Circuit Analysis & Fundamentals
Voltage Divider Calculator
Output voltage of a two-resistor divider from the input and both resistors — or solve for the resistor that gives a target output. Shows the divider schematic and loading caveat.
About this calculator
A voltage divider is two resistors in series across a source: the voltage at their junction is a fixed fraction of the input, set only by the resistor ratio. The rule is V_out = V_in · R2 / (R1 + R2) — the output takes the share of the input that R2 represents of the total series resistance. It is the most common way to scale a voltage down, set a reference, or bias a node.
This calculator works both directions. Give it the input and both resistors and it returns the output voltage, the series current, and the power in each resistor. Switch modes and give it a target output and it solves for the R2 you need. The schematic labels your values so the circuit and the numbers stay together.
The caveat that trips people up is loading. The formula above assumes nothing draws current from the output tap — an ideal, open-circuit output. The moment you connect a load, that load sits in parallel with R2, lowers the effective bottom resistance, and drops the output below the unloaded value. A divider is a good voltage reference only when the load resistance is much larger than R2 (a rule of thumb is at least ten times), or when it feeds a high-impedance input like an op-amp or ADC. For delivering real current, a divider is the wrong tool — use a regulator. This tool reports the unloaded (open-circuit) output and flags the assumption so you size R1 and R2 with loading in mind.
Design notes & common mistakes
- The output is unloaded (open-circuit). Any load sits in parallel with R2 and pulls the output DOWN — size R_load ≥ 10·R2, or buffer with an op-amp follower.
- Only the RATIO sets the output. 1 k/1 k and 1 M/1 M both halve the voltage — but the 1 k pair draws 1000× the current and resists loading better.
- A divider is not a power supply. It cannot hold its voltage while delivering current; for that, use a regulator.
- R1 is the top (series) resistor, R2 the bottom (across the output). Swapping them inverts the ratio.
Assumptions
- Two ideal resistors in series across an ideal source; DC or low frequency (no reactance).
- The output is unloaded (open circuit) — no current is drawn from the tap.
- Resistors are exact; tolerance and temperature coefficient are not applied.
When to use this calculator
Appropriate for
- Scaling a voltage down to a reference or bias level feeding a high-impedance input
- Choosing R1/R2 for a target output ratio, or sizing R2 for a specific output
- Teaching the voltage-division rule and the effect of loading
Not suitable for
- Supplying real load current — a divider sags under load; use a regulator
- Driving a low-impedance load without buffering (the output drops below the computed value)
- AC dividers with reactance or precision references without tolerance analysis
What this calculator does not cover
- Unloaded output only — a real load in parallel with R2 lowers the output; this tool does not model the loaded value.
- Two-resistor divider; multi-tap dividers must be worked node by node.
- Resistive and DC/low-frequency only — no capacitive/inductive dividers or frequency dependence.
- Ideal resistors: tolerance stack-up and temperature drift are not included.
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
What is the voltage divider formula?
V_out = V_in × R2 / (R1 + R2), where R1 is the top (series) resistor and R2 is the bottom resistor across which the output is taken. The output is the fraction of the input that R2 represents of the total series resistance.
Why does my divider output drop when I connect something to it?
Because the load sits in parallel with R2 and lowers the effective bottom resistance, reducing the ratio. A divider only holds its computed (open-circuit) output when the load impedance is much larger than R2 — a rule of thumb is at least ten times, or buffer it with an op-amp follower.
Can I use a voltage divider to power a circuit?
No. A divider cannot hold its output voltage while delivering meaningful current — it sags under load and wastes standing power. To supply current at a fixed voltage, use a voltage regulator, not a divider.
References
- Irwin, J. D., Basic Engineering Circuit Analysis, 11th ed. (voltage division)
- Sedra, A. & Smith, K., Microelectronic Circuits, 8th ed. (dividers and loading)
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