Voltage Divider Calculator
Enter the input voltage and resistors R1 and R2 to get the output of a voltage divider. You can also see how far the output drops when a load is attached, and the current and power in the resistors.
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Results
- Output voltage (no load)
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- Divider ratio R2/(R1+R2)
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- Output voltage (with load)
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- Current
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- Power in R1
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- Power in R2
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- Output resistance R1∥R2
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Current and power are for the unloaded divider. Set the load to 0 to calculate without a load.
See it as a graph
Other inputs stay at their current values while the chosen input varies from 1/4 to 4 times its current value. The red dot is the current value.
Deriving the divider formula
When R1 and R2 are in series across an input voltage Vin, the same current I = Vin/(R1 + R2) flows through both. The output is the voltage across R2, so Vout = I·R2 = Vin·R2/(R1 + R2). The fraction R2/(R1 + R2) is called the divider ratio. With the defaults of Vin = 5 V and R1 = R2 = 10 kΩ, the ratio is 0.5, the output is 2.5 V and the current is 5 V ÷ 20 kΩ = 250 µA. Each resistor dissipates I²R, or 625 µW. The formula holds when no current is drawn from the output (no load).
Loading and output resistance
Connecting a load RL to the output puts it in parallel with R2, and the output voltage drops. Seen from the output, the divider behaves like a source of Vin·R2/(R1 + R2) in series with R1 and R2 in parallel (R1∥R2), its Thevenin equivalent. With the defaults, the output resistance is 5 kΩ. Adding RL = 10 kΩ pulls the output down to about 1.67 V, while RL = 100 kΩ gives about 2.38 V. To keep the drop within 1%, RL must be at least 99 times the output resistance.
Choosing resistor values
The ratio of the resistors sets the output voltage, but their absolute size is a trade-off. Smaller resistors are less affected by the load but draw more current and dissipate more power. At 5 V, for example, 1 kΩ + 1 kΩ draws 2.5 mA and 100 kΩ + 100 kΩ draws 25 µA, but the latter has an output resistance of 50 kΩ and is more sensitive to the load and to leakage currents. Battery-powered designs generally use larger values, and heavier loads call for smaller ones. Resistor tolerance also becomes output error, so use tighter-tolerance resistors when accuracy matters.
Where dividers fit, and where they don't
A voltage divider suits jobs that draw almost no current: scaling a voltage down to an ADC's input range, setting the bias of a transistor or op-amp, or deriving a comparison voltage from a reference. With R1 = 20 kΩ and R2 = 10 kΩ, for example, the ratio is 1/3, so 9 V can be measured as 3 V. Some ADC datasheets give a maximum source impedance, so keep the output resistance below it or add a buffer or a capacitor. A divider is not suited to powering loads whose current changes, such as motors or microcontrollers; use a regulator for that.
Practice analog circuit questions
FAQ
How do I choose resistor values for a voltage divider?
First set the ratio of R1 to R2 from the divider ratio you need, then choose their size. Keep the output resistance R1∥R2 well below the resistance of the load, such as an ADC input, and keep the current and power acceptable. Values from a few kΩ to a few hundred kΩ are common; when driving an ADC, also check the source-impedance conditions in its datasheet.
Why does the output voltage drop when I connect a load?
The load RL is in parallel with R2, so the lower leg's combined resistance becomes smaller than R2. Put another way, the divider has an output resistance of R1∥R2, and load current through it causes a voltage drop. Connecting RL = 10 kΩ to the default divider (5 kΩ output resistance) pulls the output from 2.5 V down to about 1.67 V.
What is the difference between a voltage divider and a regulator?
A divider's output changes with the input voltage and the load current, and current keeps flowing through the resistors even with no load. A regulator uses feedback to hold its output nearly constant as the input voltage and load current change. As a rule, use a divider to scale a voltage down for measurement and a regulator to supply power to a circuit.