Voltage divider calculator
Output voltage and source impedance of a divider, the closest standard resistor pair for a target voltage, and feedback values for an adjustable regulator.
- Ratio Vout / Vin
- 0.2481
- Source impedance (R1 ∥ R2)
- 2.481 kΩ What an ADC sampling capacitor or a load sees.
- Divider current
- 902.3 µA
- Power in R1
- 8.14 mW
- Power in R2
- 2.69 mW
How this is calculated
Vout = Vin · R2 / (R1 + R2) R1 top, R2 bottom Loaded: R2 → R2 · RL / (R2 + RL) Source impedance = R1 · R2 / (R1 + R2) FB to ground (buck, LDO): Vout = Vref · (1 + R1 / R2) OUT to ADJ (LM317): Vout = Vref · (1 + R2 / R1) + Iadj · R2
- Find resistors
- R2 is held in the decade you enter, and R1 is the series value either side of the ideal ratio. The table compares the best pair in E12, E24, E48 and E96.
- Regulator
- R1 always runs from the output to the pin and R2 from the pin to ground; the two kinds of regulator differ in where the reference sits. A buck converter or most LDOs hold the FB pin at Vref above ground, so Vref is across R2. An LM317 holds Vref between OUT and ADJ, so Vref is across R1 (the datasheet's 240 Ω) and the adjust pin current flows on through R2. LM317 reference 1.25 V typical (1.2 to 1.3 V), adjust current 50 µA typical, 100 µA maximum; the pair search and the output include it.
- ADC loading
- A load across R2 lowers the output. A warning shows when the error passes 1%. A switched capacitor ADC also wants a low source impedance to settle within its sampling time: check its datasheet limit against R1 ∥ R2.
- 0.8 V preset
- The 0.8 V reference is the TPS54331 buck converter (0.772 to 0.828 V). Use the value from your regulator’s datasheet.
- IEC 60063, Preferred number series for resistors and capacitorsEdition 3.0, 2015-03-27
- Texas Instruments, LM317 3-Pin Adjustable Regulator datasheet, SLVS044Z, Equation 1 and electrical characteristicsRevised April 2025
- Texas Instruments, TPS54331 datasheet, SLVS839H, section 7.3.2 Voltage ReferenceRevised October 2023
Questions
The ideal ratio R1 / R2 is 0.515. 5.1 kΩ over 10 kΩ gives 3.311 V, 0.3% high; 47 kΩ over 91 kΩ, both E24, gives 3.297 V, 0.09% low. E24 is not a subset of E96, so a finer series does not always find a closer pair.
Put 240 Ω from OUT to ADJ and the setting resistor from ADJ to ground: Vout = 1.25 V × (1 + R2 / 240 Ω) plus 50 µA times R2. With 2.4 kΩ to ground that is 13.75 V, plus 0.12 V from the adjust current, about 13.87 V. Swap the two and you get about 1.39 V.
The ADC input loads R2. With 100 kΩ over 100 kΩ and a 100 kΩ input, the bottom becomes 50 kΩ and the output drops from half to a third of Vin. Lower the divider resistance or buffer it.
Large enough to keep the divider current small, small enough that load current, leakage and ADC sampling do not disturb the output. 10 kΩ to 100 kΩ suits most ADC inputs at a few volts; check the source impedance your ADC datasheet allows.