Free voltage divider calculator
Scale any voltage down with two resistors and see the output, current, and power in each resistor updated live, as you type.
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Electrical calculations are for reference only. Always consult a licensed electrician for wiring work. Follow applicable electrical codes.
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What a voltage divider is and how it works
A voltage divider is two resistors wired in series across a source, used to produce a smaller voltage from a larger one. The full input voltage sits across the pair; the output is the voltage measured across the lower resistor. Because the same current flows through both resistors, each one drops a share of the input in proportion to its resistance — so picking the two values sets the fraction you keep. This voltage divider calculator takes your input voltage and the two resistor values and returns the output voltage, the current flowing through the chain, and the power dissipated in each resistor.
The upper resistor is usually labelled R1 (between the input and the output node) and the lower one R2 (between the output node and ground). The output is read at the junction between them. Swap the two and the output fraction inverts, which is why the order matters: Vout is always measured across R2, the resistor nearer ground.
The voltage divider formula and where it comes from
The formula falls straight out of Ohm's law applied twice. The series current is the input voltage divided by the total resistance, and the output is that current multiplied by R2.
The derivation in one line
Two series resistors add to a total of R1 + R2, so by Ohm's law the current is I = Vin / (R1 + R2). That current flows through R2, and the voltage across R2 is I × R2 — substitute the current back in and you get Vout = Vin × R2 / (R1 + R2). The output is therefore the input scaled by the ratio R2 / (R1 + R2), a pure number between 0 and 1. See the Ohm's law calculator for the V = I × R relationship this is built on.
The two-resistor divider relation Vout = Vin × R2 / (R1 + R2) is a direct consequence of Kirchhoff's voltage law and Ohm's law. See Wikipedia's "Voltage divider" article for the full derivation and the loaded-divider case.The three inputs explained
Only three numbers drive the result, and each one moves the output in a predictable way.
Input voltage (Vin)
The voltage applied across the whole series pair, in volts. The output is a fixed fraction of Vin, so doubling the input doubles the output while the ratio stays the same. A divider scales — it does not regulate, so if Vin wanders the output wanders with it.
R1 — the upper resistor
The resistor between the input and the output node, in ohms. Increasing R1 takes a bigger share of the input voltage for itself and lowers the output. R1 is what you raise when you want a smaller output fraction.
R2 — the lower resistor
The resistor between the output node and ground, in ohms. The output is measured across R2, so increasing R2 raises the output toward Vin. The output equals the input only in the limit where R2 dwarfs R1.
A worked voltage divider example
Sam has a 12 V supply and a sensor input that should see about 8 V. He picks R1 = 10 kΩ for the upper resistor and R2 = 20 kΩ for the lower one, and wants to confirm the output, the current draw, and how much heat the resistors make before building it.
Step 1 — Add the resistors to get the total
R1 + R2 = 10 kΩ + 20 kΩ = 30 kΩ. This is the total resistance the supply sees.
Step 2 — Find the series current
I = Vin / (R1 + R2) = 12 V ÷ 30,000 Ω = 0.0004 A = 0.4 mA. The same 0.4 mA flows through both resistors.
Step 3 — Multiply the current by R2 for the output
Vout = I × R2 = 0.0004 A × 20,000 Ω = 8 V. The same answer comes from the ratio directly: 12 V × 20 kΩ / 30 kΩ = 8 V.
Step 4 — Check the power in each resistor
P = I² × R, so R1 dissipates 0.0004² × 10,000 = 1.6 mW and R2 dissipates 0.0004² × 20,000 = 3.2 mW — trivial heat, well within any standard 1/8 W resistor.
Choosing R1 and R2 — ratio first, then size
Designing a divider is a two-step decision. First the ratio R2 / (R1 + R2) sets the output fraction. Then the absolute size of the resistors sets the current and how stiff the output is. Two dividers with the same ratio give the same unloaded output but behave very differently once anything is connected.
The ratio table below gives the output for a 5 V input at common resistor ratios. Each Vout is simply 5 V × R2 / (R1 + R2), so the same ratios scale to any input.
| R1 | R2 | Ratio R2/(R1+R2) | Vout from 5 V |
|---|---|---|---|
| 1 kΩ | 9 kΩ | 0.90 | 4.50 V |
| 1 kΩ | 3 kΩ | 0.75 | 3.75 V |
| 1 kΩ | 2 kΩ | 0.667 | 3.33 V |
| 1 kΩ | 1 kΩ | 0.50 | 2.50 V |
| 2 kΩ | 1 kΩ | 0.333 | 1.67 V |
| 3 kΩ | 1 kΩ | 0.25 | 1.25 V |
| 4 kΩ | 1 kΩ | 0.20 | 1.00 V |
| 9 kΩ | 1 kΩ | 0.10 | 0.50 V |
Computed from Vout = 5 V × R2 / (R1 + R2). Equal resistors halve the input; the output rises toward 5 V as R2 grows relative to R1.
For the absolute size, there is a trade-off. High values (tens of kΩ to MΩ) waste almost no power but make the output a weak, high-impedance node that picks up noise and is easily disturbed by the circuit it feeds. Low values (tens to hundreds of ohms) give a stiff, noise-resistant output but burn more power as continuous heat. Around 10 kΩ is a common compromise for a reference divider. Pick real parts from the standard E12/E24 resistor series rather than arbitrary values.
The loading effect — why a divider is for reference, not for power
This calculator, like the formula, assumes nothing is connected to the output — an unloaded divider. The moment you attach a real load, the load draws its own current, and that current does not come from nowhere. Electrically, the load resistance sits in parallel with R2, lowering the effective R2 and pulling the output below the value the formula predicts. This is the single most common way a divider design fails.
Take the worked example again: R1 = 10 kΩ, R2 = 20 kΩ, Vout = 8 V unloaded. Connect a 10 kΩ load across the output and R2 becomes 20 kΩ in parallel with 10 kΩ, which is 6.67 kΩ. The new output is 12 V × 6.67 kΩ / (10 kΩ + 6.67 kΩ) = 4.80 V — a 40% error, not 8 V. The divider sagged because its own current was tiny next to the load's.
The conclusion is the rule every analog designer learns: a resistive divider is for setting a reference voltage or sensing, where the thing reading the output draws essentially no current (a high-impedance op-amp input or microcontroller ADC). It is not a way to power a load. To drive a real load at a divided voltage, follow the divider with a buffer such as an op-amp voltage follower, or use a proper voltage regulator. For long power runs the relevant loss is conductor resistance instead — see the voltage drop calculator.
Using a voltage divider as a sensor
The divider's biggest everyday use is not making a fixed reference but reading a sensor. Replace one of the two resistors with a component whose resistance changes with a physical quantity, and the output voltage becomes a direct, readable signal of that quantity.
- Potentiometer — a single three-terminal divider in one part. The wiper taps a continuously variable point along a resistive track, so turning the knob sweeps the output smoothly from 0 V to Vin. Volume controls and joystick axes are potentiometer dividers.
- Thermistor — a resistor whose value changes with temperature. Put it in a divider with a fixed resistor and the output voltage tracks temperature, ready for a microcontroller's analog input.
- Photoresistor (LDR) — resistance falls as light increases, so a divider with an LDR produces a voltage that rises or falls with brightness for automatic lighting.
- Strain and force sensors — resistive strain gauges are read as dividers, and four of them form the more precise Wheatstone bridge for tiny resistance changes.
Because a microcontroller's analog input is high-impedance, it draws almost no current — exactly the unloaded condition the divider formula assumes — which is why sensor dividers read accurately while power dividers do not.
Common voltage divider mistakes
- Using a divider to power a load. The output sags as soon as current is drawn. Use it for a reference or sensor, and buffer or regulate anything that needs real current.
- Measuring across the wrong resistor. Vout is the voltage across R2, the resistor to ground. Read across R1 and you get Vin minus the output, which is the complement, not the divided value.
- Choosing values too high. Mega-ohm dividers waste no power but become noisy, slow, and easily disturbed by stray capacitance and input bias currents.
- Choosing values too low. A divider running tens of milliamps wastes power continuously and can overheat small resistors — always check the I² × R power the calculator reports.
- Forgetting the source can shift. A divider scales the input; it does not stabilise it. If Vin is unregulated, the divided output drifts in exactly the same proportion.
How accurate is this voltage divider calculator
The arithmetic is exact. For the input voltage and resistor values you enter, Vout = Vin × R2 / (R1 + R2) is the precise output of an ideal, unloaded divider, with the current and per-resistor power computed to full floating-point precision.
Real circuits differ from the ideal for physical reasons, not arithmetic ones. Stock resistors carry a tolerance — typically ±1% for metal-film and ±5% for carbon-film parts — so the built divider lands near the calculated value rather than exactly on it; tolerances in R1 and R2 can stack. Resistor values also drift slightly with temperature, and any load on the output lowers it through the loading effect described above. Treat the calculated figure as the unloaded design target, choose tight-tolerance parts where the reference matters, and keep the divider current well above any load current. For the underlying theory, Wikipedia's "Voltage divider" article gives the full derivation from Ohm's and Kirchhoff's laws.
Frequently asked questions about the free voltage divider calculator
About this Voltage divider calculator
This voltage divider calculator runs entirely in your browser — nothing you enter is sent anywhere. Enter the input voltage and the two series resistor values and it returns the output voltage across R2, the current through the chain, and the power dissipated in each resistor, all from Vout = Vin × R2 / (R1 + R2) and recalculating instantly as you type.
It pairs with the Ohm’s law calculator, the series and parallel resistor calculator, and the voltage drop calculator. Browse the full set of free calculators for more electronics tools.