Voltage Divider Calculator
Divider output volts, current, and power from Vin, R1, R2.
What it does
Two resistors split a voltage in proportion to their sizes. Put 12 V across 2k over 1k and the 1k bottom resistor takes one third: 4 V out with 4 mA through the string. Sensor scaling, reference levels, and bias points all start here.
The tool above runs the unloaded divider. Enter source volts with R1 on top and R2 at the bottom, and it prints output volts, string current, and power total with the per-resistor split. Defaults give 4 V at 4 mA with 48 mW shared 32 and 16.
University treatments match term for term. MIT OCW derives the loop rule to Vout = R2/(R1+R2) x Vin, Georgia State HyperPhysics adds loaded versus open output with power, and an Oklahoma electronics lab builds lab dividers near 3.3 V from the same form.
Loads are the fine print. Anything hung across R2 draws extra current and drags Vout below the printed value. Keep divider current ten times the load current or buffer with an op amp when the load bites.
How to use
- Enter the source voltage.
- Enter R1 (top) in ohms.
- Enter R2 (bottom) in ohms.
- Read Vout first, then current.
- Read total power and the per-resistor split.
- Derate: keep loads 10x lighter than the divider.
Formula
- Vin
- Source volts, 0 to 1,000.
- R1
- Top resistor toward the source, ohms.
- R2
- Bottom resistor to ground; output reads here.
Unloaded form: no load across R2. With a load RL, use R2||RL in place of R2. MIT OCW and HyperPhysics print this exact derivation.
Worked example
Example: 12 V across 2k and 1k
Defaults: 12 V with 2k over 1k resistors.
- Inputs: 12 V source, R1 2,000, R2 1,000 ohms.
- Total: 3,000 ohms. Current: 12 / 3,000 = 4 mA.
- Output: 12 x 1,000 / 3,000 = 4 V.
- Power: 12 x 0.004 = 48 mW (R1 32, R2 16).
- Tool shows 4 V, 4 mA, 48 mW total.
Reference table
| Vin / R1 / R2 | Vout | Power |
|---|---|---|
| 12 / 2k / 1k | 4 V at 4 mA | 48 mW (32 + 16) |
| 9 / 1k / 2k | 6 V at 3 mA | 27 mW (9 + 18) |
| 5 / 10k / 10k | 2.5 V at 0.25 mA | 1.3 mW |
| 12 / 1k / 3k | 9 V at 3 mA | 27 mW (9 + 18) |
Limits
- Unloaded only. Real loads need the parallel form.
- Resistive DC only. Capacitance and AC differ.
- Ignores tolerance bands on real resistors.
- Heat drift and ratings sit outside the math.
Common mistakes
- Reading output across the wrong resistor.
- Loading R2 and trusting unloaded Vout.
- Swapping R1/R2 labels between references.
- Starving the divider versus the load.
- Ignoring resistor power ratings.
Try it yourself
Quick checks. Answers sit tucked under each question.
9 V with 1k over 2k?
Total 3,000. I = 9/3,000 = 3 mA. Vout = 9 x 2,000/3,000 = 6 V. P = 27 mW (9 + 18).
5 V with 10k over 10k?
Total 20,000. I = 0.25 mA. Vout = 2.5 V. P = 1.3 mW total, 0.6 mW each.
Questions
How is output voltage figured?
Source times R2 over R1 plus R2. Here 12 x 1,000/3,000 = 4 V across R2.
How is divider current figured?
Source over total resistance. Here 12 / 3,000 = 4 mA through both resistors.
Which resistor is R2?
R1 is the top resistor toward Vin; R2 is the bottom to ground. Output reads across R2.
What happens with a load attached?
It sags. Load current steals from R2, so loaded output reads lower than this page.
How stiff should the divider be?
Ten times the load current or more keeps sag small. Starved dividers drift with every load change.
How is power split?
Source times current, split by resistance share. Here 48 mW total: 32 in R1, 16 in R2.
Sources
- MIT OCW 8.02x: Voltage, Current, Resistance notes (divider derivation)
- Georgia State HyperPhysics: Voltage Divider
- Univ. of Oklahoma: Electronics Lab, Voltage Dividers