# Ohmforge Voltage Divider Calculator

> Solve Vout, Vin, R1 or R2 of a divider, with load, power per resistor and E-series pairs. Solves any one unknown of a two-resistor voltage divider (Vout, Vin, R1 or R2), optionally with a load resistance on the output. Returns currents, the power in each resistor, the output (Thevenin) resistance and how far the load pulls the output down, checks the result with Ohmforge's DC simulator and lists the closest standard E-series resistor pairs with their error.

Part of usefulweb.site (https://usefulweb.site/). Part of [Ohmforge](https://ohmforge.usefulweb.site/) (ownerApp: ohmforge). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://ohmforgevoltagedivider.usefulweb.site/ (interactive tool at https://ohmforgevoltagedivider.usefulweb.site/tool)
- JSON API: POST https://ohmforgevoltagedivider.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://ohmforgevoltagedivider.usefulweb.site/api/v1/tool)
- OpenAPI: https://ohmforgevoltagedivider.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://ohmforgevoltagedivider.usefulweb.site/mcp - tool `ohmforge_voltage_divider`

## Inputs
- solveFor (select): Solve for
- vin (number): Input voltage Vin - Ignored when you solve for Vin.
- vout (number): Output voltage Vout - Target output; ignored when you solve for Vout.
- r1 (text): R1 (top, Vin to Vout) - Engineering notation: 4.7k, 4k7, 1M. Ignored when you solve for R1.
- r2 (text): R2 (bottom, Vout to ground) - Ignored when you solve for R2.
- rload (text): Load resistance (optional) - Resistance connected from Vout to ground, e.g. the input of the next stage. Empty = no load.
- series (select): Standard resistor series

## Method
- Unloaded: Vout = Vin · R2 / (R1 + R2). With a load RL the bottom leg becomes R2 ∥ RL = R2·RL / (R2 + RL); the same formula is rearranged for Vin, R1 or R2 (R2 = 1 / (1/R2eff − 1/RL) with R2eff = R1·Vout / (Vin − Vout)).
- Power: P(R1) = I² · R1 with the input current I = Vin / (R1 + R2 ∥ RL); P(R2) = Vout² / R2. The output resistance seen by the load is R1 ∥ R2.
- The circuit is then solved with Ohmforge's DC operating point (Modified Nodal Analysis) as an independent check of the formula.
- E-series pairs: for every R2 of the chosen IEC 60063 series between 100 Ω and 1 MΩ (widened to include your values) the exact R1 is computed and its two neighbouring series values are tried; pairs whose total resistance is within ×3 of the solved R1 + R2 come first (same current level), then they are ranked by |Vout error| and closeness of the total to R1 + R2.

## Limits
- Nominal values only: real resistors deviate by their tolerance (and drift with temperature), and the source and load must be resistive and constant. A divider is not a power supply: for loads that draw significant or changing current use a regulator or a buffer.