Voltage Divider Calculator

The divider equation

Two resistors in series across a supply produce a tap voltage proportional to the lower resistor's share of the total:

Vout = Vin × R2 / (R1 + R2)

R1 is the top resistor, between V_in and the tap. R2 is the bottom resistor, between the tap and ground. Only the ratio sets the output voltage — but the absolute values decide how much current you waste, how much the output sags under load, and how much noise the node picks up.

Loading is the thing that catches people out

The equation above assumes nothing is connected to the tap. Attach a load and it appears in parallel with R2, pulling the output down:

Vout = Vin × (R2 ∥ RL) / (R1 + (R2 ∥ RL))

A useful shorthand is the divider's output impedance, which is R1 in parallel with R2. As long as the load is at least ten times that figure, the sag stays under about 10%; at a hundred times it is under 1%. A 100 kΩ/100 kΩ divider has a 50 kΩ output impedance, so it can only feed something drawing microamps — an op-amp input or a CMOS gate, not a relay coil.

Choosing the total resistance

The trade-off runs in both directions:

• Low total resistance (say 1 kΩ) gives a stiff output that tolerates loading and shrugs off noise pickup, at the cost of continuous current — 12 mA from a 12 V rail, permanently.
• High total resistance (say 1 MΩ) wastes almost nothing, which matters in battery devices, but the node becomes a high-impedance antenna, picks up mains hum and capacitively-coupled switching noise, and is disturbed by input bias currents and leakage.

For a general-purpose reference or an ADC input divider, 10 kΩ to 100 kΩ total is the usual compromise. For battery monitoring where quiescent current is critical, 1 MΩ with a small capacitor across R2 works well — the cap lowers the AC impedance the ADC sees while keeping the DC current tiny.

Tolerance stacking

Divider accuracy depends on both resistors. Two independent ±1% parts give a worst-case ratio error near ±2%, and about ±1.4% RMS. If the ratio matters more than the absolute values — as it usually does — a resistor array or divider network is worth the money, because the elements are laser-trimmed against each other and track over temperature far better than two separate parts.

What a divider cannot do

A voltage divider is not a power supply. It cannot regulate, it has no load rejection, and its output falls with any current drawn. Do not use one to power a microcontroller, drive a relay, or step a rail down for a load with varying current. For those jobs use a linear regulator or a buck converter. Dividers belong where the load draws essentially no current: ADC sense inputs, feedback nodes, bias points, reference taps and attenuators.

Common applications

• Battery voltage sensing. Scale a 12 V or 24 V rail down into a 3.3 V ADC range. Use 1% parts and keep the total high to save current.
• Level shifting a slow signal down. Fine for a 5 V logic output feeding a 3.3 V input at low speed. Not fine for fast edges — the divider's impedance and the input capacitance form a low-pass filter.
• Setting a regulator's feedback. Adjustable regulators use a divider to set output voltage; the datasheet fixes the reference voltage and usually specifies a maximum bottom-resistor value.
• Biasing a transistor or op-amp to mid-rail. Standard practice in single-supply analogue circuits, usually with a decoupling capacitor on the tap.

Frequently asked questions

How do I calculate a voltage divider?

Vout = Vin × R2 / (R1 + R2), where R1 is the top resistor between the input and the tap, and R2 is the bottom resistor between the tap and ground. Only the ratio sets the output voltage; the absolute values decide current draw and loading behaviour.

Why is my divider output lower than calculated?

Almost always loading. Anything connected to the tap sits in parallel with R2 and pulls the output down. Compare your load resistance to the divider's output impedance (R1 in parallel with R2) — you want the load to be at least ten times higher.

Can I power a circuit from a voltage divider?

No. A divider has no regulation and its output collapses as soon as meaningful current is drawn. Use a linear regulator or a buck converter for anything that draws more than a few microamps. Dividers are for sensing and biasing, not supplying power.

What resistor values should I use in a divider?

For most sensing and biasing work, a total of 10 kΩ to 100 kΩ is a good compromise between wasted current and noise immunity. Go lower for stiff outputs and noisy environments; go up to 1 MΩ for battery monitoring where quiescent current matters, and add a small capacitor across the bottom resistor.

Does a voltage divider work for AC signals?

Yes at low frequencies, but the divider's impedance forms a low-pass filter with any stray and input capacitance, so fast edges get rounded off. For high-speed signals a compensated divider — with a small capacitor across the top resistor — keeps the ratio flat with frequency.

How accurate is a divider made from 1% resistors?

Worst case the ratio error approaches ±2% and typical RMS error is about ±1.4%, because both resistors contribute. If the ratio matters, use a matched resistor array — the elements are trimmed against each other and track over temperature far better than two separate parts.