Unloaded output voltage
5 V
Free divider design tool
Voltage Divider Calculator
Calculate ideal and loaded divider output voltage, current, and resistor power from practical resistor-divider inputs.
Divider and load check
Voltage divider inputs
Unloaded and loaded output
Quick examples
Loaded output voltage
5 V
Divider current unloaded
0.005 A
Source current loaded
0.005 A
Equivalent bottom resistance
N/A
Loaded total resistance
N/A
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What is a voltage divider calculator?
A voltage divider calculator calculates the output voltage taken from two resistors connected in series across a supply. It is one of the most common ways to create a lower reference voltage from a higher input when the output only has to feed a light load.
In practice, people often use a divider to bias a transistor input, create an ADC or comparator reference, scale a signal, or derive a simple lower-voltage tap from a fixed DC source. A useful divider tool should therefore show not just the ideal unloaded result, but also what happens when a real load resistor is connected to the output.
That is why this calculator includes both unloaded and loaded output voltage plus current and resistor power information.
Why a real voltage divider can sag under load
A textbook divider gives an ideal output based only on the resistor ratio. But in real circuits, the output node is often connected to another load. That load draws current and effectively sits in parallel with the bottom resistor, which changes the divider ratio and reduces the output voltage.
This matters because a divider that looks correct on paper may not hold the expected voltage once it actually feeds something. If the load resistance is too low compared with the divider resistors, the output can drop much more than expected.
Unloaded output is the ideal ratio
Without a connected load, the output voltage depends only on the resistor split.
A load changes the effective bottom resistance
The load sits in parallel with the lower resistor and pulls the output lower.
Current draw changes the divider behavior
More load current means more voltage sag at the divider output.
A divider is not a regulator
Resistor dividers are best for light-load references, not for powering substantial loads directly.
How the voltage divider formulas work
The basic divider formula is:
Core formula
Vout = Vin × R2 / (R1 + R2)
When a load is attached, the bottom resistor is replaced by the equivalent parallel combination of the bottom resistor and the load. That new equivalent resistance is then used in the same divider relationship.
Step 1: Calculate the ideal unloaded output
Use the basic resistor ratio without any external load connected.
Step 2: Combine the load with the lower resistor
If a load is present, it forms a parallel resistance with the bottom resistor.
Step 3: Recalculate the loaded output
Use the new effective lower resistance to find the actual output voltage under load.
Step 4: Check current and resistor power
Power dissipation matters for resistor ratings, especially with lower-value divider networks.
Quick reference examples for divider planning
These examples show the kinds of jobs where a divider calculator is actually useful.
| Example | What it helps you check |
|---|---|
| 12 V to about 5 V tap | Useful for quick signal scaling or reference generation. |
| 24 V divider feeding a 10k load | Shows how much the real output drops once the divider is actually loaded. |
| ADC input scaling | Useful when stepping a higher input down into a safer measurement range. |
| Bias network design | Helpful when selecting resistor ratios for transistor or comparator bias points. |
| Resistor wattage check | Important when lower resistor values increase current and heat dissipation. |
How to use this voltage divider calculator
- 1
Enter the input voltage
Use the real source voltage that appears across the whole divider.
- 2
Enter the top and bottom resistor values
R1 is the upper resistor from the source to the tap. R2 is the lower resistor from the tap to ground.
- 3
Add the load resistor if the output feeds something real
This is the practical step that shows whether the output stays close to the ideal divider value.
- 4
Compare unloaded and loaded output voltage
The difference shows how much the load is pulling the divider off the ideal target.
- 5
Check currents and resistor power
Use the current and wattage outputs to verify that the divider is safe and sensible for the job.
Real-world uses, edge cases, and limitations
Useful for references and signal scaling
A divider works well for light-load nodes such as bias points, sensor scaling, and reference taps.
Useful for load-sag checks
Loaded-output calculation quickly shows whether the divider is stiff enough for the connected circuit.
Useful for power dissipation checks
Resistor power helps verify that the divider does not overheat or waste excessive current.
Not a regulated power source
If the output must drive changing loads or hold a tight voltage, use a buffer or regulator instead of a plain divider.
Frequently asked questions
- What is the voltage divider formula?
- The standard unloaded formula is Vout = Vin × R2 / (R1 + R2).
- Why does the output voltage drop under load?
- Because the load sits in parallel with the lower resistor and changes the effective divider ratio.
- Can I use a voltage divider to power a circuit?
- Only for very light loads. For significant current draw or stable output, a regulator or buffer is usually better.
- Why check resistor wattage?
- Because lower-value dividers can waste more current and cause the resistors to dissipate more heat.
Check the ideal tap voltage and the real loaded result
Use this voltage divider calculator to calculate both the ideal divider output and the real loaded output, along with current and resistor dissipation, so the result is practical for actual circuit design.