Voltage Divider Tool | Celtic Engineering Solutions

Celtic Engineering Toolbox

Voltage Divider Tool

Design and analyze simple two-resistor voltage dividers. Compute Vout and current from Vin, R1, and R2, or solve for a resistor given a target output voltage. Intended for sense and reference dividers, not power supplies.

Voltage Divider Calculator

Choose a mode, enter the known values, and we’ll compute the missing one, along with divider current and power in each resistor.

Mode

Input voltage Vin (V)

Target output Vout (V) – used in solve modes

R1 (top resistor, Ω)

R2 (bottom resistor, Ω)

Quick presets

How to use this tool

A voltage divider is just two resistors in series. You tap the middle to get a fraction of the input voltage. This helper keeps the math and current visible.

Pick a mode:
- Compute Vout – you already know Vin, R1, and R2.
- Solve R2 – you know Vin, your target Vout, and R1.
- Solve R1 – you know Vin, your target Vout, and R2.
Enter the voltages in volts (V) and resistors in ohms (Ω). Use whole numbers, not kΩ (for example 4.7 kΩ → 4700).
Click Calculate.
Read:
- Output voltage Vout (V)
- Divider current (A and mA)
- Power in R1 and R2 (W)
- R1:R2 ratio

Example 1 – 12 V down to 5 V sense input

• Mode: Solve R2
• Vin = 12 V, target Vout = 5 V
• Choose R1 = 10 kΩ

The tool solves R2 ≈ 7.7 kΩ. Using a real 7.5 kΩ or 8.2 kΩ resistor gives a Vout close to 5 V, with only a small sense current.

Example 2 – 5 V down to 3.3 V for a logic input

• Mode: Solve R2
• Vin = 5 V, target Vout = 3.3 V
• Choose R1 = 3.3 kΩ

The tool solves R2 ≈ 4.9 kΩ. A standard 4.7 kΩ gives a Vout slightly under 3.3 V, still high enough for many 3.3 V inputs.

Divider formulas and what they assume

For a simple two-resistor divider with R1 on top and R2 on the bottom (output taken at the junction, referenced to ground):

Vout = Vin × R2 / (R1 + R2)
I_divider = Vin / (R1 + R2)
P_R1 = I² × R1
P_R2 = I² × R2

This tool assumes the load connected to Vout draws negligible current compared to the divider. If the load’s input resistance is comparable to R2, it will pull the output down and the math above will no longer be accurate.

Rule of thumb. For a clean sense divider, make the input’s resistance at least 10× higher than R2. For very accurate work, model the load as a parallel resistor or choose a much stronger (lower-resistance) divider and accept the extra current.

Design checklist for voltage dividers

Use this as a quick sanity check after you calculate values.

Purpose. Use dividers for sensing, level detection, and references — not for powering significant loads. For power rails, use regulators or converters.
Current vs. battery life. Lower resistances give better noise immunity and load tolerance, but they burn more current continuously. Decide what you can afford.
Resistor power rating. Compare P_R1 and P_R2 with each resistor’s power rating (¼ W, ½ W, etc.). Use at least a 2× margin in real hardware.
Safety and fault conditions. Consider what happens if the load is shorted, or if Vin rises above its nominal value. Ensure the resistors and downstream circuitry are still safe.
Tolerance and accuracy. With ±1% or ±5% resistors, Vout also varies. If your threshold is tight, consider precision resistors or a different approach (reference + buffer).

Examples from Celtic Engineering Solutions

These examples are simplified versions of actual design choices where a quick divider check clarified limits and tradeoffs.

Example A

Monitoring a 24 V rail with a microcontroller ADC

An ADC channel could accept up to 3.3 V. The supply rail to monitor was nominally 24 V with some headroom.

• Vin = 24 V, target Vout ≈ 3 V at nominal
• Choose R1 = 100 kΩ, solve R2
• R2 ≈ 15 kΩ → divider current ≈ 0.21 mA

The ADC saw a safe voltage with modest divider current, and a simple calibration factor turned the reading into volts.

Example B

Sense input derived from a 12 V line

A digital input needed to read “line present” from a 12 V rail. The input pin preferred 5 V logic levels.

• Vin = 12 V, target Vout ≈ 5 V
• R1 = 18 kΩ, solve for R2
• R2 ≈ 11.5 kΩ → actual Vout with 11 kΩ ≈ 4.9 V

The input saw a solid logic HIGH when the line was present and close to 0 V when not.

Example C

Battery monitor for a 4-cell Li-ion pack

A pack up to about 16.8 V (4 × 4.2 V) needed to be monitored by a 3.3 V ADC.

• Vin(max) ≈ 16.8 V, Vout(max) target ≈ 3.2 V
• Choose R1 = 82 kΩ, solve R2
• R2 ≈ 18 kΩ → max divider current ≈ 0.17 mA

The ADC saw the full battery range without exceeding its input limit, while keeping quiescent current acceptable for a battery product.

Example D

Trim reference for a comparator threshold

A comparator needed a reference at about 1.2 V from a 5 V rail.

• Vin = 5 V, target Vout ≈ 1.2 V
• R2 chosen as 10 kΩ for low noise
• Solve R1 → R1 ≈ 31.7 kΩ, implemented with 31.6 kΩ

The threshold was stable and easy to reproduce across units with standard-tolerance resistors.

Unsure if a divider is the right tool?

If your divider is feeding an ADC, a long cable, or anything safety-relevant, this tool is a starting point, not the final answer. Send a short note with your voltages, target device, and load details, and we’ll help you confirm whether a divider is appropriate or if a different approach makes more sense.

Send one question about your divider →

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