Voltage Divider Calculator with Standard Resistor Values
Find the standard E6 to E192 resistor pair for a target voltage, a ratio or a regulator's feedback divider, ranked by error. For each pair it shows what decides whether the divider actually works: the current it draws, the power in each resistor, the worst-case output from resistor tolerance, and the output impedance an ADC or a load sees. Every input lives in the URL, so a copied link reproduces the calculation.
- Divider current
- 363 µA
- 12 V across R1 + R2 = 33.1 kΩ
- Resistor power
- 3.15 mW / 1.2 mW
- R1 / R2
- Worst case, ±1% parts
- 3.251 V to 3.347 V
- ±1.45% of nominal
- Output impedance
- 6.598 kΩ
- R1 in parallel with R2 (Thevenin)
With 1% resistors the output can sit anywhere in ±1.45% of nominal, 53 times this pair's nominal error. A closer ratio buys nothing now; tighter tolerance parts do.
The link reproduces these exact inputs - paste it in a design review or a pull request.
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Ranked E24 pairs
Every standard pair inside the window, closest first. Pairs that are the same ratio scaled by ten (10 kΩ over 20 kΩ and 100 kΩ over 200 kΩ) show once, at the total nearest the middle of the window.
| R1 (top) | R2 (bottom) | Divider output | Error | Current | Output Z |
|---|---|---|---|---|---|
| 24 kΩ | 9.1 kΩ | 3.299 V | -0.027% | 363 µA | 6.598 kΩ |
| 180 kΩ | 68 kΩ | 3.29 V | -0.293% | 48.4 µA | 49.35 kΩ |
| 47 kΩ | 18 kΩ | 3.323 V | +0.699% | 185 µA | 13.02 kΩ |
| 200 kΩ | 75 kΩ | 3.273 V | -0.826% | 43.6 µA | 54.55 kΩ |
| 39 kΩ | 15 kΩ | 3.333 V | +1.010% | 222 µA | 10.83 kΩ |
| 150 kΩ | 56 kΩ | 3.262 V | -1.147% | 58.3 µA | 40.78 kΩ |
| 220 kΩ | 82 kΩ | 3.258 V | -1.264% | 39.7 µA | 59.74 kΩ |
| 43 kΩ | 16 kΩ | 3.254 V | -1.387% | 203 µA | 11.66 kΩ |
| 62 kΩ | 24 kΩ | 3.349 V | +1.480% | 140 µA | 17.3 kΩ |
| 160 kΩ | 62 kΩ | 3.351 V | +1.556% | 54.1 µA | 44.68 kΩ |
| 27 kΩ | 10 kΩ | 3.243 V | -1.720% | 324 µA | 7.297 kΩ |
| 100 kΩ | 39 kΩ | 3.367 V | +2.027% | 86.3 µA | 28.06 kΩ |
The equation, and how the search works
With R1 from the input to the tap and R2 from the tap to ground, the output is Vin × R2 / (R1 + R2). For a regulator's feedback divider the regulator holds the tap at its reference instead, so the output is Vref × (1 + R1 / R2). Either way the job is the same: find two values you can actually buy whose ratio is as close as possible to the one you need. For every value of R2 in the chosen series, from 1 Ω to 10 MΩ, the calculator computes the ideal R1, tries the two standard values either side of it, keeps the pairs whose total fits your window, and ranks them by the error on the voltage you asked for.
Tolerance usually matters more than the ratio
Once a pair is within a few tenths of a percent, a closer ratio stops mattering, because the resistors themselves are not the value printed on them. The worst case shown on this page puts both resistors at opposite ends of their tolerance, the arrangement that moves the output most. With 1% parts that is ±1.45% on a 3.3 V output from 12 V, about fifty times the 0.03% nominal error of the best E24 pair. If the result has to be accurate, buy 0.1% resistors, calibrate in firmware, or use a precision divider network whose two resistors track each other.
Choosing the window
A low total resistance wastes current: a 33 kΩ divider across 12 V draws 360 µA all the time, which is nothing on a mains product and a measurable share of the budget on a coin cell. A high total resistance saves current but raises the output impedance, so the divider becomes sensitive to leakage, to the input current of whatever reads it, and to noise. Between 10 kΩ and 1 MΩ is a sensible default for an ADC input. For a battery monitor the current budget is the better limit, and the answer is often a high-value divider with a capacitor at the tap, or a divider switched off by a transistor between readings.
ADC loading and the output impedance
The input of a microcontroller's SAR ADC is a small sampling capacitor that connects to the pin for a short sampling window. The divider has to recharge it within that window, through its output impedance, which is R1 and R2 in parallel. Datasheets express this as a maximum source impedance for each sampling time. Past it, readings come out low and depend on the previous channel. A capacitor of 10 to 100 nF from the tap to ground fixes this for slow signals such as battery voltage, because it holds enough charge to fill the sampling capacitor without the divider having to. It also slows the divider down, so it is the wrong fix for a fast signal: buffer that with an op amp instead.
Frequently asked questions
How do I get 3.3 V from 12 V with a voltage divider?
The output is Vin × R2 / (R1 + R2), so you need a ratio of 3.3 / 12 = 0.275. With E24 values and a total between 10 kΩ and 1 MΩ, the closest pair is R1 = 24 kΩ on top and R2 = 9.1 kΩ to ground: 3.299 V nominal, 0.03% low, drawing about 360 µA. With 1% resistors the worst case spans 3.251 V to 3.347 V, so the tolerance sets the accuracy, not the ratio. Use a divider to feed a high-impedance input such as an ADC or a comparator, never as a power supply.
What resistor values give 3.3 V from 5 V?
In E24, R1 = 47 kΩ and R2 = 91 kΩ give 3.297 V, 0.09% low, at 36 µA. In E96 the best pair inside 10 kΩ to 1 MΩ is 13.7 kΩ over 26.7 kΩ at 3.305 V, 0.14% high. A finer series is not automatically closer: E96 does not contain every E24 value, which is why the calculator compares all six series for your target.
How do I choose feedback resistors for a regulator's output voltage?
An adjustable regulator holds its feedback pin at Vref, so Vout = Vref × (1 + R1 / R2), with R1 from the output to the pin and R2 from the pin to ground. For a 0.8 V reference and a 3.3 V output, R1 / R2 must be 3.125. With a 10 kΩ to 200 kΩ total, the E24 pair 75 kΩ and 24 kΩ is exact, while the best E96 pair, 35.7 kΩ and 11.5 kΩ, lands 0.5% low at 3.284 V. Keep the divider current well above the pin's leakage, and stay inside the R2 range the datasheet recommends.
What divider resistance should I use in front of an ADC?
As low as your current budget allows, because the ADC sees the two resistors in parallel as its source impedance. Scaling a 4.2 V Li-ion cell to 1.0 V on a 1 to 10 µA budget gives 1.15 MΩ over 357 kΩ in E96: only 2.8 µA, but a 272 kΩ source impedance, far above what most microcontroller SAR ADCs accept. Put a 100 nF capacitor from the tap to ground so it supplies the sampling charge, and sample slowly; or buffer the tap with an op amp.
Why does my divider measure lower than calculated?
Usually because something is loading it. A load resistance sits in parallel with R2 and pulls the output down. The 24 kΩ and 9.1 kΩ divider that gives 3.299 V unloaded reads 3.095 V into a 100 kΩ load, more than 6% low. Enter the load in the calculator to see the loaded value, or lower the divider resistance until the load is at least a hundred times its output impedance.
If the board is the product
Two resistors is a five-minute decision. The measurement chain around them (reference, filtering, ADC sampling, calibration) is where a product's accuracy is won or lost, and that is our day job: PCB design and layout explains how we work and what a board costs in engineering hours. For the power side of the same board, the LDO thermal calculator and LED resistor calculator are next door. If a number here decides something real, a free 30-minute technical call is the fastest way to check it against someone who has shipped the board.