Voltage Divider Calculator
Free online voltage divider calculator.
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What is Voltage Divider Calculator?
A voltage divider is a pair of series resistors that scales an input voltage down to a lower output voltage. This calculator computes Vout = Vin Γ R2 / (R1 + R2) instantly, along with the divider current, the power dissipated in each resistor, and optional loaded-output analysis β plus a reverse solver that finds the R2 needed for a target voltage using E24 standard values.
Key Benefits
- Design sensor and ADC reference dividers without manual algebra
- Verify resistor power ratings before parts go up in smoke
- Predict output sag when the divider drives a real load
- Snap ideal resistor values to real E24 series parts
- Works fully offline β nothing is uploaded
Common Use Cases
- β’Scaling a 5V or 3.3V sensor signal into an ADC input range
- β’Building a reference voltage for comparators or op-amps
- β’Measuring battery voltage on a microcontroller
- β’Level-shifting logic signals between different voltage domains
- β’Choosing safe resistor pairs for hobby and prototype circuits
Design a Resistor Voltage Divider
- Enter Vin, R1, and R2: In standard mode, type the input voltage, the top resistor, and the bottom resistor in ohms β the output voltage, divider ratio, current, and power in each resistor appear instantly, e.g. 5 V across two 10 kOhm parts gives 2.5 V.
- Model a real load: Tick the load checkbox and enter RL to see the loaded output voltage, computed from Req = (R2 x RL) / (R2 + RL) in parallel with R2 β useful for checking how far the output sags.
- Or reverse-solve for R2: Switch to Find R2 mode, enter your target Vout and fixed R1, and read the ideal R2, the closest E24 standard value, and the actual Vout with its percentage error.
- Verify with the step-by-step math: Scroll to the derivation panel to see every formula with substituted numbers, then Copy the result summary for your notes β all calculations stay in your browser.
Key Features
- Real-time Vout, divider ratio, and current as you type
- Per-resistor power dissipation for safe part selection
- Loaded-divider mode with parallel RL
- Reverse-solve R2 for a target Vout with E24 snapping
- Step-by-step derivations with substituted numbers
Frequently Asked Questions
Which formula does the divider use?
The unloaded divider Vout = Vin x R2 / (R1 + R2). Enable the load option and the tool first computes Req = (R2 x RL) / (R2 + RL) in parallel with R2, showing how much the output sags under a real load.
Can it tell me which resistor to buy for a target voltage?
Yes β switch to Find R2 mode, enter your target Vout and fixed R1, and it solves R2 = (Vout x R1) / (Vin - Vout), snaps the ideal value to the closest E24 standard resistor, and reports the actual Vout and percentage error you would get.
Why is the error percentage sometimes shown in amber?
When the closest E24 standard value deviates from your target by more than 5% the error turns amber, prompting you to adjust R1 or use a finer series such as E96 or a trimmer for tighter tolerance.
Does it show power dissipation in the resistors?
Yes β it computes the divider current I = Vin / (R1 + R2) and the power in each resistor (I squared x R), so you can tell whether standard 1/4 W parts are safe or you need a higher wattage. Everything runs locally in your browser.