Voltage Divider Calculator
Choose what to solve for, fill in the other values and add an optional load resistance. Leave the load at 0 for an unloaded divider.
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Formula
Vout = Vin × R2 ÷ (R1 + R2)- With a load
RLacross R2:R2' = R2 × RL ÷ (R2 + RL), then use R2' in place of R2. A load of 0 means no load. Vin = Vout × (R1 + R2') ÷ R2'R1 = R2' × (Vin ÷ Vout − 1)R2' = R1 × Vout ÷ (Vin − Vout), thenR2 = R2' × RL ÷ (RL − R2')when a load is present (needsR2' < RL).- Divider current
I = Vin ÷ (R1 + R2')and powerVin × I.
Worked example
12 V in, R1 = 10 kΩ, R2 = 5 kΩ: Vout = 12 × 5000 ÷ 15000 = 4 V, current 0.8 mA, power 9.6 mW. Add a 10 kΩ load: R2' = 5000 × 10000 ÷ 15000 = 3333.3 Ω, Vout = 12 × 3333.3 ÷ 13333.3 = 3 V, and the current becomes 0.9 mA.
Frequently asked questions
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2). With 12 V, R1 = 10 kΩ and R2 = 5 kΩ, Vout = 12 × 5000 ÷ 15000 = 4 V.
How does a load change the output?
A load in parallel with R2 lowers the bottom resistance to R2 × RL ÷ (R2 + RL). A 10 kΩ load on the example above makes it 3.33 kΩ, so Vout drops from 4 V to 3 V.
How do I choose R1 for a target voltage?
Rearrange to R1 = R2 × (Vin ÷ Vout − 1). For 12 V in, 4 V out and R2 = 5 kΩ: R1 = 5000 × (3 − 1) = 10 kΩ.
When is a voltage divider a bad idea?
When the load current is not small compared with the divider current. A good rule is a load at least 10 times R2. For power, use a regulator, since a divider wastes current and its output sags with load.
Can I rely on this for a real design?
It is for learning and design checks, not safety certification. Resistor tolerance (for example ±1%) shifts Vout, and each resistor must stay within its power rating.