Building a Voltage Divider with Resistors

Building a Voltage Divider with Resistors

A voltage divider turns a higher voltage into a lower one using two resistors.¹

The consequences of a badly designed voltage divider are usually subtle: an ADC reading that is 5–10% high, a bias point that drifts with temperature, or a voltage reference that collapses when loaded. Understanding the formula, the loading effect, and the power implications gives you a voltage divider that works correctly across your full operating range.

The formula and divider ratio

Vout = Vin × R2 / (R1 + R2), where R1 is the top resistor (from Vin to Vout) and R2 is the bottom resistor (from Vout to ground). Rearranging: R2 / (R1 + R2) is the divider ratio. For Vin = 5V and Vout = 3.3V target: ratio = 3.3/5 = 0.66. Setting R1 = 10kΩ: R2 = R1 × ratio / (1 − ratio) = 10kΩ × 0.66 / 0.34 ≈ 19.4kΩ. Round to the nearest E24 value (20kΩ). The actual Vout with 10kΩ and 20kΩ is 5 × 20/30 = 3.33V, close enough for most applications.¹

Setting the divider ratio before choosing parts

Pick the ratio first, then choose R1 and R2 for a divider from standard resistor values that preserve it. The absolute values matter next, because they set quiescent current and loading tolerance. Once you have the target ratio, scan the E12 or E24 series table for a pair of values whose ratio matches within a few percent, then calculate the actual Vout to confirm it falls within your acceptable window before committing to a specific pair.

A useful trick is to pick R1 as a value already on your board, then solve for R2, because that reduces the component count and keeps the divider inside your existing reel. If the solved R2 lands awkwardly between two E24 steps, shift R1 up or down a decade to bring R2 back onto a stocked value rather than ordering a one-off part. CapyToolkit's calculator lets you sweep a few R1 choices and watch the resulting ratio, so you can settle on a pair that is both accurate and already in your kit.

Loading effect and impedance

Connecting a load in parallel with R2 reduces the effective bottom resistance and lowers Vout. If your load has resistance Rload, the parallel combination of R2 and Rload replaces R2 in the formula. This loading effect is the most common source of systematic error in voltage divider designs, and it is invisible unless you calculate the parallel resistance explicitly.²

Keeping loading error below 10%

Consequently, a 10kΩ/10kΩ divider producing 2.5V from 5V will output only 1.67V when loaded with a 10kΩ load, a 33% drop. The rule of thumb: make R1 + R2 at least 10× smaller than the load impedance to keep the loading effect below 10%. For ADC inputs with impedance above 100kΩ, a 10kΩ/10kΩ divider is adequate. For op-amp inputs, even higher divider resistances work. When the divider feeds a microcontroller ADC with a sample-and-hold capacitor, the source impedance must be low enough to charge the sampling capacitor within the acquisition time; otherwise the reading will be systematically low, and adding a buffer capacitor at the divider output or reducing the divider resistance solves this problem.

Common mistakes and practical tips

Choosing resistors too high (100kΩ range) in a divider that feeds a low-impedance load is the most common mistake, causing Vout to droop under load. Conversely, choosing resistors too low wastes current: a 100Ω/100Ω divider across 12V draws 60 mA and dissipates 720 mW continuously, which is unacceptable in battery circuits.

Selecting stable E24 values

Picking E24 preferred values closest to the calculation result, rather than searching for unobtainable exact values, is the practical approach. For temperature stability, use matched resistors from the same manufacturer and batch to ensure both temperature coefficients track together, keeping the ratio stable even as both values drift. This matched-pair approach is especially important in battery-monitoring dividers where the ADC reading directly drives fuel-gauge calculations, because a ratio error of just 2% translates into a misleading state-of-charge estimate that confuses users.

For a one-off hobby build, mixed parts are usually fine, because a few percent of ratio drift over temperature never shows up in a reading you are eyeballing. The matched-batch discipline only earns its keep when the divider feeds a fuel gauge or a calibrated reference, where that same few percent becomes a visible error on a display. Keeping a strip of same-batch divider pairs labelled together avoids the temptation to grab whichever 10kΩ is nearest, which is how mismatch creeps into an otherwise careful design.

Temperature tracking in matched resistor pairs

When both resistors in a divider drift equally with temperature, the ratio stays constant even as individual values shift. This tracking property is why using resistors from the same manufacturer, the same tolerance grade, and ideally the same production batch produces better divider stability than mixing parts from different suppliers or tolerance grades.³

Metal film resistors typically drift 50–100 ppm/°C. Two 10kΩ metal film resistors with identical 50 ppm/°C coefficients both increase by 25Ω over a 50°C rise: the ratio 10 025 / (10 025 + 10 025) stays exactly 0.5, because ratio tracking is perfect when temperature coefficients match. A 10kΩ carbon film (±200 ppm/°C) paired with a 10kΩ metal film (±50 ppm/°C) produces a ratio shift of 150 ppm/°C over temperature, generating a 7.5 mV output error at 5V across 50°C. For ADC references where 0.1% stability matters, use matched parts from the same batch.

Voltage dividers for sensor interface circuits

NTC thermistors form natural voltage dividers when connected with a fixed pull-up resistor. A 10kΩ NTC paired with a 10kΩ fixed pull-up across 3.3V produces a junction voltage spanning roughly 0.3V to 2.8V over a 0°C to 100°C temperature range, depending on the thermistor's beta coefficient. This voltage drives an ADC input directly without additional signal conditioning.

Photoresistors (LDRs) work the same way: a 10kΩ pull-up paired with an LDR ranging from 500Ω in bright light to 500kΩ in darkness covers most of the ADC input range at 3.3V. Choosing the pull-up to match the midpoint of the sensor's resistance range places the output voltage near 50% of supply at the target measurement condition, maximising the usable ADC swing across the sensor's operating range.

When to use this

Use this guide whenever you need to level-shift a voltage for an ADC input, create a reference voltage for a comparator, bias a transistor base, or reduce a battery voltage to a measurable range. Recalculate whenever your load impedance or supply voltage changes.

Examples

Vin = 12V, Vout target = 3.3V for ADC. Ratio = 3.3/12 = 0.275.
R1 = 33kΩ, R2 = 12kΩ (closest E12 pair).
Actual Vout = 12 × 12/(33+12) = 3.2V.

12V battery mapped to 3.2V at ADC input. Current draw: 12V / 45kΩ = 267 µA, which is negligible for battery life.

Use 10kΩ/20kΩ divider: Vout = 5 × 20/30 = 3.33V. Op-amp non-inverting input has 10MΩ impedance, so loading is negligible.

3.33V reference with 0.3% loading error at the op-amp input. Acceptable for a buffer application.