The voltage divider formula
Two resistors in series across a voltage split it in proportion to their values. Tapping the junction between them gives:
Vout = Vin × R2 / (R1 + R2)
where R1 is the top resistor (from Vin to the tap) and R2 is the bottom (from the tap to ground). Only the ratio matters for the voltage — 10 k/10 k and 1 M/1 M both halve Vin — but the absolute values set how much current flows and how the divider behaves under load.
Worked examples
Halve it: 5 V with R1 = R2 = 10 kΩ gives 2.5 V, drawing 0.25 mA.
3.3 V from 5 V: R1 = 10 kΩ, R2 = 20 kΩ gives 5 × 20/30 = 3.33 V.
The loading trap
A divider's output sags as soon as you draw current from it, because the load resistance sits in parallel with R2. Rule of thumb: keep the load impedance at least 10× R2 for the output to stay within ~10% of the unloaded value, or buffer the tap with an op-amp follower. This is why a divider is fine for setting a reference into a high-impedance ADC or op-amp input, but a poor way to "drop" a supply feeding a real load — use a regulator for that.
Frequently asked questions
- Which resistor is R1 and which is R2?
- R1 is the upper resistor between Vin and the output tap; R2 is the lower one between the tap and ground. Swapping them gives 1 − the original ratio.
- How do I pick actual resistor values?
- Use the reverse solver above — enter Vin and the output you want, and it finds the closest standard E-series pairs. Then check the loading rule against whatever the output feeds.
- Can I use a potentiometer instead?
- Yes — a pot is an adjustable divider, with the wiper as the tap. The same formula applies with R1 and R2 being the two halves of the track.
- What about dividing an AC or high-frequency signal?
- Resistive dividers work for AC too, but stray and input capacitance form a low-pass filter at high frequencies; scope probes add a compensating capacitor across R1 to keep the ratio flat. For pure reactance, see the reactance calculator.
- Need the current and power?
- Shown above for any R1/R2. For a deeper dive on volts, amps, and watts, the Ohm's law calculator pairs well.