Voltage Divider Calculator
What does R1/R2 actually give you at Vout — and how much does connecting a real load pull that down? Everything runs in this page — nothing is uploaded.
Divider output
A real load in parallel with R2 pulls Vout down from the unloaded value — the higher R1/R2 are relative to the load, the worse the sag. Fill this in to see the real-world number, not just the textbook one. Know the load and want R1/R2 sized to resist it? See the divider-for-a-load calculator.
Design for a target output
Pick R1, say what Vout you want, and get the R2 you need — exact, and the nearest resistor you can actually buy.
Common questions
How much does a load pull down a divider's output?
It depends on how the load resistance compares with R2, since a load at Vout sits electrically in parallel with R2. With this page's own defaults — 12 V in through R1 = 10 kΩ and R2 = 10 kΩ — the unloaded output is 6.0000 V. Connect a 10 kΩ load and the effective R2 halves to 5 kΩ, so the output falls to 4.0000 V, a 33.33% drop. A load much larger than R2 barely moves the output; a load equal to R2 cuts it by a third, and a load smaller than R2 sags it further still.
Why does the target-output tool suggest a different R2 than the exact math gives?
Because real resistors only come in a fixed set of standard values — the E24 series this calculator snaps to — not any number the arithmetic happens to produce. Asking for 3.3 V out of a 12 V supply through a 10 kΩ R1, for example, calls for an exact R2 of 3793.10 Ω, a value nobody sells. The nearest E24 value is 3.9 kΩ, which the calculator recommends instead; wired up, it actually delivers 3.3669 V, about 2.03% high of the 3.3 V target, and the price of using a resistor you can actually buy.
Why shouldn't I use a plain resistive divider to power something?
Because, as the first question shows, its output voltage changes the moment real current is drawn — it is a voltage reference, not a regulator. It's a reasonable choice for feeding a high-impedance point such as an ADC input, which draws very little current; for a load that draws real current, use a proper linear or switching regulator instead. It also wastes power on its own: at the page's default 12 V, 10 kΩ / 10 kΩ, the divider dissipates 3.6 mW in R1 and 3.6 mW in R2 even before anything is connected to Vout.