Voltage Divider Calculator
Solve divider ratios with nearest E24/E96 resistor values.
Vout = Vin · R2 / (R1 + R2) — unloaded output
| R1 | R2 | Vout | Error |
|---|---|---|---|
| 1.37k | 2.67k | 3.3045 V | 0.14 % |
| 13.7k | 26.7k | 3.3045 V | 0.14 % |
| 549 | 1.07k | 3.3045 V | 0.14 % |
| 5.49k | 10.7k | 3.3045 V | 0.14 % |
| 590 | 1.15k | 3.3046 V | 0.14 % |
This tool works a resistive divider in both directions: compute the unloaded output of a given R1/R2 pair, or — the harder everyday problem — search the standard E24/E96 value series for the pairs that best hit a target ratio, ranked by error. 12 V through equal 10 kΩ resistors gives 6 V; 5 V → 3.3 V lands within 1 % with ordinary E96 parts.
How it works
Two resistors in series split a voltage in proportion to resistance: Vout = Vin · R2 / (R1 + R2), with R2 the grounded (bottom) resistor. The finder normalizes your target ratio, walks R2 through the chosen IEC 60063 series at your preferred impedance level, snaps the ideal companion R1 to its nearest standard value, and re-computes the exact output each pair produces. Sorting by relative error surfaces combinations that are often better than the “obvious” pair.
The result is the unloaded output; anything connected to the tap sits in parallel with R2 and lowers it — the classic reason a divider measures fine on the bench and sags in circuit.
Worked example
Vout = 12 × 10k / 20k = 6.000 V
target 5 V → 3.3 V (E96, ~10 k): e.g. R1 5.11k / R2 9.76k → 3.283 V (−0.5 %)
Parameters
| Parameter | Value | Notes |
|---|---|---|
| Formula | Vout = Vin·R2/(R1+R2) | unloaded |
| E24 | 24 values/decade | ±5 % family (IEC 60063) |
| E96 | 96 values/decade | ±1 % family |
| Search | best pairs by |error| | top 5 shown |
| Loading rule | load ≫ R2 | ≥100× or buffer |
Python implementation
# Voltage divider: Vout = Vin * R2 / (R1 + R2)
def vout(vin, r1, r2):
return vin * r2 / (r1 + r2)
assert vout(12, 10_000, 10_000) == 6.0
# 5 V -> 3.3 V with E96 parts: R1 = 5.11k, R2 = 9.76k (error < 1 %)
print(vout(5, 5110, 9760)) # ≈ 3.283 VFAQ
▸Why can't I buy a resistor for the exact ratio I need?
Resistors come in standardized IEC 60063 series: E24 has 24 values per decade (±5 % families), E96 has 96 (±1 % families). Your exact ratio usually falls between values, so the practical question is which standard pair lands closest — exactly what the finder tab answers, with the residual error shown.
▸How do I choose the impedance level (the R2 magnitude)?
It is a power-vs-noise trade. Low resistance wastes current through the divider (5 V across 2 kΩ total burns 2.5 mA continuously); high resistance makes the output sensitive to load and ADC input current. Around 10 kΩ total is the common middle ground for sensing dividers.
▸Why does my measured Vout differ from the calculation?
The formula assumes an unloaded output. Any load — an ADC input, a following stage — appears in parallel with R2 and pulls Vout down. Keep the load impedance at least 100× R2, or buffer the divider with an op-amp follower.
▸Does resistor tolerance add to the table's error figure?
Yes, independently. The table shows the nominal-value error; actual parts add their tolerance on top. With ±1 % parts the worst-case ratio error adds roughly ±2 % around the nominal figure, so pick pairs whose nominal error is comfortably inside your budget.
▸Is my data uploaded?
No. The search runs locally in your browser.