Voltage Divider Calculator

How a voltage divider works

When to use this: scaling a signal down to something a microcontroller can read, creating a mid-rail bias point, setting a reference voltage, or working out why the 3.3 V you designed for is measuring 2.1 V on the bench.

Two resistors in series across a supply carry the same current. Because they share that current but have different resistances, the supply voltage splits between them in proportion to their values. Tap the junction between them and you get a fraction of the input:

Vout = Vin × R2 / (R1 + R2)

Only the ratio sets the output. A 10 kΩ/10 kΩ divider and a 1 MΩ/1 MΩ divider both halve the input. What the absolute values change is everything else — how much current the divider wastes, how stiff the output is, and how much a load disturbs it.

The loading effect — the part that catches people out

The formula above assumes nothing draws current from the output. The moment you connect something, its input resistance appears in parallel with R2, the effective lower leg gets smaller, and the output drops:

R2′ = R2 ∥ RL = (R2 × RL) / (R2 + RL)
Vout = Vin × R2′ / (R1 + R2′)

This is not a small correction. Take a textbook 10 kΩ/10 kΩ divider on 12 V — nominally 6 V out. Connect a 1 kΩ load and the lower leg becomes 909 Ω, so the output collapses to 1.04 V. That is an 83% error, and it is the single most common reason a divider "doesn't work".

Switch on the load option in the calculator above and watch what a realistic load does to your design. Most calculators hide this on a separate page, if they cover it at all.

The 10× rule

If you cannot make the load large relative to R2 — and often you cannot, because the load is whatever it is — the answer is a buffer, not a different resistor ratio. An op-amp wired as a voltage follower presents a near-infinite input impedance to the divider and drives the real load from its output.

Output impedance

Looking back into the output node, with the supply treated as a short to ground, you see R1 in parallel with R2:

Zout = R1 ∥ R2

This is the Thévenin resistance of the divider, and it is what determines how easily the output is disturbed. A 10 kΩ/10 kΩ divider has a 5 kΩ output impedance. A 1 MΩ/1 MΩ divider with the same ratio has 500 kΩ — same voltage, hugely different behaviour.

High output impedance causes three practical problems: loading error, susceptibility to noise pickup, and slow settling when driving any capacitance. That last one bites when feeding an ADC — the sample-and-hold capacitor has to charge through your divider, and most ADC datasheets specify a maximum source impedance of a few kilohms for exactly this reason.

Worked examples

Scaling a 12 V signal for a 3.3 V ADC

You need to measure a 12 V rail with a microcontroller whose ADC tops out at 3.3 V. Pick a ratio that keeps the maximum input just under full scale:

Required ratio = 3.3 / 12 = 0.275
Choose R2 = 3.3 kΩ, R1 = 8.2 kΩ (both E24 values)
Vout = 12 × 3300 / 11500 = 3.44 V

Slightly over 3.3 V, so scale down a little or accept clipping at the very top of the range. The divider draws 12 / 11500 = 1.04 mA and has an output impedance of 2.36 kΩ — low enough for most ADCs to sample directly.

Battery monitor that doesn't flatten the battery

Same job, but the divider is permanently across a battery. Now the standing current matters more than the output impedance:

R1 = 470 kΩ, R2 = 100 kΩ
Vout = 12 × 100/570 = 2.11 V
I = 12 / 570 kΩ = 21 µA

21 µA drains about 0.18 Ah per year — negligible against any real battery. The trade-off is an 82 kΩ output impedance, which is too high for most ADCs to sample directly. The usual fix is a small capacitor across R2 to act as a charge reservoir for the sample-and-hold, or a MOSFET to switch the divider on only when you need a reading.

Choosing values

The ratio is dictated by the job. The absolute values are a three-way trade-off:

A reasonable default for signal work is a total in the 10 kΩ to 100 kΩ range. Go lower when the output must be stiff or the environment is electrically noisy; go higher when standing current matters and you can tolerate buffering.

Tolerance stacking

Both resistors have tolerance, and in the worst case they drift in opposite directions. Two ±5% resistors give a divider ratio that can be off by roughly ±10%. Two ±1% parts give about ±2%.

If the ratio matters more than the absolute values — which it usually does — a matched pair or a resistor network in a single package will track far better than two loose parts, because they share a substrate and a temperature.

What a divider cannot do

• Power anything. A divider is a signal technique. Any real current draw collapses the output, and the resistors waste power continuously regardless. Use a regulator.
• Provide a stable reference. The output tracks the input exactly, so supply ripple and drift pass straight through. Use a shunt reference or a dedicated voltage reference IC.
• Level-shift bidirectionally. A divider steps a signal down but does nothing in reverse. For bidirectional level shifting on an I²C bus you need a MOSFET translator.
• Work at high frequency without care. Stray capacitance across the resistors turns the divider into a frequency-dependent network. Oscilloscope probes solve this with a trimmer capacitor across the upper resistor — that is what you are adjusting when you compensate a probe.