CALTRX

Voltage Divider Calculator

Calculate voltage divider output voltage and resistor values. Enter V_in and two resistor values — or enter V_in, V_out target, and one resistor to find the other.

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Enter your values above to see the results.

Tips & Notes

  • ✓The output voltage ratio = R2 / (R1 + R2). To get 3.3 V from 5 V: ratio = 3.3/5 = 0.66. Choose R2 = 6.8 kΩ, R1 = 3.3 kΩ (use 3.3 kΩ standard) → V_out = 5 × 6800/(3300+6800) = 3.37 V.
  • ✓Load the divider with a resistor at least 10× R2 to maintain accuracy. A 10 kΩ divider (R1+R2=10 kΩ) loaded with 10 kΩ has effective R2 = 5 kΩ, shifting V_out significantly. For ADC inputs (very high impedance), this is not a concern.
  • ✓Current through the divider I = V_in / (R1 + R2). Use high-value resistors (10 kΩ–100 kΩ) for battery circuits to minimize quiescent current. A 100 kΩ total divider on a 12 V supply draws only 0.12 mA continuously.
  • ✓For precision voltage references, use matched resistors (same temperature coefficient and tolerance). A ±1% tolerance divider targeting 3.3 V from 5 V can produce outputs from 3.17 V to 3.43 V — ±3.9% — due to tolerance stack-up.
  • ✓Potentiometers are voltage dividers with adjustable taps. The wiper moves between ground and V_in; wiper voltage = V_in × (wiper position / total resistance). This is how volume controls and position sensors work.

Common Mistakes

  • ✗Ignoring load current when calculating divider output — a load connected to V_out adds a parallel resistance with R2, reducing effective R2 and pulling V_out lower. Always check that load resistance is at least 10× R2.
  • ✗Choosing resistor values too low — a 100 Ω / 220 Ω divider from 5 V draws 15 mA continuously, wasting 75 mW as heat. Use 10 kΩ–100 kΩ total resistance for static dividers with no current requirement.
  • ✗Using a voltage divider as a voltage regulator — a voltage divider has poor load regulation. When load current changes, V_out changes significantly. Use a voltage regulator IC for stable voltage references under varying loads.
  • ✗Forgetting R1 position — R1 connects between V_in and V_out; R2 connects between V_out and GND. Swapping them gives V_out = V_in × R1/(R1+R2), the complement of the intended output.
  • ✗Not accounting for temperature drift in precision applications — resistor values change with temperature. Using resistors with matched temperature coefficients (from the same batch or same manufacturer lot) minimizes the ratio change over temperature.

Voltage Divider Calculator Overview

The voltage divider is the simplest and most versatile building block in analog electronics. It appears in biasing networks, ADC input conditioning, sensor interfaces, volume controls, and reference generation — everywhere a fixed fraction of a voltage is needed.

Voltage divider formula:

V_out = V_in × R2 / (R1 + R2) | I_divider = V_in / (R1 + R2) | Power = V_in² / (R1 + R2)
EX: Scale 12 V signal to 3.3 V ADC input. Ratio = 3.3/12 = 0.275. Choose R2 = 10 kΩ → R1 = 10k × (12/3.3 − 1) = 26.36 kΩ → use 27 kΩ → V_out = 12 × 10/(27+10) = 3.243 V (1.7% low — acceptable)
Finding R1 for a target output:
R1 = R2 × (V_in/V_out − 1) | R2 = R1 / (V_in/V_out − 1)
EX: Want 2.5 V from 9 V, choose R2 = 22 kΩ → R1 = 22k × (9/2.5 − 1) = 22k × 2.6 = 57.2 kΩ → use 56 kΩ → V_out = 9 × 22/(56+22) = 9 × 0.282 = 2.538 V
Common voltage scaling applications:
ApplicationV_inV_out TargetR1R2Current
5 V → 3.3 V (logic)5 V3.3 V5.1 kΩ10 kΩ0.33 mA
12 V → 5 V (ADC)12 V5 V14 kΩ10 kΩ0.5 mA
12 V → 3.3 V (ADC)12 V3.3 V27 kΩ10 kΩ0.32 mA
48 V → 3.3 V (monitor)48 V3.3 V130 kΩ10 kΩ0.34 mA
Loading effect — impact of load resistance on V_out:
Load R_LEffective R2 (R2=10k)V_out Error (from 5V, R1=10k)
Infinite (no load)10 kΩ0% — V_out = 2.5 V
1 MΩ9.9 kΩ−0.5% — V_out = 2.49 V
100 kΩ9.09 kΩ−4.6% — V_out = 2.38 V
10 kΩ (= R2)5 kΩ−33% — V_out = 1.67 V
1 kΩ909 Ω−82% — V_out = 0.44 V
The loading effect table reveals the critical design rule: use R_L ≥ 10× R2 to keep error below 10%. For microcontroller ADC inputs (impedance 100 kΩ+), a 10 kΩ total divider is well within limits. For op-amp inputs (impedance 1 MΩ+), virtually any divider resistor value works without loading concerns. Only when driving low-impedance loads does divider design require careful analysis — or a buffer op-amp stage between the divider and load.

Frequently Asked Questions

V_out = V_in × R2 / (R1 + R2), where R1 is the top resistor (connected to V_in) and R2 is the bottom resistor (connected to GND). The output is taken at the junction between R1 and R2. Example: V_in = 12 V, R1 = 8.2 kΩ, R2 = 3.9 kΩ → V_out = 12 × 3900 / (8200 + 3900) = 12 × 3900/12100 = 12 × 0.322 = 3.87 V. The output is always between 0 V (R2=0) and V_in (R1=0).

First choose R2 (typically 10 kΩ for general use, higher for battery conservation, lower for faster response with capacitive loads). Then calculate R1 = R2 × (V_in/V_out − 1). Example: target V_out = 3.3 V from V_in = 5 V, choose R2 = 10 kΩ → R1 = 10,000 × (5/3.3 − 1) = 10,000 × 0.515 = 5,152 Ω → nearest standard: 5.1 kΩ (E24). Verify: V_out = 5 × 10/(5.1+10) = 3.311 V (0.3% off target — acceptable).

When a load resistor R_L is connected to V_out, it appears in parallel with R2. Effective R2 = R2 × R_L / (R2 + R_L), which is always less than R2. This reduces V_out below the unloaded value. Example: 10 kΩ / 10 kΩ divider from 12 V → unloaded V_out = 6 V. Add 10 kΩ load (equal to R2): effective R2 = 5 kΩ → V_out = 12 × 5/(10+5) = 4 V. A 10:1 rule: use R_L ≥ 10 × R2 to keep loading error below 10%. For R_L = 100 kΩ and R2 = 10 kΩ: error ≈ 9%.

Voltage dividers appear throughout electronics. Bias networks: set DC operating point for transistor amplifiers — both R1 and R2 are calculated to provide the required base bias voltage. ADC input scaling: scale a 0-12 V signal to 0-3.3 V for a microcontroller ADC using a divider ratio of 3.3/12 = 0.275. Sensor interfaces: NTC thermistors form one leg of a voltage divider — output voltage changes with temperature. Reference voltage generation: precision references use voltage dividers with trimmer resistors to calibrate output. Potentiometers: the adjustable voltage divider is the basis of volume controls, joysticks, and throttle position sensors.

A battery voltage monitor uses a divider to scale the battery voltage to within the ADC range. Example: monitor a 12 V lead-acid battery (range 10.5–14.4 V) with a 3.3 V ADC. Maximum V_in = 14.4 V; target maximum V_out = 3.3 V. Ratio = 3.3/14.4 = 0.229. Choose R2 = 22 kΩ; R1 = 22,000 × (14.4/3.3 − 1) = 22,000 × 3.364 = 74,000 Ω → use 75 kΩ. Verify: V_out at 14.4 V = 14.4 × 22/(75+22) = 14.4 × 0.2268 = 3.267 V (within ADC range). At 10.5 V: V_out = 10.5 × 0.2268 = 2.38 V. Total current: 14.4/(75+22) kΩ = 0.148 mA — acceptable for continuous monitoring.

A voltage divider provides a fixed ratio of input voltage — if V_in changes by 10%, V_out changes by 10%. It has poor load regulation — when load current increases, V_out decreases. It is suitable for signal scaling with high-impedance loads (ADC inputs, oscilloscope probes) and biasing circuits where load current is constant and known. A voltage regulator maintains constant output voltage despite input voltage variation and load current changes. Linear regulators (7805, LM317) and switching regulators (buck, boost) actively control their output. Use voltage dividers for signal conditioning; use regulators for power supply rails.