Voltage Divider Calculator

Calculate the output voltage of a two-resistor voltage divider, the current it draws from your supply, and how much power each resistor dissipates. Includes a reverse-design mode for picking R1 and R2 to step any input voltage down to a target (e.g. dividing a 12 V battery to fit Arduino's 5 V ADC).

How it works

A voltage divider is just two resistors in series across your supply. The voltage at the junction between them is a fraction of the input:

Vout = Vin × R2 / (R1 + R2)

Practical example: a 9 V battery and two 10 kΩ resistors gives V_out = 9 × 10/(10+10) = 4.5 V at the junction. To get exactly 5 V from 9 V, use R1 = 4.7 kΩ and R2 = 5.6 kΩ → V_out ≈ 4.9 V.

When (not) to use a voltage divider

Voltage dividers work for signal-level voltages — sensor outputs, ADC inputs, reference voltages. They do not work as power supplies because:

• The output voltage sags under load. The moment you draw current through V_out, R2 effectively becomes R2 in parallel with your load. V_out drops. Math no longer applies.
• Wastes power continuously. Current flows through the divider 24/7 even when no signal is being read.
• For real power conversion, use a buck converter (DC-DC step-down), a linear regulator (e.g. LM7805), or an LDO. They cost $1–3 and don't sag.

Battery monitoring example

To measure a 12 V lead-acid battery on a 5 V Arduino ADC:

1. Maximum expected battery voltage: ~13.8 V (when charging)
2. Arduino ADC max: 5 V
3. Divider ratio: 5 / 13.8 ≈ 0.36, so V_out = V_in × 0.36
4. Pick R1 = 10 kΩ, then R2 = R1 × ratio / (1 − ratio) = 10 × 0.36/0.64 ≈ 5.6 kΩ
5. Verify: 13.8 × 5.6/(10+5.6) = 4.95 V ✓
6. Current draw: 13.8/(10k+5.6k) = 0.88 mA — about 1 mA continuous (fine for sensing, but on battery think about adding a MOSFET to disable the divider when not measuring)

In Arduino code, scale back: actualVolts = analogRead(A0) × 5.0/1023.0 / 0.36

Choosing R1 + R2 (impedance vs power)

The ratio R1:R2 sets the output voltage. The total R1 + R2 sets:

• How much power the divider wastes — P = V_in² / (R1+R2). Bigger total = less waste.
• The output impedance = R1 || R2. Bigger total = higher output impedance = the divider is "weaker" and easier to be loaded down by whatever you connect to V_out.

Rule of thumb for Arduino ADC inputs: keep output impedance under 10 kΩ (so combined R1 || R2 ≤ 10 kΩ). For 12 V → 5 V, that means R1 ≈ 10 kΩ, R2 ≈ 5.6 kΩ — what we worked out above.

Related

• LED Resistor Calculator — current limiting for LEDs
• AC Dimmer Power Calculator — AC RMS calculations
• Nano V3 CH340 — Arduino board with 6 ADC inputs (A0–A5) at 0–5 V
• STM32 Blue Pill — 10× 12-bit ADC at 0–3.3 V