty for none.
Output voltage, Vout = Vin × R2 ÷ (R1 + R2)
- Ratio, R2 ÷ (R1 + R2)
- 0.333
- Current through R1
- 400 µA
- Power from the supply
- 4.8 mW
The formula
The same current flows through both resistors, so each takes a share of the voltage in proportion to its resistance. To get a Vout you want, rearrange it for R2:
A worked example
To read a 12 V battery on a 5 V Arduino input, scale it down by 3. With R1 = 20 kΩ and R2 = 10 kΩ:
Only 12 ÷ 30 kΩ = 0.4 mA flows through the pair, so it barely drains the battery.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Voltage dividers” is lesson 7 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 13 before it first.
Or start from the very first lesson, no account neededThe ideas behind it
- Voltage divider explainedTwo resistors that split a voltage: the formula, potentiometers, and why a load drags the output down.
- How to measure a 12 V battery with a voltage dividerScale 12 V down to 4 V for a meter or a microcontroller, without draining the battery.
- Ohm's law explainedV = I × R: set any two of voltage, current and resistance and the third follows.
Questions people ask
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), where R1 is the top resistor (to Vin) and R2 is the bottom one (to ground). Vout is the voltage across R2.
Why is my divider's output lower than the formula says?
Whatever you connect to the output sits in parallel with R2 and lowers it. Keep the load at least ten times bigger than R2, or buffer the output with an op-amp. Put the load into the calculator to see the drop.
What resistor values should I use?
The ratio sets Vout; the size sets the current. 10 kΩ-range resistors suit most sensors and Arduino inputs: a few hundred microamps flow, and a high-impedance input barely loads it.
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