Work out a divider's output voltage, or the standard E24 and E96 resistor pair for a target voltage, with load, current, power and tolerance. It also sets regulator feedback resistors, including LM317-style parts.
Accepts 4.7k, 4k7, 10k or 1M.
Accepts 4.7k, 4k7, 10k or 1M.
Whatever the output drives, such as an ADC input or a transistor base.
Output voltage (Vout)
3.333 V
Vout = Vin × R2 / (R1 + R2) = 5 V × 0.6667
Vout = Vin × R2 / (R1 + R2), with R1 from the input to the output and R2 from the output to ground. To get 3.3 V from 5 V, the ratio is 0.66: E24 resistors R1 = 4.7 kΩ and R2 = 9.1 kΩ give 3.297 V (-0.088%). Anything connected to the output must have a resistance well above R1 ∥ R2, or the voltage drops.
With nothing connected to the output, the same current flows through both resistors: I = Vin / (R1 + R2). The output is the drop across R2, Vout = I × R2, which gives the voltage divider rule:
Vout = Vin × R2 / (R1 + R2)
Solved for one resistor, R2 = R1 × Vout / (Vin − Vout) and R1 = R2 × (Vin − Vout) / Vout. The ratio can only fall between 0 and 1: resistors alone cannot raise or invert a voltage. Seen from the output, the divider is a source of Vout behind a resistance of R1 ∥ R2 = R1 × R2 / (R1 + R2), and that resistance decides how much a load pulls it down.
12 V across R1 = 10 kΩ and R2 = 4.7 kΩ: Vout = 12 × 4.7 / 14.7 = 3.837 V. The divider draws 816 µA and its output resistance is 3.197 kΩ.
With R1 = 10 kΩ, R2 = 10k × 3.3 / (5 − 3.3) = 19.41 kΩ. The nearest E24 value, 20 kΩ, gives 3.333 V; the nearest E96 value, 19.6 kΩ, gives 3.311 V.
10 V into R1 = 8 kΩ and R2 = 2 kΩ gives 2 V unloaded. Connect a 10 kΩ load and the bottom leg becomes 2k ∥ 10k = 1.667 kΩ, so the output falls to 1.724 V, a 13.8% drop, because the load is only 6.3 times the 1.6 kΩ output resistance.
Best pairs with R1 + R2 between 10 kΩ and 100 kΩ. Error is the nominal output against the target, before resistor tolerance.
| Vin → Vout | E24 pair (R1 / R2) | Output | Best 1% pair (E24 or E96) | Output |
|---|---|---|---|---|
| 5 V → 3.3 V | 4.7 kΩ / 9.1 kΩ | 3.297 V (-0.088%) | 8.25 kΩ / 16 kΩ | 3.299 V (-0.031%) |
| 5 V → 2.5 V | 16 kΩ / 16 kΩ | 2.5 V (0%) | 15.8 kΩ / 15.8 kΩ | 2.5 V (0%) |
| 3.3 V → 1.65 V | 16 kΩ / 16 kΩ | 1.65 V (0%) | 15.8 kΩ / 15.8 kΩ | 1.65 V (0%) |
| 12 V → 5 V | 18 kΩ / 13 kΩ | 5.032 V (+0.65%) | 18.2 kΩ / 13 kΩ | 5 V (0%) |
| 12 V → 3.3 V | 24 kΩ / 9.1 kΩ | 3.299 V (-0.027%) | 24 kΩ / 9.1 kΩ | 3.299 V (-0.027%) |
| 9 V → 3.3 V | 62 kΩ / 36 kΩ | 3.306 V (+0.19%) | 43 kΩ / 24.9 kΩ | 3.3 V (+0.013%) |
| 24 V → 5 V | 9.1 kΩ / 2.4 kΩ | 5.009 V (+0.17%) | 12 kΩ / 3.16 kΩ | 5.003 V (+0.053%) |
| 24 V → 3.3 V | 47 kΩ / 7.5 kΩ | 3.303 V (+0.083%) | 18 kΩ / 2.87 kΩ | 3.3 V (+0.013%) |
| 48 V → 3.3 V | 27 kΩ / 2 kΩ | 3.31 V (+0.31%) | 18 kΩ / 1.33 kΩ | 3.303 V (+0.08%) |
Vout = Vref × (1 + R1 / R2) for a regulator whose FB pin sits at Vref. Best 1% pair, searched within a factor of about three of the total a 10 kΩ R2 would give; FB bias current ignored. Your regulator's datasheet has the real Vref and any limits on R2.
| Output | Vref = 0.6 V | Vref = 0.8 V | Vref = 1.25 V |
|---|---|---|---|
| 1.8 V | 20 kΩ / 10 kΩ 1.8 V (0%) | 15 kΩ / 12 kΩ 1.8 V (0%) | 3.3 kΩ / 7.5 kΩ 1.8 V (0%) |
| 2.5 V | 47.5 kΩ / 15 kΩ 2.5 V (0%) | 25.5 kΩ / 12 kΩ 2.5 V (0%) | 10 kΩ / 10 kΩ 2.5 V (0%) |
| 3.3 V | 16.2 kΩ / 3.6 kΩ 3.3 V (0%) | 24 kΩ / 7.68 kΩ 3.3 V (0%) | 16 kΩ / 9.76 kΩ 3.299 V (-0.025%) |
| 5 V | 110 kΩ / 15 kΩ 5 V (0%) | 35.7 kΩ / 6.8 kΩ 5 V (0%) | 30 kΩ / 10 kΩ 5 V (0%) |
| 12 V | 300 kΩ / 15.8 kΩ 11.99 V (-0.063%) | 140 kΩ / 10 kΩ 12 V (0%) | 82 kΩ / 9.53 kΩ 12.01 V (+0.046%) |
The LM317 holds 1.25 V between OUT and ADJ, so TI writes Vout = Vref × (1 + R2 / R1) + Iadj × R2 with R1 (OUT to ADJ) = 240 Ω. Output shown with Iadj at its 50 µA typical and 100 µA maximum.
| Target | Exact R2 | Standard R2 | Output (typ. Iadj) | Output (max. Iadj) |
|---|---|---|---|---|
| 1.8 V | 104.6 Ω | 105 Ω | 1.802 V | 1.807 V |
| 2.5 V | 237.7 Ω | 237 Ω | 2.496 V | 2.508 V |
| 3.3 V | 389.9 Ω | 390 Ω | 3.301 V | 3.32 V |
| 5 V | 713.2 Ω | 715 Ω | 5.01 V | 5.045 V |
| 9 V | 1.474 kΩ | 1.47 kΩ | 8.98 V | 9.053 V |
| 12 V | 2.044 kΩ | 2.05 kΩ | 12.03 V | 12.13 V |
| 15 V | 2.615 kΩ | 2.61 kΩ | 14.97 V | 15.1 V |
10 V into two equal resistors (5 V unloaded). Each cell is the loaded output and its error.
| Load | 1 kΩ / 1 kΩ | 10 kΩ / 10 kΩ | 100 kΩ / 100 kΩ |
|---|---|---|---|
| 1 MΩ | 5 V (-0.05%) | 4.98 V (-0.5%) | 4.76 V (-4.8%) |
| 100 kΩ | 4.98 V (-0.5%) | 4.76 V (-4.8%) | 3.33 V (-33%) |
| 10 kΩ | 4.76 V (-4.8%) | 3.33 V (-33%) | 0.833 V (-83%) |
| 1 kΩ | 3.33 V (-33%) | 0.833 V (-83%) | 0.098 V (-98%) |
| Scenario | Divider | Result |
|---|---|---|
| Arduino Uno battery monitor, 3S Li-ion (12.6 V full) Output resistance 6.667 kΩ, under the 10 kΩ the ATmega328P ADC is optimized for. Each 10-bit count at a 5 V reference is 14.6 mV of battery voltage. | 20 kΩ / 10 kΩ | 4.2 V at full charge, 420 µA |
| ESP32 battery monitor, 1S Li-ion (4.2 V) Inside the 150 to 2450 mV range Espressif gives for the highest attenuation. With 50 kΩ of source resistance, add the 100 nF capacitor Espressif suggests. | 100 kΩ / 100 kΩ | 2.1 V, 21 µA |
| 5 V to 3.3 V logic, one direction 10–90% rise time into an assumed 10 pF input. | 1 kΩ / 2 kΩ 10 kΩ / 20 kΩ | 3.33 V, 15 ns 3.33 V, 146 ns |
| 24 V rail (up to 30 V) into a 3.3 V ADC 30 V maps to 3.0 V, leaving headroom below 3.3 V. | 27 kΩ / 3 kΩ | 3 V at 30 V, 2.4 V at 24 V |
| LM317 set to 5 V R1 = 240 Ω from OUT to ADJ, Iadj 50 µA typical. | 240 Ω / 715 Ω | 5.01 V |
| Buck converter, 3.3 V from a 0.8 V reference R1 from VOUT to FB, R2 from FB to ground. | 24 kΩ / 7.68 kΩ | 3.3 V (0%) |
| Op-amp mid-supply bias on 5 V Decouple the midpoint with a capacitor; it is a 50 kΩ source. | 100 kΩ / 100 kΩ | 2.5 V, 25 µA |
| Transistor voltage divider bias 12 V supply; the base loads the divider with about (β + 1) × RE, here 101 kΩ for β = 100 and RE = 1 kΩ. | 47 kΩ / 10 kΩ | 2.11 V unloaded, 1.95 V at the base |
| 10 kΩ potentiometer as a divider The wiper's output resistance peaks at mid-travel. | 5 kΩ + 5 kΩ at center | Vin / 2 behind 2.5 kΩ |
Say you want 5 V at 100 mA from 12 V and try R1 = 68 Ω, R2 = 100 Ω. Unloaded the output is 7.14 V. With a 50 Ω load (100 mA at 5 V) it falls to 3.95 V, and if the load doubles its current it falls to 2.73 V. Meanwhile the divider burns 0.86 W with no load at all. A regulator holds its output as the load changes; a buck converter does it efficiently. Dividers are for measuring, biasing and setting references, where the load current is tiny and steady.
The calculator uses ideal DC circuit theory: an ideal source, linear resistors and a resistive load. The worst case puts both resistors at opposite tolerance extremes, which is pessimistic but safe; temperature drift (TCR) adds to it. It does not model an ADC's sampling capacitor, leakage into high-value nodes, or AC behaviour, which a SPICE simulation can check. For a regulator's feedback divider, the datasheet's Vref tolerance usually limits accuracy more than the resistors.
In the ProtoFlow desktop app, start with a sentence such as "ESP32 with a battery voltage monitor on a 1S Li-ion cell". The AI drafts the schematic with catalog parts and wires the divider node to the ADC pin. Set the resistor values, then run an operating-point simulation in the built-in ngspice engine to read the node voltage and currents, or sweep the battery source from empty to full with a DC sweep. Review mode has the AI check each subcircuit, divider ratios and LED resistors included, against how the parts are actually connected. When you move to the board, value-only resistors are matched to catalog parts with the exact value and package, and the BOM compares LCSC, DigiKey and Mouser. Download ProtoFlow free for macOS, Windows or Linux.
Vout = Vin × R2 / (R1 + R2), where R1 runs from the input to the output and R2 from the output to ground. It comes from Ohm’s law: the same current Vin / (R1 + R2) flows through both resistors, so the voltage across R2 is that current times R2. For example 12 V with R1 = 10 kΩ and R2 = 4.7 kΩ gives 3.837 V.
Rearrange the formula: R2 = R1 × Vout / (Vin − Vout). For 5 V to 3.3 V with R1 = 10 kΩ, R2 = 10k × 3.3 / 1.7 = 19.41 kΩ. That is not a standard value, so use 20 kΩ (E24) or 19.6 kΩ (E96), or let the calculator search pairs.
With E24 values, R1 = 4.7 kΩ and R2 = 9.1 kΩ give 3.297 V. 10 kΩ over 20 kΩ gives 3.333 V. For a fast logic signal, scale both down, for example 1 kΩ and 2 kΩ.
Yes. The load sits in parallel with R2 and pulls the output down. A divider behaves like a source of Vout behind R1 ∥ R2, so keep the load at least 10 times that resistance for no more than 9.1% error. 10 V into 8 kΩ / 2 kΩ gives 2 V, but a 10 kΩ load drags it to 1.724 V.
Vin / (R1 + R2) with no load. A 10 kΩ / 20 kΩ divider on 5 V draws 167 µA; a 100 kΩ / 100 kΩ divider on a 4.2 V battery draws 21 µA, which matters for battery life.
For a one-way signal from a 5 V output to a 3.3 V input at modest speed, yes. Not for bidirectional or open-drain lines such as I²C, not for raising 3.3 V to 5 V, and not for fast edges with high-value resistors, because the input capacitance slows the edge. Use a level shifter for those.
Use Vout = Vref × (1 + R1 / R2), with R1 from the output to FB and R2 from FB to ground. TI’s SLVA477 picks R2 from the divider current (at least 100 times the FB bias current), then R1 = R2 × (Vout / Vref − 1). With a 0.8 V reference, 24 kΩ over 7.68 kΩ gives 3.3 V.
No. Its output follows the load: the 68 Ω / 100 Ω example above falls from 7.14 V to 3.95 V with a 50 Ω load. Use an LDO or a buck converter to power anything.
Each tap sits at Vin × (resistance below the tap) / (total resistance). Three 10 kΩ resistors on 12 V give taps at 8 V and 4 V. The calculator’s 3+ resistors mode handles up to six.
Yes, if the divider runs from the negative rail up to a positive one so the tap stays positive. Between −12 V (through 56 kΩ) and +3.3 V (through 10 kΩ), the tap sits at 0.982 V, inside a 3.3 V ADC’s range: Vout = Vbottom + (Vtop − Vbottom) × R2 / (R1 + R2).
Two capacitors in series divide an AC voltage by their reactances, so the output across C2 is Vin × C1 / (C1 + C2), the inverse of the resistor case. It does not work at DC, where leakage decides the split. This calculator covers resistive dividers.
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