Voltage divider calculator
Solve for Vout, or start from the Vout you want and get the five best standard resistor pairs — with the error, the current they burn and the range they will really give you at ±1, 5 or 10 %.
Best standard pairs
The formula
Field solver: solve ∇·(ε∇V) = 0 over the cross-section twice (real dielectrics, then vacuum) for C and C₀: Z0 = 1 / (c·√(C·C₀)), Eeff = C / C₀ Pair: Zdiff = 2·Zodd, Zcomm = Zeven / 2 Instant first answer: Hammerstad–Jensen / Kirschning–Jansen / Cohn closed forms, bare copper
When to use it
A resistive divider is the right tool for sensing a voltage — battery monitoring on an ADC pin, a feedback node, a logic-level reference that draws next to nothing. It is the wrong tool for powering anything: the output moves with the load, and the resistors dissipate the full divider current all the time. If the load current is not far below the divider current, use a regulator.
Pitfalls
- Tolerance stacks: two 5 % resistors give up to about ±5 % on Vout in the worst direction, which is why the table shows the real range, not just the nominal value.
- A load in parallel with R2 lowers Vout. Enter it and the table recomputes; the rule of thumb is a divider current at least ten times the load current.
- High-value dividers (megohms) save battery but feed an ADC slowly: check the ADC’s input impedance or add a capacitor across R2.
- The same ratio in a different decade is the same divider. Set the current or total resistance range to pick the decade you can afford.
Need the whole board designed, not just the divider?