Voltage Divider Calculator
Built by Johnny Alto · free, no sign-up · updated July 26, 2026
Enter your input voltage and two resistor values to get the output voltage of a divider, plus the current it draws and the power each resistor dissipates. Flip to solve mode to find the second resistor for a target output. It is the everyday circuit for scaling a voltage down to a level a microcontroller can read.
Output Vout0volts
Divider current0mA
Total power0mW
This assumes no load on the output. Drawing current from Vout lowers it, so keep the divider current well above what the load draws, or buffer the output with an op-amp. Higher resistor values waste less power but are more sensitive to loading and noise.
How a voltage divider works
A voltage divider is two resistors in series across a voltage. The output is tapped between them, and it carries a fraction of the input set by the ratio of the resistors. The formula is the input voltage times R2 divided by the sum of R1 and R2. With 5 volts in, a 1k ohm R1 and a 2k ohm R2 give 3.33 volts out, because R2 is two thirds of the total resistance.
The same current flows through both resistors, equal to the input voltage divided by the total resistance. That current, times the input voltage, is the power the divider burns, which is why very low resistor values waste energy as heat while very high values save power but pick up noise.
The catch: loading
The clean formula only holds when nothing draws current from the output. Connect a load and it sits in parallel with R2, pulling the output voltage down. The rule of thumb is to make the divider current at least ten times the load current so the sag stays small, or to buffer the output with an op-amp follower when precision matters. Dividers are perfect for scaling a signal into a microcontroller's input range but poor as a power source.
Recommended gear
Electronics bench gear
Frequently asked questions
How do you calculate a voltage divider?
Output voltage equals the input voltage times R2 divided by the sum of R1 and R2. For 5 volts with a 1k R1 and 2k R2, the output is 3.33 volts.
How do I pick resistor values for a target voltage?
Fix R1, then R2 equals R1 times the target output divided by the difference between input and output. Use the solve mode above to get the value directly.
Why is my divider output lower under load?
A load draws current and sits in parallel with R2, dropping the output. Keep the divider current at least ten times the load current, or buffer the output with an op-amp.
Should I use high or low resistor values?
Low values waste power as heat but resist loading and noise; high values save power but are more sensitive. Values in the kilohm to tens of kilohm range are a common balance for signal dividers.
More calculators
This calculator gives estimates for planning only; always confirm against your own measurements and species research. Some links are to our buying guides, which contain affiliate links. As an Amazon Associate I earn from qualifying purchases.
← All calculators · Buying guides · About · Affiliate Disclosure · Privacy Policy