Voltage Divider Calculator
Get the output voltage from the input voltage and two resistors, and find the E24 standard resistor pair closest to the output you need.
With a load connected
Formula and cautions
Vout = Vin × (R2)/(R1 + R2). A load on the output sits in parallel with R2 and pulls the voltage down. A load of at least ten times R1 and R2 in parallel keeps the error under 10%; ten times R2 is a simpler, more generous bound. Dividers suit inputs that draw almost no current, such as a microcontroller ADC pin.
Try it: what a load does to a divider
Connect a load RL across R2 and watch the output drop below the ideal value. Drag RL to see where the load stops mattering.
- Output without load
- 3.33 V
- Output with RL
- 3.13 V
- Change from the ideal
- −6.25%
- Source resistance Rth = R1 ∥ R2
- 6.67 kΩ
- Load current
- 31.3 µA
- RL for under 1% error
- 660 kΩ
Seen from its output, a divider behaves like a source of the unloaded voltage Vth with the resistance R1 ∥ R2 in series. A load forms a second divider with that resistance, so the output always drops a little.
Rule of thumb: keep RL at least 10 times Rth for under 10% error and about 100 times for 1%. That is why dividers suit high-impedance inputs such as an ADC pin or a meter, and why smaller divider resistors hold up better under load at the cost of more current.
Worked example: 5 V down to 3.3 V
- Required ratio: (3.3 V)/(5 V) = 0.66
- The E24 pair whose (R2)/(R1 + R2) is closest to 0.66: R1 = 4.7 kΩ, R2 = 9.1 kΩ
- Output: Vout = 5 V × (9.1 kΩ)/(13.8 kΩ) = 3.297 V (0.09% error)
Common divider resistor pairs (E24)
| Conversion | R1 | R2 | Actual output | Error |
|---|---|---|---|---|
| Conversion5 V → 3.3 V | R14.7 kΩ | R29.1 kΩ | Actual output3.297 V | Error0.09% |
| Conversion12 V → 5 V | R118 kΩ | R213 kΩ | Actual output5.032 V | Error0.65% |
| Conversion12 V → 3.3 V | R124 kΩ | R29.1 kΩ | Actual output3.299 V | Error0.03% |
| Conversion24 V → 5 V | R19.1 kΩ | R22.4 kΩ | Actual output5.009 V | Error0.17% |
| Conversion9 V → 5 V | R112 kΩ | R215 kΩ | Actual output5.000 V | Error0.00% |
| Conversion5 V → 2.5 V | R15.1 kΩ | R25.1 kΩ | Actual output2.500 V | Error0.00% |
| Conversion3.3 V → 1.8 V | R110 kΩ | R212 kΩ | Actual output1.800 V | Error0.00% |
| Conversion24 V → 3.3 V | R147 kΩ | R27.5 kΩ | Actual output3.303 V | Error0.08% |
E24 standard values
| 1.0 | 1.1 | 1.2 | 1.3 | 1.5 | 1.6 | 1.8 | 2.0 |
| 2.2 | 2.4 | 2.7 | 3.0 | 3.3 | 3.6 | 3.9 | 4.3 |
| 4.7 | 5.1 | 5.6 | 6.2 | 6.8 | 7.5 | 8.2 | 9.1 |
Bold values are also in the E12 series. Real parts are these values times 10, 100, 1k, 10k… (for example 4.7 → 47 Ω or 4.7 kΩ).
Checked on our bench: the ratio holds, the meter intrudes
The numbers below are computed from values the site owner recorded in lab classes. Measured values are left exactly as recorded.
The ratio below comes from the measured voltages on a 1 kΩ and 3 kΩ divider. It divides by the sum of the two resistor voltages rather than the supply, which leaves out the drop across the ammeter that was also in the loop.
| Supply | VR1 (1 kΩ) | VR2 (3 kΩ) | Measured ratio | vs nominal 0.75 |
|---|---|---|---|---|
| Supply10 V | VR1 (1 kΩ)2.480 V | VR2 (3 kΩ)7.503 V | Measured ratio0.7516 | vs nominal 0.75+0.21% |
| Supply16 V | VR1 (1 kΩ)3.963 V | VR2 (3 kΩ)11.856 V | Measured ratio0.7495 | vs nominal 0.75−0.07% |
The ratio matched nominal within 0.3%. A divider's output is set by the ratio, not the absolute values, so when both resistors lean the same way their errors cancel.
A high-value divider changes when you measure it. The gate divider in the MOSFET bias guide (560 kΩ, 160 kΩ, 10 V) gives 2.222 V in this calculator. Touch R2 with an ordinary multimeter of 10 MΩ input resistance and R2 acts like 157.5 kΩ, so the meter shows 2.195 V (−1.2%). On the 1 kΩ divider in the table the same effect is under 0.01%. When you measure a divider in the hundreds of kΩ, put "R2 in parallel with the meter's input resistance" into the R2 field. This last figure is a calculation, not a measurement.
Designing a divider
From target voltage to a resistor pair
- Enter the input voltage and the two resistors (R1 on top, R2 to ground) to see the unloaded output.
- To design one, type the output you want in the target field and take the suggested E24 pair.
- Look at the load graph: if whatever you connect to the output draws current, the real output is lower than the unloaded value.
Why the output reads lower than calculated
- Powering something from a divider. A divider is a reference, not a supply. The output sags as soon as the load draws current comparable to the divider current. Use a regulator for anything that draws more than a few hundred microamps.
- Values far too large or too small. Megohm dividers pick up noise and are pulled down by a meter's 10 MΩ input; 100 Ω dividers waste power. For a microcontroller ADC input, keep the source resistance (R1 and R2 in parallel) around 10 kΩ or less.
- Forgetting tolerance. With two 5% resistors the ratio can be off by close to 5%, so a "3.3 V" output might read anywhere from about 3.15 V to 3.45 V.
Choosing resistor sizes
A rule that keeps loading error under about 10% is to make the load resistance at least ten times R1 and R2 in parallel. For a battery monitor, larger values (100 kΩ range) cut the standby drain, and a 100 nF capacitor from the output to ground keeps the ADC reading steady. When you measure the output with a multimeter, remember the meter itself is a load; on a high-value divider it can drag the reading down by a few percent.
Frequently asked questions
Which resistors drop 5 V to 3.3 V?
R1 = 10 kΩ and R2 = 20 kΩ give about 3.33 V, and both are E24 standard values. Enter your target voltage above to see other pairs.
Can I power a motor or LED from a voltage divider?
It is not recommended. The output voltage sags as soon as load current flows, and the resistors waste power. Use a voltage regulator for power, and keep dividers for inputs that draw almost no current, such as an ADC pin.