How to use the voltage divider calculator
A resistive divider scales a voltage down with two resistors: R1 from the input to the output node and R2 from the output node to ground. The Compact view asks for the input voltage and the output you want, then searches standard E-series values for the closest resistor pair. The Full view lets you choose what to solve for (Vin, Vout, R1 or R2, or auto, which solves for the one value you leave empty), enter resistors by hand, and edit every tolerance contribution.
Typical uses: scaling a 12 V rail or a battery voltage into an ADC input, setting the feedback voltage of a regulator, or building a threshold for a comparator.
Formulas
- Output voltage:
Vout = Vin × R2 / (R1 + R2) - Divider current:
I = Vin / (R1 + R2) - Power in each resistor:
P1 = I² × R1, P2 = I² × R2 - Source (Thevenin) resistance seen by the load:
Rth = R1 × R2 / (R1 + R2) - Solving for a resistor:
R1 = R2 × (Vin / Vout − 1) and R2 = R1 × Vout / (Vin − Vout)
Tolerance stack
Resistor error comes from more than the printed tolerance. The calculator combines five contributions: initial tolerance, aging, thermal drift, humidity and matching (defaults 1 %, 0.5 %, 0.25 %, 0.15 % and 0.2 %). Worst case adds them; RSS takes the square root of the sum of squares. The combined tolerance is then applied to both resistors in opposite directions (R1 high with R2 low, and the reverse) to give the minimum and maximum output voltage, and the spread as a percentage of nominal.
Limits
- The output is treated as unloaded. A load in parallel with R2 lowers Vout.
- Input voltage tolerance and ripple are not included, only resistor error.
- The resistor search stays within one decade of the scale you set, so it will not pair 4.7 kΩ with 470 kΩ. Change the scale to search another decade.
- Power is DC dissipation only. Check the resistor's voltage rating for high-voltage dividers.
Measuring battery voltage on a low-power board? Use the battery life calculator to see what the divider current costs in runtime, or browse the KiCad design blocks for tested power-monitoring circuits.
Frequently asked questions
Does the calculator account for the load on the divider output?
No. It assumes an unloaded output. Use the Thevenin resistance it reports: if the input that reads the divider (ADC, comparator, feedback pin) draws current or has a sample capacitor, keep its input resistance well above R1 || R2, or buffer the divider.
What is the difference between RSS and worst-case tolerance?
Worst case adds the initial, aging, thermal, humidity and matching contributions linearly. RSS (root sum of squares) assumes they are independent and takes the square root of the sum of their squares, which gives a smaller, statistically realistic band. Use worst case for safety limits and RSS for typical spread.
Which E-series values does the resistor search use?
Standard E12, E24, E48 or E96 values within one decade, scaled by the value you enter (for example 1000 searches 1 kΩ to 9.76 kΩ in E96). It tries every bottom resistor in that decade, picks the nearest top resistor and its two neighbours, and lists the ten pairs with the smallest output error.
How much current does a divider waste?
I = Vin / (R1 + R2), the whole time it is powered. A 100 kΩ + 33 kΩ divider on 12 V draws about 90 µA. On battery devices, use higher values or switch the divider off between measurements.