Ohmforge Voltage Divider Calculator

What it is for

A two-resistor divider turns a higher voltage into a lower one: a 24 V rail into something a 3.3 V microcontroller input can read, a battery voltage into an ADC range, a bias point for a transistor. The formula is short, but the practical questions are not: which standard resistors get closest, how much current does the divider waste, and how far does the next stage pull the output down?

Choose what to Solve for (Vout, Vin, R1 or R2) and fill in the other three values. R1 is the top resistor from Vin to Vout, R2 the bottom one from Vout to ground. An optional Load resistance models whatever is connected to the output. The tool solves the circuit, checks it with Ohmforge's DC simulator and lists standard pairs from the series you choose (E6 to E192).

It does not replace a regulator. If the load draws a large or changing current, a divider is the wrong part.

Worked example

You want to measure a 24 V supply with a 3.3 V ADC. You picked R1 = 100k to keep the current small, and the ADC input behaves like 1 MΩ. Settings: Solve for R2, Vin 24 V, Vout 3.3 V, R1 100k, load 1M, series E24.

The tool returns R2 = 16.2 kΩ, and the simulated output is 3.3 V, so formula and simulator agree. The table shows 207 µA from the supply, 4.285 mW in R1 and 672.2 µW in R2. Without the ADC the same resistors would give 3.346 V, so the load costs 1.38 %. The output resistance (R1 ∥ R2) is 13.94 kΩ, which is what the ADC's sample capacitor sees.

The closest E24 pair is R1 = 51 kΩ, R2 = 8.2 kΩ at 3.301 V (+0.0305 %), followed by 62k/10k at +0.148 %. Pairs at a similar total resistance to your 100k design are listed first, so the suggestions keep roughly the same current.

How to read the result

• Divider metrics: the four quantities (the solved one is highlighted), the ratio Vout/Vin and the Simulated Vout from the operating point. If those two ever disagree, something in the input is odd.
• Currents and power: the current from Vin, through R2 and into the load, the power of each resistor, and with a load the unloaded output and the drop it causes.
• Closest pairs: standard values with their output and signed error. The same ratio one decade up or down is shown once.
• The SPICE netlist and an Ohmforge project download let you open the divider in the workbench.

Tips and common mistakes

• Enter resistances with engineering notation: 4.7k, 4k7 and 1M all work.
• Keep the divider at least ten times lower in resistance than the load if the output must stay stable; the callout warns when the load pulls more than 5 %.
• High resistances save current but raise the output resistance. ADC inputs and op-amp bias currents then cause errors.
• The pair error ignores tolerance. With E24 (5 %) parts the real ratio can be off by several percent; use E96 parts for precise dividers.
• Check the power line: a 12 V divider with 100 Ω resistors burns over half a watt.

FAQ

Why is the loaded output lower than the formula I learned?

The load sits in parallel with R2, so the bottom leg becomes R2 ∥ RL. The tool uses that combined value everywhere, including when it solves R1 or R2.

Can the output be higher than the input?

No. A resistive divider can only reduce a voltage; the tool rejects a Vout at or above Vin.

What does "Simulated Vout" add?

It solves the same netlist with Ohmforge's simulator instead of the formula. It is an independent check that the circuit described is the one you meant.

Which series should I choose?

E12 or E24 for everyday 5–10 % parts, E96 for 1 % metal-film resistors. Higher series give smaller errors but need parts you may not have in the drawer.