Voltage Divider Calculator (with Load)

Background.

A microcontroller's analog input tops out at 3.3 V and you want it to read a 12 V battery. The usual answer is two resistors in series across the battery, with the input wired to the joint between them. That is a voltage divider. Its output is a fixed fraction of the input, R2 ÷ (R1 + R2), where R1 runs from the supply down to the tap and R2 from the tap to ground. With 10 kΩ on top and 4.7 kΩ below, 12 V becomes 3.8367 V, a little too much for that input, which is exactly the kind of thing worth knowing before you solder.

The same pair of resistors sets the output of adjustable regulators through their feedback pins, biases transistor stages, gives comparators a threshold, shifts 5 V logic down to 3.3 V, and, with one resistor swapped for a thermistor, light-dependent resistor or potentiometer, turns a physical quantity into a voltage. The formula is identical in every case, and so is its blind spot: it assumes nothing draws current from the tap. Connect anything and it sits in parallel with R2, the effective lower resistance falls, and the output sags. A divider that reads perfectly on a multimeter can read a quarter low once the real circuit is attached.

That is why the load is an input here, not an afterthought. Next to the ideal output you get the divider's output impedance, the output with your load connected, the percentage it sagged and a plain verdict on whether that is acceptable. You also see the current the pair draws at all times and the power it turns into heat, which are the two prices you pay for a stiffer output.

You can check a divider you already have, or design one by giving R1 and the voltage you want and letting R2 be solved. The model assumes a stiff supply, steady or slowly changing voltages, and resistors at exactly their nominal values; the FAQs below cover what changes when those assumptions fail.

What is voltage divider calculator?

A voltage divider is two resistances in series across a voltage source, with the output taken from the node between them. Because there is only one path, the same current flows through both resistors, and by Ohm's law that current is I = Vin ÷ (R1 + R2). The output is the voltage dropped across the lower resistor, I × R2, which gives Vout = Vin × R2 ÷ (R1 + R2). Only the ratio of the two values matters to this unloaded voltage, so 10 kΩ over 4.7 kΩ and 10 MΩ over 4.7 MΩ give the same answer.

The ratio does not tell the whole story, because the two pairs behave very differently once something is connected. Seen from the tap, a divider behaves like an ideal source of Vout in series with a resistance equal to R1 and R2 in parallel, its Thévenin equivalent. Whatever you connect forms a second divider with that internal resistance. A potentiometer is the same circuit with a sliding tap, and a sensor divider is the same circuit with one resistor that changes with temperature, light or position.

How to use this calculator.

1. 01Choose the mode: the output voltage if you already have both resistors, or the lower resistor R2 if you know the voltage you want and have picked R1.
2. 02Enter the input voltage across the whole divider, the supply rail or battery feeding the top of R1, not the output you are aiming for.
3. 03Enter R1, the upper resistor between the supply and the tap, and either R2, the lower resistor between the tap and ground, or the target output voltage. Swapping R1 and R2 gives a plausible but wrong answer: 12 V across 4.7 kΩ over 10 kΩ gives 8.16 V instead of 3.84 V.
4. 04Enter the resistance of whatever the tap drives: the input resistance of an ADC, meter, comparator or transistor stage from its datasheet. Leave it at 0 if nothing is connected; 0 here means an open circuit, not a short to ground.
5. 05Read the loaded output and the droop beside the ideal figure, and check the verdict. If the droop is too large, scale both resistors down by the same factor, which keeps the voltage and cuts the output impedance, or buffer the tap with an op-amp follower.
6. 06In design mode, round the solved R2 to a resistor you can buy, switch to the first mode and run it again with that value to see the voltage you will actually get.

The formula.

Unloaded output: Vout = Vin × R2 ÷ (R1 + R2). Rearranged for design, R2 = R1 × Vout ÷ (Vin − Vout). The denominator shows why a divider can only step down: as the target approaches the supply, Vin − Vout shrinks toward zero and R2 heads for infinity; past it, R2 would be negative. A target at or above the input voltage is therefore refused, and a boost converter, charge pump or transformer is needed instead.

The current through the pair is I = Vin ÷ (R1 + R2), and it flows whether or not anything is connected. The power it dissipates is P = Vin × I = Vin² ÷ (R1 + R2), all of it as heat in the two resistors.

The output impedance is R_th = R1 × R2 ÷ (R1 + R2), the two resistors in parallel, which is what the tap looks like with the supply replaced by a short. A useful identity follows from the divider equation: R_th = R1 × (Vout ÷ Vin). With a load R_L connected, R2 is replaced by R2 in parallel with R_L and the divider equation is applied again. The same result can be written as Vout × R_L ÷ (R_th + R_L), and the droop as a fraction of the unloaded output is R_th ÷ (R_th + R_L). That last expression shows that the sag depends on one number only, the ratio k = R_L ÷ R_th. The verdict bands translate directly: under 1% droop needs k above 99, under 5% needs k above 19, and k below 4 means a sag of 20% or more, at which point the load is setting the voltage as much as the resistors are. The familiar "load at least ten times the output impedance" rule gives 1/11, about 9.1%, which is only good enough for rough work.

Resistor tolerance is not included, because each part is taken at its nominal value. Its effect is easy to estimate: the fractional error in Vout is roughly (1 − Vout/Vin) multiplied by the combined tolerance of the two resistors, so a divider that drops most of the supply is the most sensitive.

A worked example.

Frequently asked questions.

References& sources.

1. [1]OpenStax (Rice University), "University Physics Volume 2", §10.2 "Resistors in Series and Parallel". Equation 10.2 gives the series rule RS = R1 + R2 + … = Σ Ri and the common-current constraint the divider relation is derived from; Equation 10.3 gives the parallel rule used for the loaded case and states that the parallel equivalent "is less than the smallest of the individual resistances". Retrieved and read 2026-07-29.
2. [2]"Series and parallel circuits", reference article, retrieved and read 2026-07-29. Independent statement of both rules used here: R = Σ Ri in series, R = (Σ 1/Ri)⁻¹ in parallel with the two-resistor form R = R1R2/(R1 + R2) — the expression this page uses for the divider's Thevenin output impedance and for the loaded lower arm. Agrees with OpenStax term for term.
3. [3]Vishay BCcomponents, "MRS16, MRS25 Professional Thin Film Leaded Resistors", document 28724, revision 07-Mar-16. Read directly from the PDF on 2026-07-29. Source of the maximum-operating-voltage figures quoted in the high-voltage-divider answers: Umax AC/DC of 200 V (MRS16) and 350 V (MRS25), alongside rated dissipation P70 of 0.4 W and 0.6 W and a peak permissible film temperature of 155 °C — a voltage limit that is specified separately from, and often binds before, the power limit.
4. [4]Vishay, "D/CRCW e3 Standard Thick Film Chip Resistors", document 20035, revision 14-Apr-2026. Read directly from the PDF on 2026-07-29. Gives the surface-mount maximum operating voltages used on this page — 75 V (0402 and 0603), 150 V (0805), 200 V (1206, 1210, 1218), 400 V (2010), 500 V (2512) — together with rated dissipation quoted as P70 and a 155 °C film limit.
5. [5]IEC 60063:2015, "Preferred number series for resistors and capacitors", third edition, 27 March 2015. Defines the E24 (±5 %) and E96 (±1 %) value sets referred to when turning a solved R2 into a resistor you can actually buy. Retrieved 2026-07-29 via the reference summary of the series values and their standard tolerances.
6. [6]Vishay Draloric / Beyschlag / BCcomponents, "Resistor Color Card", document VMN-MS6212-1501 (2015). Read directly from the PDF on 2026-07-29. Prints the E24 and E96 preferred-value tables under the heading "According to IEC 60063" — the source used to check that a solved R2 such as 3,793 Ω has a nearest stock value of 3.9 kΩ in E24 and 3.83 kΩ in E96.