Voltage Divider Calculator
Two resistors, one scaled-down voltage. Find the output of a divider from its resistors, or work backward to the resistor you need for a target voltage — with the unloaded current draw and division ratio.
Output is measured across R2. Enter resistances in ohms — 10 kΩ is 10000.
Formula & how it works
Output is Vout = Vin × R2 ÷ (R1 + R2). Rearranged, R2 = R1 × Vout ÷ (Vin − Vout) and R1 = R2 × (Vin − Vout) ÷ Vout. The unloaded current is Vin ÷ (R1 + R2).
Worked example
With 9 V in and two 10 kΩ resistors, Vout = 9 × 10k ÷ 20k = 4.5 V, drawing 0.45 mA. Need 3.3 V from a 5 V rail with a 10 kΩ top resistor? R2 works out to about 19.4 kΩ, so a standard 18 kΩ gets you close.
Using dividers well
Ratio first, values second
A divider only cares about the ratio between its resistors, not the exact values — 1 k and 2 k divide the same as 10 k and 20 k. So set the ratio to hit your target voltage, then pick actual values based on how much current you can spare, since lower resistances draw more.
The loading trap
The classic mistake is tapping a divider to power something. As soon as a load draws current comparable to what flows through the divider, the output sags well below the calculated value. Dividers shine as references for high-impedance inputs — an analog-to-digital pin, an op-amp, a comparator — not as little power supplies.
Trading current for stability
Smaller resistors make the divider stiffer, meaning a given load disturbs it less, but they waste more power as constant current flow. Larger resistors sip almost nothing but are easily upset by loading and noise. Choosing values is a balance between those two, guided by the current figure here.