Three calculators, one panel.
Pick a mode below — everything recalculates live as you type, and every result explains itself with the formula behind it.
The supply or signal you're dividing — 5 V logic, a 9 V battery, a 24 V rail…
Sits between Vin and the output tap. Bigger R1 → smaller output.
Sits between the tap and ground — the output appears across it. Bigger R2 → bigger output.
Whatever the output feeds. Leave empty for an ideal, unloaded tap — add a value to see the real voltage.
The supply going in — it must sit above your target for a passive divider.
The voltage you want at the tap — strictly between 0 V and Vin.
The resistor between Vin and the output tap — it stays fixed while R2 is solved.
Between Vin and the output tap.
Between the tap and ground — the output appears across it.
Used for the suggested real-world replacements below — E24 (±5%) is the everyday workhorse.
The supply you're dividing from.
The voltage you want at the tap — the finder matches it as closely as the series allows.
The tolerance family your resistors come from — finer series, closer matches.
Keeps the divider current sensible — the finder picks the decade closest to this.
From supply to answer in four steps.
No calculate button, no waiting — the panel reacts to every keystroke. Here's the quickest route from "I have a supply" to "I have my resistors".
Pick a mode
Output voltage for the classic calculation, Find a value to work backwards from a target, or Standard pairs when you want resistors you can actually buy.
Enter what you know
Type the supply and resistances — the Ω / kΩ / MΩ selectors keep big numbers tidy. Add a load resistance to see how the output really behaves.
Read the live results
Vout, current, power and output resistance update as you type, each with the formula substituted using your own numbers, so every step is checkable.
Snap to standard values
Real resistors come in E-series steps. The solver suggests the nearest E6–E96 replacements, and the pair finder ranks full combinations by voltage error.
Three formulas run the whole panel.
Every number the panel shows you comes from one of these — and the calculator prints them substituted with your own values.
The divider equation
R1 sits between the supply and the tap, R2 between the tap and ground. The output is simply R2's share of the total — which is why two equal resistors always give exactly half the input.
Loaded divider
Vout = Vin × R2∥RL ÷ (R1 + R2∥RL)
A load sits in parallel with R2 and drags the output down. Rule of thumb: keep the load at least 10× R2 and the sag stays in the single-digit percents.
Current, power & stiffness
Rout = R1·R2 ÷ (R1 + R2)
The divider burns power constantly — smaller resistors give a stiffer, lower-impedance output but drink more current. Totals between 1 kΩ and 100 kΩ are the usual sweet spot.
Quick answers.
The questions that come up again and again when building dividers — answered in plain language, with the numbers to back them up.
- Vout is always ≤ Vin — dividers only step down.
- Equal resistors give exactly half the input.
- Keep the load at least 10× R2 to limit sag.
- Totals of 1 kΩ–100 kΩ fit most projects.
What is a voltage divider?+
Two resistors chained in series across a voltage source, with the output taken from the point between them. The pair turns the input into a smaller, predictable fraction of itself, which is why dividers show up everywhere — reading sensors with an ADC, shifting a 5 V signal into a 3.3 V input, or setting a reference.
A potentiometer is simply an adjustable divider: the wiper taps somewhere along a resistive track.
What is the voltage divider formula?+
Vout = Vin × R2 ÷ (R1 + R2) — the output is R2's share of the total resistance.
Example: 9 V across R1 = 10 kΩ and R2 = 10 kΩ gives Vout = 9 × 10 ÷ 20 = 4.5 V. Swap in R2 = 20 kΩ and you get 9 × 20 ÷ 30 = 6 V. The calculator shows this formula substituted with your numbers on every result.
Which resistor is R1 and which is R2?+
R1 is the top resistor, between the supply (Vin) and the output tap; R2 is the bottom one, from the tap down to ground. The output is always measured across R2.
Getting them backwards gives the complement — if R2 ÷ (R1 + R2) should be 30% but you swap the resistors, you'll get 70% instead.
Can a voltage divider step voltage up?+
No. A passive resistor divider can only produce an output between 0 V and Vin — the fraction R2 ÷ (R1 + R2) can never exceed 1. To get more voltage than you put in you need active circuitry: a boost converter, a charge pump, or an amplifying op-amp stage.
Can I power a device from a voltage divider?+
Not reliably. As soon as the device draws current it becomes a load in parallel with R2, pulling the output down — often far down. Dividers also burn power continuously in the resistors themselves.
For power, use a regulator (linear or buck/boost); save dividers for signals feeding high-impedance inputs like ADC pins, comparator inputs and op-amp buffers.
How do I choose R1 and R2 values?+
It's a trade-off. Small resistors (say a 1 kΩ total) give a stiff output that resists loading but waste milliamps; large ones (1 MΩ total) sip current but sag easily and pick up noise. Totals between 1 kΩ and 100 kΩ suit most projects.
If a load is attached, keep it at least 10× R2, and check the power figures — the calculator shows dissipation per resistor so you can pick suitable wattage ratings.
Why is my measured output lower than the calculated one?+
Four usual suspects: the load itself (add it in the calculator's load field to see the real value), resistor tolerance — ±5% parts can each be off, shifting the ratio, while ±1% parts fix this — the supply's own output impedance adding to R1, and a high-impedance divider being loaded by the multimeter itself. The loaded mode models the first and most common cause.
Is this calculator free, and is my data sent anywhere?+
Completely free, with no sign-up and no limits. Every calculation runs locally in your browser — nothing you type is uploaded, logged, or sent to a server.