The voltage divider calculator above computes the output of a two-resistor divider, sizes either resistor to hit a target output, and models what happens once you connect something to the tap. That last part is the one that matters in practice. The unloaded formula is trivial arithmetic that any textbook gives, and it is also the reason so many dividers fail in real circuits: the number it predicts is only correct while nothing is drawing current, which is rarely the case.
Arb Digital builds free tools that model the failure mode as well as the ideal case. This one shows the loaded and unloaded outputs side by side, gives the shift between them as a percentage, and reports the divider's output impedance so you can judge in advance whether a given load will disturb it. It also reports current and power, because a divider that gives the right voltage while quietly wasting milliwatts is a poor design in a battery-powered product.
What This Voltage Divider Calculator Does
A voltage divider is two resistors in series across a supply, with the output taken from the junction between them. Because the same current flows through both, the supply voltage splits between them in proportion to their resistances. OpenStax University Physics Volume 2, section 10.2 on resistors in series and parallel, puts it directly: the current is the same in each resistor, and individual resistors in series do not receive the full source voltage but divide it, with the total drop equal to the sum of the individual drops.
The tool works in three directions. Forwards, it takes both resistors and returns the output. Backwards, it takes a target output and one resistor and returns the other, which is the mode you want when designing rather than analysing. In every mode it also applies a load resistance if you give it one, recomputing the output with the load in parallel with the lower resistor.
The two bars show how the supply splits between the upper and lower resistor, which is a quicker sanity check than reading the numbers. If your intended output is a small fraction of the supply, most of the voltage should appear across R1, and a bar showing the opposite means the resistors are the wrong way round.
How to Use It
- Pick a mode. Analysis if you have both resistors, one of the design modes if you know the output you want and one resistor value.
- Enter the supply voltage across the whole divider. That is the voltage from the top of R1 to the bottom of R2, not the output.
- Set each resistor with its own unit. Kilohms is the default because that is the range most signal-level dividers live in.
- Add the load resistance if anything is connected. Leave it at zero to mean nothing is connected. For a load specified as a current draw, divide the expected output by that current to get an equivalent resistance.
- Compare the output impedance with the load. As a working rule, a load ten times the output impedance shifts the result by roughly a tenth; a hundred times shifts it by about a hundredth.
The Formula: How a Voltage Divider Is Calculated
Unloaded, Vout = Vin × R2 ÷ (R1 + R2). Rearranged for design, R2 = R1Vout ÷ (Vin − Vout) and R1 = R2(Vin − Vout) ÷ Vout. With a load RL connected, R2 is replaced by the parallel combination R2RL ÷ (R2 + RL) and the same formula applied. Output impedance is the parallel combination of the two resistors seen from the tap, R1R2 ÷ (R1 + R2).
Work the defaults. With 12 V across two 10 kΩ resistors, the output is 12 × 10 ÷ 20 = 6 V, exactly half, as symmetry demands. The current is 12 ÷ 20,000 = 0.6 mA, and the total power is 12 × 0.0006 = 7.2 mW, split evenly between the two resistors. The output impedance is 10 × 10 ÷ 20 = 5 kΩ.
Now connect a 10 kΩ load. R2 in parallel with the load is 5 kΩ, so the output becomes 12 × 5 ÷ 15 = 4 V. The divider you designed for 6 V now delivers 4 V, a shift of a third. That is not a subtle error or a tolerance issue; it is the divider behaving exactly as the physics requires, and it is the single most common surprise for anyone using one for the first time.
Why Loading Ruins a Divider
A divider is not a voltage source. It behaves like a source of the unloaded output voltage in series with a resistance equal to the two resistors in parallel, which is what the output impedance figure reports. Draw current through that internal resistance and the voltage sags, exactly as a battery with high internal resistance does. The result falls straight out of Kirchhoff's rules, set out in OpenStax University Physics Volume 2, section 10.3: the current entering the tap must equal the current leaving it, so any current taken by the load has to come out of what was flowing through the lower resistor.
The practical rule follows immediately. If the load is much larger than the output impedance, the sag is small; if it is comparable, the sag is enormous. A load equal to the output impedance halves the effective lower resistance and pulls the output down dramatically. A load a hundred times larger shifts it by roughly one per cent, which is usually acceptable.
This is why dividers are fine for feeding a high-impedance input, such as an operational amplifier input or a microcontroller analogue-to-digital converter with an input impedance in the megohms, and useless as a power supply for anything that draws real current. Trying to run a device from a divider means the output voltage changes whenever the device's consumption changes, which for anything with a variable current draw makes the supply completely unstable. A regulator is the right answer there, not a resistor pair.
Choosing the Resistance Scale, Not Just the Ratio
The ratio sets the output voltage, but the absolute values matter for everything else, and an infinite number of pairs give the same ratio. Two 10 Ω resistors and two 10 MΩ resistors both give exactly half the supply, and they behave completely differently.
Low values give a low output impedance, so loading barely disturbs them, but they draw substantial current continuously. A 12 V supply across two 10 Ω resistors draws 600 mA and burns 7.2 W permanently, which is disastrous in anything battery-powered and needs resistors physically large enough to survive. High values draw almost nothing — two 10 MΩ resistors on the same supply consume 0.6 microamps — but the output impedance is 5 MΩ, so almost any load disturbs them, and they become vulnerable to problems that do not exist at lower impedances.
Those problems are worth naming. Very high impedance nodes pick up noise readily, since a tiny induced current produces a large voltage. Printed circuit board leakage, flux residue and humidity all form unintended parallel resistances that matter at megohm levels. And any capacitance at the output, including the input capacitance of whatever you are feeding, forms a low-pass filter with the output impedance, so a 5 MΩ source driving 20 pF has a time constant of 100 microseconds and cannot follow anything fast. Sampling an analogue-to-digital converter through a high-impedance divider without a buffer is a classic source of readings that are consistently low.
Tolerance, Temperature and Real Resistor Values
Divider ratios are more forgiving of tolerance than people expect, and only in one specific way. If both resistors drift by the same proportion — as they do with temperature if they are the same type and at the same temperature — the ratio is unaffected and the output does not move. That is why matched pairs and resistor networks exist, and why a divider can be more stable than either resistor alone.
Independent tolerance is a different matter. Two resistors each specified at five per cent can, in the worst case, be five per cent high and five per cent low, and the ratio error approaches ten per cent. For a divider feeding a measurement, that error propagates directly into the reading. One per cent parts reduce it to about two per cent in the worst case, and calibrating in software against a known input removes it entirely.
There is also the practical matter of what values you can actually buy. Standard series give a fixed set of preferred values, so the exact R2 that hits your target output usually does not exist as a single part. The design modes here return the ideal value; rounding to the nearest available one and then running the analysis mode to see what output it actually produces is the right workflow. Series and parallel combinations can hit intermediate values, and the resistor combination calculator works those out. The resistor color code calculator confirms what a part in front of you actually is.
How This Differs From the Adjacent Circuit Tools
The boundary in one sentence: this page computes how a series pair splits a supply voltage between them, while the Ohm's law calculator relates a single voltage, current and resistance without any network at all. The two are related — the divider result follows from applying Ohm's law twice — but the question each answers is different.
The resistor combination calculator reduces networks of resistors to a single equivalent value and is what you need when a divider leg is made of several parts. The LED resistor calculator sizes a single series resistor for a fixed forward voltage, which is a current-limiting job rather than a voltage-dividing one. The electrical power calculator handles dissipation in more detail, and the voltage drop calculator deals with unwanted loss along a cable run, which is a divider you did not intend to build. For component values, the resistance converter rescales units and the NE555 timer calculator covers a common circuit where resistor ratios set timing rather than voltage.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Ignoring the load — anything connected across the lower resistor sits in parallel with it, and a load equal to the output impedance moves the output enormously.
- Using a divider as a power supply — the output changes whenever the load current changes, which makes it unusable for anything that draws varying current.
- Choosing only the ratio — the absolute values set the current, the wasted power and the output impedance, and the same ratio can be sensible or useless depending on scale.
- Going too high in resistance — megohm dividers pick up noise, suffer from board leakage and cannot charge the input capacitance of what they feed quickly enough.
- Forgetting power dissipation at low values — a low-impedance divider on a high supply can dissipate watts continuously and needs resistors rated for it.
Related Free Tools From Arb Digital
For the underlying relationship, use the Ohm's law calculator, and for networks the resistor combination calculator. Identify parts with the resistor color code calculator and rescale values with the resistance converter. The LED resistor calculator covers current limiting, the electrical power calculator covers dissipation, the NE555 timer calculator covers timing, and the voltage drop calculator handles loss along a cable run. Everything else is in the free online tools hub.
Frequently Asked Questions
Almost always because something is loading it. Anything connected to the tap sits in parallel with the lower resistor and reduces the effective resistance, which pulls the output down. Enter the load resistance and the calculator will show the shifted value.
The two resistors in parallel, seen from the output. A divider behaves like a source of the unloaded voltage in series with that resistance, which is why loads comparable to it disturb the output so badly.
Not reliably. The output falls as the load draws more current, so any device with a varying consumption sees a varying supply. Dividers suit high-impedance inputs such as amplifier inputs and analogue-to-digital converters, not power delivery.
Both. The ratio sets the output voltage, but the absolute values set the current drawn, the power wasted and the output impedance. The same ratio can be a sensible design or an unusable one depending on the scale chosen.
Large enough that the wasted current is acceptable, small enough that the output impedance is well below the load impedance and the node is not vulnerable to noise or board leakage. Signal-level dividers commonly sit between one kilohm and a hundred kilohms.
Only the ratio matters, so drift affecting both resistors equally cancels out. Independent tolerance does not: two five per cent resistors can give a ratio error approaching ten per cent in the worst case.
Round to the nearest preferred value and run the analysis mode to see what output that actually gives. Series or parallel combinations can reach intermediate values when the rounding error is too large.
Because the converter's sampling capacitor has to charge through the divider's output impedance. With a megohm-scale source and a short sampling window there is not enough time, so the reading settles below the true value. Buffering or lowering the impedance fixes it.
This tool is provided for educational and estimating use. It models an ideal two-resistor divider with purely resistive loading and does not account for component tolerance, temperature drift, stray capacitance or the input characteristics of any specific device, so treat its output as a design starting point rather than a verified circuit.
