Two resistors in series can turn a 12 V supply into a polite 8 V, or scale a sensor signal down to what a microcontroller pin can tolerate. This humble circuit — the voltage divider — is everywhere in electronics, and its arithmetic, while simple, is easy to fumble when several candidate resistor pairs are on the bench. The voltage divider calculator works out the output voltage for a given input voltage and pair of series resistors, so you can test combinations on screen before committing them to solder.
Quick Answer
The output voltage is Vout = Vin × R2 / (R1 + R2), where R1 is the resistor on the input side and R2 is the resistor between the output tap and ground. The output is always the fraction R2/(R1 + R2) of the input.
Why Two Resistors Split a Voltage
Resistors in series carry the same current, because there is only one path for charge to flow. That current is set by the total resistance: I = Vin / (R1 + R2). Ohm's law then gives the voltage dropped across each resistor in proportion to its size — V = I × R — so the larger resistor takes the larger share of the supply voltage. The tap between the two resistors sits at the voltage dropped across R2, and dividing the two Ohm's-law expressions is what produces the divider formula. The current cancels out entirely, which is why the formula needs only the ratio of the resistors, not their absolute values alone.
Equal resistors split the input exactly in half, whatever their value. A pair in the ratio 1:2 delivers two-thirds or one-third of the input, depending on which resistor sits on the ground side. The divider therefore sets a fraction, not a fixed voltage: if the input wanders, the output wanders proportionally, a behaviour that is a feature for sensor scaling and a bug for powering circuits that expect a steady rail.
How to Use the Voltage Divider Calculator
- Note your input voltage — the full supply that will appear across both resistors together.
- Identify which resistor connects the input to the output tap (this is R1) and which connects the tap to ground (this is R2); swapping them computes the complementary fraction.
- Enter the input voltage and the two resistances, using the same unit for both resistors — both in ohms, or both in kilohms.
- Run the calculation and read the output voltage at the tap.
- If the fraction is off, adjust the ratio rather than the absolute values: keep the proportion and scale both resistors together to change current draw without changing the output.
- Check the current through the pair (input voltage divided by total resistance) is sensible for your resistors' power ratings.
Worked Example: 12 V Down to 8 V
Take Vin = 12 V, R1 = 1 kΩ and R2 = 2 kΩ. Apply the formula: Vout = 12 × 2 / (1 + 2) = 12 × 2/3 = 8 V.
Verify with the series current: I = 12 V / 3 kΩ = 4 mA. The drop across R1 is 4 mA × 1 kΩ = 4 V, and across R2 it is 4 mA × 2 kΩ = 8 V. The drops sum to the full 12 V supply, and the tap sits 8 V above ground — matching the formula. That self-check, drops summing to the supply, catches most wiring misunderstandings before they reach a circuit board.
- 5 V with R1 = 10 kΩ, R2 = 10 kΩ → 2.5 V, exactly half
- 9 V with R1 = 4.7 kΩ, R2 = 4.7 kΩ → 4.5 V
- 24 V with R1 = 3 kΩ, R2 = 1 kΩ → 6 V, one quarter of the input
- 3.3 V with R1 = 1 kΩ, R2 = 2.2 kΩ → about 2.27 V
Choosing the Actual Resistor Values
The formula fixes only the ratio, leaving a real design choice: how large should the pair be in absolute terms? Scale the worked example up by a hundred — R1 = 100 kΩ, R2 = 200 kΩ — and the output is still 8 V, but the standing current falls from 4 mA to 40 µA, a hundredfold saving in wasted power that matters in battery devices. The catch runs the other way: a high-resistance divider is a weak source. Its output impedance is roughly R1 and R2 in parallel (about 67 kΩ in the scaled version), so noise couples in easily and any load disturbs the voltage, as the caveats below quantify. Sensor circuits feeding slow, high-impedance inputs happily use hundreds of kilohms; anything driving a real load wants kilohms or less.
A potentiometer is this circuit made adjustable — one resistive track, with the tap as a sliding R1/R2 boundary — which is why volume knobs and trim pots behave exactly as the formula predicts across their travel.
The Loading Trap and Other Real-World Caveats
The formula assumes the tap drives nothing. The moment a load connects to the output, that load sits in parallel with R2, dragging the effective resistance down and the output voltage with it. A load ten times R2 already shifts the output noticeably; the working rule is to keep the load resistance at least ten times R2, or buffer the output. Component tolerance is the second reality check: two resistors marked 5% can land the ratio several percent off the nominal calculation, so precision work uses 1% parts. Third, mind the power: each resistor dissipates P = V²/R with its own share of the voltage, and a pair chosen with megohm values to save current can end up too feeble to drive an analogue input, whose own input resistance then loads the tap. Finally, a divider is not a voltage regulator — it cannot hold its voltage up while supplying meaningful current, and treating it as a cheap power supply is the classic beginner disappointment.
More Electronics Helpers
Once a divider stage is feeding other circuitry, current and power numbers usually follow, and the Watts to Amps Calculator converts between the two for exactly those checks. This tool and its companions live in the science section, alongside the physics and wave calculators that share the same circuit-theory roots.