Voltage Divider Calculator

Step-by-Step Guide

1. 1Enter Input Voltage (Vin): Type the supply voltage feeding the divider circuit, in volts.
2. 2Enter R1 (Top Resistor): Input the resistance between the input voltage and the output tap point, in Ω, kΩ, or MΩ.
3. 3Enter R2 (Bottom Resistor): Input the resistance between the output tap point and ground. Vout is measured across this resistor.
4. 4Read the Output Voltage and Ratio: View Vout, the R2 ÷ (R1 + R2) ratio, and a full step-by-step breakdown of the calculation.
5. 5Check Current and Power Dissipation: Review the current through the divider and power dissipated in each resistor to confirm wattage ratings are sufficient.

What This Calculator Provides

• ✓Real-time calculation as you type
• ✓Full step-by-step calculation breakdown
• ✓Current through the divider (mA)
• ✓Power dissipation in R1 and R2 (mW)
• ✓Resistance unit support: Ω, kΩ, MΩ
• ✓Six built-in presets (Arduino, ESP32, logic-level)
• ✓Calculation history
• ✓Export calculation as a text file
• ✓Private: your inputs are not collected or stored
• ✓No signup required

Pro Tips

• 💡Keep total resistance (R1 + R2) low relative to your load's input impedance — a good rule of thumb is at least 10× lower — so the load doesn't meaningfully pull the output voltage down from the calculated no-load value.
• 💡For battery-powered circuits, keep total resistance high (tens to hundreds of kΩ) to minimize the divider's own current draw, since that current flows continuously whenever the circuit is powered, even with no load connected.
• 💡Always check the power dissipation result against your resistor's wattage rating, especially for low total resistance values. A 1/4-watt resistor is fine for most logic-level dividers but can be exceeded in low-impedance test or power circuits.
• 💡Use standard E12 or E24 resistor values (like 4.7kΩ, 3.3kΩ, 10kΩ) rather than the mathematically exact ratio — off-the-shelf resistors have a tolerance of 1-5% anyway, so hitting an odd exact value isn't worth sourcing a custom part.
• 💡Add a small capacitor (0.1μF is common) across R2 if you're feeding the output into an ADC, to filter noise — this doesn't change the calculated Vout but improves reading stability in practice.

Common Mistakes to Avoid

• ✕Swapping R1 and R2. Vout is measured across R2, the bottom resistor — putting the wrong resistor in the wrong position inverts the ratio and gives a completely different output voltage than intended.
• ✕Ignoring load sag when the downstream device draws meaningful current. The standard formula assumes no load; connecting anything with a comparable or lower impedance than R2 in parallel will pull Vout below the calculated figure.
• ✕Using a voltage divider to supply real power, like charging a battery or driving a motor. Dividers waste power as heat and cannot maintain voltage under significant current draw — use a regulator or converter for anything beyond a signal-level connection.
• ✕Forgetting to check resistor power ratings on low-resistance dividers. A divider built with 100Ω resistors at 12V dissipates hundreds of milliwatts — well above a standard 1/4-watt resistor's limit — and will overheat or fail.
• ✕Mixing resistance units without converting first. Entering R1 as "4.7" in ohms when you meant 4.7kΩ produces a ratio off by a factor of 1,000 — always confirm the unit selector matches the value you intended.