Developer Tools
Voltage Divider Calculator
Solve any unknown in a two-resistor voltage divider. Get Vout, R1, R2, current, power per resistor, loaded output, and the nearest E12 and E24 standard values.
What do you want to solve for?
Given Vin, R1, and R2, find the output voltage at the divider midpoint.
Known values
Input voltage applied across R1 + R2.
Solving for this value.
Upper resistor, between Vin and the Vout junction.
Lower resistor, between the Vout junction and ground.
A real divider drives something. Adding RL recomputes the loaded output as Vin times (R2 parallel RL) over R1 plus (R2 parallel RL). Keep RL much larger than R2 for a clean reference voltage.
Quick presets
Click any preset to load its values. You can edit them after loading.
Nearest standard resistor pairs
Exotic resistor values rarely exist on a shelf. These are the closest pairs from the IEC 60063 E12 (10% tolerance) and E24 (5% tolerance) series, plus the actual Vout each pair produces.
E12 series
10% values, 12 per decade
R1
10 kΩ
R2
10 kΩ
Resulting Vout
6 V
Error vs target: +0%
E24 series
5% values, 24 per decade
R1
10 kΩ
R2
10 kΩ
Resulting Vout
6 V
Error vs target: +0%
Formula reference
Every rearrangement of the voltage-divider relation, plus the quantities that drop out once any three of Vin, Vout, R1, R2 are known.
Solving for Vout
- from Vin, R1, R2Vout = Vin × R2 / (R1 + R2)
Solving for R2
- from Vin, Vout, R1R2 = R1 × Vout / (Vin − Vout)
Solving for R1
- from Vin, Vout, R2R1 = R2 × (Vin − Vout) / Vout
Solving for Vin
- from Vout, R1, R2Vin = Vout × (R1 + R2) / R2
Loaded output
Vout = Vin × (R2 ∥ RL) / (R1 + R2 ∥ RL)
R2 ∥ RL is the parallel combination: (R2 × RL) / (R2 + RL). The divider stops behaving like a clean reference once RL is on the same order as R2.
Current and power
I = Vin / (R1 + R2)
P_R1 = I² × R1, P_R2 = I² × R2
Always check the per-resistor power against the part's rated wattage. A 0.25 W resistor will burn out above 0.25 W.
How to use
- Pick the unknown you want to solve for: Vout, R2, R1, or Vin. The selected field will go into solving mode and the other three remain editable.
- Enter the three known values. Click the unit chips beside each input to switch between mV, V, kV for voltages and ohms, kilohms, or megohms for resistors.
- Read the result panel: the solved value highlights green, and the side cards show divider current, power in R1, and power in R2. Use Copy full report to grab a plain-text summary of every result.
- Check the nearest E12 and E24 standard resistor pairs. Each card shows the actual Vout that pair produces and the percent error versus your target, so you can pick parts that exist on a shelf.
- Turn on the load resistance toggle if a real load will be connected at Vout. Enter RL and the calculator recomputes the loaded output, the loaded current, and the effective R2 parallel RL resistance.
- Click any preset to load a common scenario like 12 V to 5 V, 5 V to 3.3 V level shift, or a battery monitor divider, then tweak the values to match your circuit.
About this tool
Voltage Divider Calculator solves the standard two-resistor voltage divider for any unknown quantity. Pick the value you want to find, enter the other three, and the tool computes Vout, R1, R2, or Vin using the rearranged form of Vout = Vin × R2 / (R1 + R2). Every result also includes the divider current, the power dissipated in each resistor, the Vout to Vin ratio, and a full step-by-step working trace so you can check the math by hand. An optional load resistance lets you see how much the output sags once a real load is connected at Vout: the calculator replaces R2 with R2 in parallel with RL and reports the loaded output, the loaded current, and the effective lower-leg resistance. To make the result actionable on a bench, the tool also reports the nearest IEC 60063 E12 (10 percent) and E24 (5 percent) standard resistor pairs and the actual Vout each pair produces, so you can pick parts that exist on a shelf instead of theoretical values. Six built-in presets cover the cases this tool sees most often: 12 V to 5 V, 5 V to 3.3 V microcontroller level shifting, half-rail biasing, 24 V to 5 V ADC scaling, a 4.2 V single-cell Li-ion monitor, and a Wheatstone half-bridge midpoint. Inputs accept mV, V, kV for voltages, and ohms, kilohms, or megohms for resistance, with comma or dot decimals and trimmed whitespace. Useful for breadboard prototypes, ADC scaling, op-amp biasing, sensor reference points, level shifters, battery low monitors, and electronics homework. Everything runs locally in your browser, so the values you enter never leave your device.
Free to use. Works in your browser. No signup, no login.
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