Voltage Divider Calculator

Voltage Divider Calculator results cover Vout, R1, R2 or Vin for two resistors or a chain of up to five, along with how a load, resistor tolerance and ADC range move your readings.

Voltage Divider Calculator for 2 to 5 Resistors

A voltage divider is two resistors in series across a source, with the output taken from the junction between them. R1 runs from Vin to the output and R2 runs from the output to ground, the same layout the Quick Setup note in the tool uses.

Makers use it to scale a battery or supply rail into a microcontroller ADC pin. Power designers use the same pair to set the output of an adjustable regulator, and students use it to check bias networks and lab circuits.

Many voltage divider calculator pages stop at the output voltage. This one keeps going and reports the divider current and resistor power, the output under a real load, the worst-case spread from tolerance, and the count an ADC will return.

Solve Forward, in Reverse, or for a Pair

Output Voltage mode takes Vin, R1 and R2 and returns Vout, with the divider ratio and its value in decibels underneath. The R2 and R1 modes work backward from a target output and also round the answer to the nearest part in your chosen E-series.

Input Voltage mode is the reverse voltage divider calculator case. Enter the voltage you measured at the tap plus both resistors, and it returns the source voltage along with the scale factor your firmware should multiply each reading by.

Best Standard Resistor Pair mode needs only Vin, the target Vout and a rough total resistance. It searches E12, E24 or E96 combinations near that total and returns the pair with the smallest error from your target.

Chain mode takes three to five resistors in series and returns the voltage at the tap you pick. It covers reference ladders and multi-threshold circuits that a two-resistor layout can't describe on its own.

Voltages are in volts, and resistances switch between Ω, kΩ and MΩ without retyping. Load, tolerance and ADC settings sit in a closed accordion, and a load of 0 means the output is left open.

The Divider Equation, Rearranged

The same current flows through both resistors, so each one drops a share of Vin in proportion to its resistance. Texas Instruments writes the relationship this way in its note on resistive feedback dividers in DC/DC converters.

$$V_{out}=V_{in}\times \frac{R_2}{R_1+R_2}$$

Solving that equation for each unknown gives the forms the other modes use. Both resistor forms divide by Vin minus Vout, so a target output equal to or above the input has no real answer.

$$R_2=R_1\times \frac{V_{out}}{V_{in}-V_{out}}$$

$$R_1=R_2\times \frac{V_{in}-V_{out}}{V_{out}}$$

$$V_{in}=V_{out}\times \frac{R_1+R_2}{R_2}$$

A divider can only scale down, so the calculator rejects a target Vout at or above Vin, and every voltage and resistor must be above zero. The most common entry mistake is swapping R1 and R2, which turns the default 2.98 V answer for 12 V, 10 kΩ and 3.3 kΩ into 9.02 V.

Divider Current and Resistor Power

The first result card shows the current that flows through the divider all the time, even with nothing connected. Each resistor's power follows from that current, which tells you whether an ordinary 1/4 W part is enough.

$$I=\frac{V_{in}}{R_1+R_2}$$

$$P=I^2\times R$$

Raising both resistors by ten times cuts the current and the power by ten, which helps battery designs. TI's same feedback note warns that very large dividers let leakage into the sensing pin pull the result low, so bigger values are not always more accurate.

Picking Standard Resistor Values

Resistors are sold in the preferred-number series of IEC 60063. E12 has 12 values per decade and pairs with 10% parts, E24 has 24 values for 5% parts, and E96 has 96 values for 1% parts.

The Closest Standard Resistor Pairs table lists up to eight combinations near your target, sorted by error, with the current each one draws. Treat it as the voltage divider calculator resistor values list when the exact answer isn't a part you can buy.

Rounding to the nearest value can push the output past a hard limit. The table below takes 12 V down to a 3.3 V ADC input with R1 fixed at 10 kΩ, and the last column assumes 1% resistors at their worst corners.

Of these four, only the 3.65 kΩ choice keeps a 3.3 V reference safe at every tolerance corner. When the output feeds an ADC or a pin with an absolute maximum, aim the target a few percent below the limit instead of right at it.

Before trusting a reading on the bench, confirm the fitted parts with the Resistor Color Code Calculator. A 3.3 kΩ and a 33 kΩ resistor differ by a single band, and that swap changes the output by a factor of several.

Chains of 3, 4 or 5 Resistors

Chain mode handles a voltage divider with 3 resistors as standard, and R4 and R5 join the string when you give them a value above zero. The Output Tap menu then lists every junction, from below R1 down to just above the last resistor.

Every resistor in a series chain carries the same current, so the voltage at a tap is Vin times the resistance below that tap, divided by the total resistance of the string.

$$V_{tap}=V_{in}\times \frac{R_{below}}{R_{total}}$$

Three 10 kΩ resistors across 12 V draw 400 µA and give 8 V below R1 and 4 V below R2. Both taps see 6.67 kΩ of output impedance, since 10 kΩ and 20 kΩ in parallel give the same value whichever side is larger.

The Voltage at Each Tap table lists every junction along with the drop across each resistor, so one run gives all the thresholds of a comparator ladder. The tap you selected is highlighted, and the hero shows how much resistance sits above and below it.

For the load, tolerance and ADC cards, everything above the tap acts as one resistor and everything below acts as the other. Series parts of the same tolerance can't drift further as a group than one part can, so the worst-case range stays a true bound for the whole chain.

Two chains or dividers fed from the same source and compared tap to tap form a bridge. The Wheatstone Bridge Calculator uses that difference between the two tap voltages to find an unknown resistance.

Output Impedance and Load Droop

Anything connected to the output sits in parallel with R2, or with everything below the tap in chain mode, so the divider behaves like a source with a resistor in series. That resistance is R1 and R2 in parallel, the same value the Parallel Resistor Calculator returns.

$$R_{th}=\frac{R_1\times R_2}{R_1+R_2}$$

$$V_{load}=V_{out}\times \frac{R_L}{R_{th}+R_L}$$

Holding droop to 1% needs a load of at least 99 times the output impedance, and 10% needs 9 times. The default pair has 2.48 kΩ of output impedance, so a 100 kΩ load drops the output from 2.977 V to 2.905 V, about 2.4%.

The card turns green when droop stays within 1% and amber when it doesn't. With no load entered, it shows the output impedance and both load thresholds instead, so you know the smallest load each accuracy level allows.

Worst-Case Tolerance Spread

A common assumption is that two resistors of tolerance T give an output error of ±2T. Texas Instruments shows in application report SLVA423 that the real worst case shrinks as the output gets closer to the input.

$$\frac{\Delta V_{out}}{V_{out}}\approx \pm 2T\times \left(1-\frac{V_{out}}{V_{in}}\right)$$

With the default 12 V to 2.977 V divider and 1% parts, that gives ±1.50% rather than ±2%. TI's own example, a 1.193 V reference scaled up to a 3.30 V rail, lands at ±1.28% with 1% resistors.

The third card skips the approximation and computes both corners exactly, R1 low with R2 high and then the reverse. SLVA423 also notes that temperature and solder heat can widen the spread, though two resistors of the same type tend to drift together and hold the ratio.

The card also lists the thermal noise of the output impedance at 25°C. Analog Devices' MT-047 noise tutorial puts a 1 kΩ resistor at about 4 nV/√Hz, and the value grows with the square root of resistance.

$$e_n=\sqrt{4\times k\times T\times R_{th}}$$

Here k is Boltzmann's constant, 1.380649 × 10⁻²³ J/K, and T is 298.15 K. The default 2.48 kΩ output impedance works out to 6.39 nV/√Hz, which only starts to matter ahead of a 16-bit converter or a high-gain amplifier.

Reading the Divider With an ADC

The fourth card converts the output into the number a converter reports. The ATmega328P datasheet behind the Arduino Uno defines the 10-bit result with 1024 steps, and the same form scales to any bit depth.

$$ADC=\frac{V_{out}\times 2^N}{V_{ref}}$$

Many sketches divide by 1023 instead, which skews every reading by about 0.1%. The default 2.977 V on a 12-bit, 3.3 V converter reads 3,696 counts, and each count stands for 3.247 mV back at the 12 V input.

Headroom is measured from the worst-case high output, not the nominal one. If that corner passes Vref the card turns red, because the reading would clip at full scale exactly when the input is at its highest.

Source impedance matters as much as range. Microchip's ADC analog input guidance says the converter is optimized for sources of 10 kΩ or less, since its sample capacitor charges through that resistance.

The calculator flags any divider above 10 kΩ of output impedance. A capacitor across R2 holds the tap steady for each sample, and the RC Time Constant Calculator shows how long it takes to settle after the input changes.

Worked Example: 12.6 V Battery Monitor

A builder on the All About Circuits forum wanted to read a 12.6 V battery through an ADC with a 2.06 V reference. The plan used 47 kΩ plus 4.7 kΩ on top and 10 kΩ on the bottom, which gives 2.042 V at full charge.

That nominal number looks safe, but with 1% resistors the worst-case high reaches 2.077 V, past the reference. The output impedance of 8.38 kΩ sits under the 10 kΩ guideline, so the problem is range rather than loading.

Swapping the top pair for a single 56 kΩ part brings full charge down to 1.909 V. The worst-case high is now 1.942 V, leaving 118 mV of headroom, while the divider draws 191 µA and its output impedance stays at 8.48 kΩ.

On a 12-bit converter that design reads 3,796 counts at full charge, with each count worth 3.32 mV of battery voltage. Entering the same values in the voltage divider calculator reproduces every figure here, and the pair table shows nearby E24 options if 56 kΩ isn't on hand.

Questions on Resistor Values and Loads

What resistor values drop 5 V to 3.3 V?

You need a ratio of 3.3 divided by 5, or 0.66, so R1 should be about 0.515 times R2. With R2 at 10 kΩ the exact R1 is 5.15 kΩ, and the nearest E24 part is 5.1 kΩ.

That pair gives 3.311 V and draws 331 µA from the 5 V rail. Its output impedance is 3.38 kΩ, which suits an ADC pin or a slow logic input such as a button or a low-speed serial line.

For fast signals like high-speed SPI, that impedance and the pin capacitance round off the edges. A logic-level translator handles those lines better than a pair of resistors does.

Can a voltage divider power a circuit?

Not well, because any load drags the output down through the output impedance. Hang a 1 kΩ load on the default 10 kΩ and 3.3 kΩ divider and the 2.977 V output collapses to 0.855 V.

The output also moves every time the load current changes, so a sensor or radio would see its supply jump around. Use a regulator for power and keep dividers for sensing and reference work, where the load is light and steady.

Driving an LED from a divider is a common version of this mistake. A single series resistor sized with the Series LED Resistor Calculator sets the LED current directly and wastes less power.

What power rating do the resistors need?

Multiply the divider current squared by each resistance, or read both values off the first card. A 48 V input scaled to 3 V with 150 kΩ and 10 kΩ draws 300 µA, putting 13.5 mW in R1 and 0.9 mW in R2.

Build the same ratio from 1.5 kΩ and 100 Ω and the current rises to 30 mA, with 1.35 W in R1. Higher values fix the heat, as long as the output impedance still suits whatever reads the tap.

At higher voltages, check each resistor's maximum working voltage as well as its wattage, and split R1 into several parts in series if needed. Dividers on mains or other hazardous sources also need proper isolation and spacing, so treat them as a safety design rather than a quick calculation.

Does a voltage divider work with AC?

A purely resistive divider scales AC the same way it scales DC, so the ratio holds for RMS, peak and instantaneous values. Stray capacitance across very large resistors starts to change that ratio at higher frequencies.

Dividers built from capacitors or inductors follow the same equation with impedance in place of resistance, and that impedance changes with frequency. The Capacitor Impedance Calculator gives the reactance to use for each leg.