Voltage divider calculator with two resistors
Calculate a DC output voltage from an upper resistor R1 and a lower resistor R2. An optional resistive load connects from the output node to ground, in parallel with R2. The result shows output voltage, source current and each divider resistor’s dissipation, while the steps also show load current and Thevenin output resistance.
Select a resistor-solving mode to find R1 or R2 for an unloaded target voltage when the other resistance is known. This is a different workflow from the general Ohm’s Law Calculator: the divider combines two resistances and a specified output node.
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Optional digital multimeters (paid link) can help check the actual resistor values and output voltage. The meter’s input resistance can itself act as a load. No purchase is required.
Connect R1, R2 and an optional resistive load
- Choose output voltage, solve R1, or solve R2.
- Enter the positive DC input voltage in volts, up to 60 V.
- For output mode, enter both resistors; for resistor-solving mode, enter the known resistor and the target voltage.
- Select Ω, kΩ or MΩ for each resistance. Changing units converts that entered value and clears the old result.
- Output mode optionally accepts a finite resistance RL from output to ground. Calculate and inspect the steps.
R1 is the upper resistor between Vin and Vout; R2 is the lower resistor between Vout and ground. Ground is the same reference for all voltages. Optional solderless breadboards (paid link) can help arrange a low-voltage test circuit. A wire short, open circuit, negative resistance or active current load is outside the finite positive-resistance model.
The target must be strictly greater than zero and less than Vin. Load controls are disabled when solving a resistor because those modes explicitly solve an unloaded divider. To check loading afterward, return to output mode and enter the solved value with both resistors.
Unloaded and loaded voltage divider formulas
With load: Rbottom = R2 || RL = 1 / (1/R2 + 1/RL)
Loaded: Vout = Vin × Rbottom / (R1 + Rbottom)
Solve R1 = R2 × (Vin/Vtarget − 1)
Solve R2 = R1 × Vtarget / (Vin − Vtarget)
Source current equals (Vin − Vout)/R1. The lower-resistor current equals Vout/R2, and load current equals Vout/RL when a load is present. Source current equals the sum of those two branch currents. Divider resistor powers are (Vin − Vout)²/R1 and Vout²/R2. The unloaded source resistance seen at the output is R1 || R2 for the modeled ideal voltage source.
Optional resistor assortments (paid link) can help compare physical components with a calculated resistance. Resistor-solving modes return exact model values and do not automatically select E12/E24 parts. Substituting a preferred nominal value changes the ratio; check the new output in forward mode.
Two-resistor voltage divider examples with and without load
Unloaded example: Vin = 12 V, R1 = 10 kΩ and R2 = 10 kΩ. Vout = 12 × 10,000 / 20,000 = 6 V. Current is 0.6 mA and each resistor dissipates 0.0036 W. The Thevenin output resistance is 5 kΩ.
Loaded example: add RL = 10 kΩ across R2. The effective lower resistance becomes 5 kΩ. Output falls to 12 × 5,000 / 15,000 = 4 V, which is 33.33333% below the unloaded output. R1/source current is 0.8 mA; R2 and load each draw 0.4 mA. R1 dissipates 0.0064 W and R2 dissipates 0.0016 W.
Resistor-solving example: for an unloaded 5 V input, 3.3 V target and R2 = 10 kΩ, R1 = 10,000 × (5/3.3 − 1) ≈ 5.151515 kΩ. With R1 = 10 kΩ instead, solving R2 gives about 19.41176 kΩ. These exact model values are not promises of a supplier’s standard components.
Loading, resistor tolerance and practical limitations
A divider is useful for a voltage ratio or a high-impedance signal input. It is not a regulated power supply: changing the load changes Vout. A purely resistive load is the only load model here. ADC sampling capacitors, input leakage, protection clamps, frequency response and startup/transient loads require additional analysis.
An ideal source and exact resistances are assumed. Resistor tolerance, temperature coefficient, voltage rating, thermal derating and source internal resistance are excluded. Check component power and voltage limits separately. Optional jumper wires (paid link) should suit the prototype connection and current requirements.
Positive resistances from 0.001 to 1 billion ohms and input voltage from 0.001 to 60 V are supported. These finite bounds are computational, not design approvals. Seven significant digits are shown without treating displayed rounded values as exact parts.
For a single LED or a series LED string, use the LED Resistor Calculator. For unrelated electrical power, use the existing Electrical Power Calculator. Related tools retain their existing functions and category assignments.
Voltage Divider Calculator FAQs
For an unloaded divider, Vout = Vin × R2 / (R1 + R2). R1 runs from input to output; R2 runs from output to ground.
Yes, in output-voltage mode. Enter the load resistance from output to ground. The model replaces the lower leg with R2 in parallel with that load and recalculates output, currents and power.
Yes, for an unloaded target voltage strictly between zero and Vin. Enter the other resistor. The solved value is an exact model value, not a preferred E-series component selection.
Yes. Select Ω, kΩ or MΩ beside each resistance. Changing a unit converts its entered value so the represented resistance stays the same, then clears the old result.
A finite resistance across R2 reduces the effective lower-leg resistance. This draws additional current and lowers output relative to the same unloaded divider.
It is not a regulated supply. Output depends on load. This model assumes a purely resistive load and ideal source; it does not model device startup, dynamic ADC loading, leakage or transient behavior.
Sources and methodology
Independent circuit equations and implementation, reviewed October 24, 2026. Manufacturer datasheets control real component ratings. The linked organizations do not endorse this tool.