Voltage Divider Calculator

1. 5 V to 3.3 V level shift — Vin 5 V · R1 1.8 kΩ · R2 3.3 kΩ → Vout ≈ 3.24 V. Suitable for an ADC input where the source is push-pull and the signal is slower than ~10 kHz.
2. Battery monitor — Vin 12 V · R1 100 kΩ · R2 33 kΩ → Vout ≈ 2.98 V on a 3.3 V ADC. Total current draw is just 90 µA — negligible for battery life.
3. Loaded divider warning — Vin 9 V · R1 10 kΩ · R2 10 kΩ → Vout = 4.5 V unloaded. Add a 5 kΩ load and Vout drops to 3.0 V — a 33 % error. Lower R values or buffer with an op-amp.

Two resistors in series share the same current. As that current flows through them, each resistor drops a share of the voltage proportional to its size. Vout is the voltage at the point between them, and it works out to Vin × R2 / (R1 + R2). That's the whole formula.

The formula assumes nothing is connected at Vout. As soon as you hook something up — say, an Arduino input pin — that thing pulls a little current and drags Vout down. The calculator's optional Rload field shows you the real Vout once a load is attached, along with the power each resistor is burning.

When the load barely sips current (an ADC pin, an op-amp input, a MOSFET gate), the drop is tiny and you can ignore it. When the load draws meaningful current, you have two choices: use smaller resistors so the divider is “stiffer”, or feed Vout into a unity-gain op-amp that copies the voltage but supplies all the load current itself.

• Make the load at least 10× bigger than your resistors — If R1 and R2 are 10 kΩ each, the load should be 100 kΩ or more. Otherwise Vout will sag noticeably.
• For sensing, use values between 10 kΩ and 100 kΩ — Smaller wastes battery; larger gets noisy and picks up interference. This range is the sweet spot for most projects.
• Check the power before you build it — A 12 V divider with two 100 Ω resistors burns over half a watt nonstop. Tiny resistors will scorch in a sealed enclosure.
• Use 1 % resistors when precision matters — Cheap 5 % resistors are fine for pull-ups and LED dimming. For ADC inputs and reference voltages, spend a few cents on 1 % parts.
• Buffer the output if anything fast is connected — Audio, logic signals, or any moving load will distort through a bare divider. Add a unity-gain op-amp after Vout — it copies the voltage and supplies the current.
• Never power a circuit from a divider — It is not a regulator. The moment your circuit draws current, Vout collapses. Use an LDO, a buck converter, or a Zener instead.