Voltage Divider Calculator
Calculate the output voltage of a two-resistor voltage divider from the input voltage and resistor values, with the current drawn and power dissipated.
How to use this calculator
- 1Enter the input voltage supplied to the divider.
- 2Enter R1, the top resistor between the input and the output node.
- 3Enter R2, the bottom resistor, across which the output is measured.
- 4Read the output voltage, the divider ratio, and the current drawn.
How it works
Voltage divider
Vout = Vin × R2 ÷ (R1 + R2) current = Vin ÷ (R1 + R2) voltage across R1 = Vin − Vout valid only under light (high-impedance) load
A voltage divider is two resistors in series across a voltage source, with the output taken from the junction between them. Because the same current flows through both resistors, the input voltage divides between them in proportion to their resistances — the larger resistor drops the larger share of the voltage. The output voltage, measured across the lower resistor R2, is therefore the input voltage multiplied by R2 divided by the total resistance R1 plus R2. This simple ratio makes the divider one of the most useful building blocks in electronics. The current flowing through the pair is set by the input voltage over the total resistance, by Ohm's law. The crucial caveat is that the formula assumes the output supplies essentially no current; if a load draws meaningful current from the output, it changes the effective resistance and the output voltage sags below the calculated value.
Worked example
With 12 volts in and resistors of 1,000 Ω (R1) and 2,000 Ω (R2), the output is 12 × 2,000 ÷ 3,000 = 8 volts — two-thirds of the input, matching R2's two-thirds share of the total resistance. The current through the divider is 12 ÷ 3,000 = 4 milliamps.
Voltage Divider Calculator: the complete guide
Splitting voltage in proportion
The voltage divider is often the first circuit an electronics student analyses, and for good reason: it captures the essential behaviour of resistors in series in one clean formula. When current flows through two resistors in a row, it is the same current through both, so by Ohm's law each resistor drops a voltage proportional to its resistance. The input voltage is thus split between them in the ratio of their values, and tapping the junction between the two gives an output that is a fixed fraction of the input. Choose the resistor ratio and you choose the fraction.
This proportionality is intuitive once seen. If both resistors are equal, the voltage splits evenly and the output is half the input. If the lower resistor is twice the upper, it takes two-thirds of the voltage. If it is much larger, the output approaches the full input; much smaller, and the output approaches zero. The divider gives a designer a simple, precise way to produce any voltage between zero and the input from a single supply, using nothing but two resistors — which is why it appears constantly in real circuits.
The loading problem
The great limitation of the voltage divider, and the source of most beginner mistakes, is loading. The clean formula assumes that nothing is drawing current from the output — that the output is measured by something with very high resistance, like the input of an op-amp or a microcontroller pin. The moment you connect a load that draws significant current, that load sits in parallel with the lower resistor, reducing the effective resistance and pulling the output voltage down below the calculated value. The heavier the load, the more the output sags.
This is why voltage dividers are used for sensing and reference, not for supplying power. They work beautifully to set a reference voltage for a comparator, to scale a sensor's signal into a readable range, or to bias a transistor, because those applications draw little current from the output. But trying to power an actual load — an LED, a motor, another circuit — from a divider fails, because the load pulls the output down and wastes energy heating the resistors. For power, a proper voltage regulator is needed. Recognising when a divider's light-load assumption holds, and when it does not, is one of the key practical skills in circuit design.
Choosing resistor values
The divider ratio sets the output voltage, but the absolute size of the resistors matters too, and it involves a trade-off. For a given ratio, small resistances draw more current and large resistances draw less. Large values waste less power and load the source lightly, which sounds ideal — but if the resistances are too high, even the tiny current drawn by whatever reads the output becomes significant relative to the divider current, reintroducing loading error, and the circuit becomes susceptible to electrical noise. Small values are stiffer and less noise-prone but waste more power as heat.
The usual compromise is to make the divider current comfortably larger than any current the output must supply — a common rule is at least ten times larger — so the load barely perturbs the ratio, while keeping the values high enough to avoid wasting excessive power. For a divider feeding a high-impedance input like a microcontroller's analogue pin, resistances in the kilohm to tens-of-kilohms range are typical. The calculator shows the current and power for your chosen values, which is exactly what you need to judge this balance: enough current for a stable output, not so much that the resistors run hot or the battery drains needlessly.
Frequently asked questions
How do I calculate a voltage divider?
The output is Vout = Vin × R2 ÷ (R1 + R2), where the output is taken across R2. With 12 V in and resistors of 1,000 Ω and 2,000 Ω, that's 12 × 2,000 ÷ 3,000 = 8 V. The input splits between the resistors in proportion to their values.
Why does my voltage divider output sag under load?
Because a load that draws current sits in parallel with R2, lowering the effective resistance and pulling the output down. The formula assumes negligible output current. Dividers are for reference and sensing, not for powering loads — use a voltage regulator for that.
What resistor values should I use?
Pick the ratio for the voltage you want, then choose absolute values so the divider current is much larger (often 10×) than any current the output supplies, to keep it stable — but high enough to avoid wasting power. Kilohm-range values suit high-impedance inputs like a microcontroller pin.
Can I power a circuit from a voltage divider?
No, not reliably. A divider can't supply meaningful current without its output collapsing, and it wastes energy as heat in the resistors. It's for setting reference voltages and scaling signals. To power a load at a lower voltage, use a voltage regulator or a DC-DC converter instead.