How this calculator works
Ohm's law links resistor drop to current. In an unloaded divider, both resistors carry the same current and the lower resistor sets the output fraction. A dropping resistor assumes a specified load current; changing that current changes the output voltage. Calculated dissipation is the electrical heat load and needs a higher practical component rating after derating.
Formula
Vdrop = I R; divider Vout = Vs R2/(R1+R2); Rdropping = (Vs-Vload)/I; P = Vdrop I.
Worked example
A 100 ohm resistor carrying 20 mA drops 2 V and dissipates 0.04 W. A 12 V source divided by 1000 ohms above 2000 ohms produces 8 V with no external load.
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Frequently asked questions
Can a divider power a varying load?
A load in parallel with the lower resistor changes the division ratio. Use a regulator or include the actual load impedance.
Is the reported power a minimum resistor rating?
It is the expected dissipation under the entered conditions. Select a component with enough margin for temperature, tolerance and transients.
What if the supply is below the desired output?
A passive resistor cannot raise voltage. That request is rejected.