Voltage Divider Calculator

Find the output of a resistor divider, or the resistor you need for a target voltage, including the effect of a load.

This free voltage divider calculator finds the output voltage of a two-resistor divider, or the resistor you need for a target voltage, using standard E24 values. It includes the loading effect of whatever is connected to the output, so you can design level shifters (5 V to 3.3 V), battery voltage sensing for an Arduino or ESP32 ADC, sensor scaling and reference voltages.

Voltage divider formula

Vout = Vin × R2 ÷ (R1 + R2)
R1 = R2 × (Vin − Vout) ÷ Vout
With a load RL: replace R2 with R2 ∥ RL = R2 × RL ÷ (R2 + RL)

Worked example: reading a 12 V battery with a 3.3 V ADC

To scale a battery voltage of up to 14.4 V (a 12 V battery while charging) down to 3.3 V, the ratio is 3.3 ÷ 14.4 = 0.229. With R2 = 10 kΩ, R1 = 10k × (14.4 − 3.3) ÷ 3.3 = 33.6 kΩ; the E24 value 33 kΩ gives 3.35 V at 14.4 V — slightly over, so 36 kΩ (3.13 V) is the safer choice. The divider draws only about 0.3 mA.

Common voltage divider resistor values

Resistor pairs for common signal-level conversions, with R2 = 10 kΩ and R1 rounded to the nearest E24 value. For signals and ADC inputs only — not for powering circuits.

Vin → target	R1	R2	Actual Vout	Divider current
5 V → 3.3 V	5.1 kΩ	10 kΩ	3.31 V	0.33 mA
3.3 V → 1.8 V	8.2 kΩ	10 kΩ	1.81 V	0.18 mA
9 V → 5 V	8.2 kΩ	10 kΩ	4.95 V	0.49 mA
12 V → 5 V	15 kΩ	10 kΩ	4.8 V	0.48 mA
12 V → 3.3 V	27 kΩ	10 kΩ	3.24 V	0.32 mA
24 V → 5 V	39 kΩ	10 kΩ	4.9 V	0.49 mA
24 V → 3.3 V	62 kΩ	10 kΩ	3.33 V	0.33 mA
48 V → 3.3 V	130 kΩ	10 kΩ	3.43 V	0.34 mA

Find the color bands of these values with the resistor color code calculator.

The loading effect

Anything connected to Vout acts as a resistor in parallel with R2, pulling the output voltage down. If your load is a microcontroller ADC or op-amp input (megaohms), the effect is tiny. If it draws real current, enter its equivalent resistance (V ÷ I) in the load field to see the actual output — and consider a regulator instead.

Frequently asked questions

What is the voltage divider formula?

Vout = Vin × R2 ÷ (R1 + R2), where R1 is the resistor between the input and the output and R2 is between the output and ground.

How do I get 3.3 V from 5 V with a voltage divider?

Choose R2, then R1 = R2 × (Vin − Vout) ÷ Vout. With R2 = 20 kΩ, R1 = 20k × 1.7 ÷ 3.3 ≈ 10.3 kΩ; the standard 10 kΩ gives 3.33 V. This is fine for level-shifting a signal into a high-impedance input, but not for powering a 3.3 V circuit.

How do I get 5 V from 12 V with resistors?

For a signal, use R1 = 15 kΩ and R2 = 10 kΩ, which gives 4.8 V. That is fine for feeding a 12 V signal into a 5 V logic or ADC input. To power a 5 V device from 12 V, a divider will not work — use a buck converter or a linear regulator such as a 7805.

How do I measure a battery voltage with an Arduino?

Scale the battery voltage below the ADC reference with a divider, then multiply the reading back up in code: Vbattery = Vadc × (R1 + R2) ÷ R2. Size the divider for the highest voltage the battery can reach, such as 14.4 V for a 12 V lead-acid battery on charge, and use high values (tens of kΩ) so it drains little current.

Can I use a voltage divider as a power supply?

No. The output voltage drops as soon as a load draws current, because the load is in parallel with R2. Use a voltage regulator (linear or switching) to power circuits; use dividers for references, sensing and signal levels.

How do I choose resistor values for a divider?

Only the ratio sets the output voltage; the absolute values trade off current and loading. Lower values waste more current but are less affected by the load. A common rule is to make the divider current at least 10× the load current, or keep R2 at least 10× smaller than the load resistance.