Voltage After Resistor Calculator: Find the Output

A voltage after resistor calculator finds the voltage at a resistor’s output node from the supply voltage, resistance values, and circuit arrangement. For a two-resistor voltage divider, enter the supply voltage and both resistances to get the output voltage and the voltage dropped across each resistor.

Formula

This calculator uses the voltage-divider formula for two resistors in series across a supply. The output is measured at the junction between the resistors, relative to the circuit’s common reference (ground):

• Output voltage: V_out = V_s × R₂ / (R₁ + R₂)
• Drop across R₁: V_R1 = V_s × R₁ / (R₁ + R₂)
• Series current: I = V_s / (R₁ + R₂)

Here, V_s is the supply voltage in volts (V), R₁ is the resistor connected from the supply to the output node, and R₂ is the resistor connected from that node to ground. Both resistances must use the same unit, such as ohms (Ω) or kilohms (kΩ). The divider current is returned in amperes (A).

Worked Example

Using the calculator’s default values, V_s = 12 V, R₁ = 10,000 Ω, and R₂ = 10,000 Ω:

1. Add the resistances: R₁ + R₂ = 10,000 + 10,000 = 20,000 Ω.
2. Find the output: V_out = 12 × 10,000 / 20,000 = 6 V, displayed as 6.00 V.
3. Find the drop across R₁: V_R1 = 12 × 10,000 / 20,000 = 6.00 V.
4. Find the current: I = 12 / 20,000 = 0.0006 A, displayed as 0.0006 A.

The equal resistors split the supply voltage equally. The two drops add to 12 V, as expected for components in series.

How the Calculator Works

In a series circuit, the same current flows through both resistors. Ohm’s law says that the voltage across a resistor equals current multiplied by resistance. Since R₂ is part of the total series resistance, its share of the supply is R₂ divided by R₁ + R₂. The output node sits across R₂, so that share is the output voltage.

For example, increasing R₂ while leaving the other values unchanged raises the output; increasing R₁ lowers it. If R₂ is much larger than R₁, the output approaches the supply voltage. If R₂ is much smaller, the output approaches zero. With positive resistor values, the unloaded output lies between zero and the supply voltage.

When to Use a Voltage Divider

A resistor divider is useful for estimating or producing a lower voltage when the load draws little current. Common examples include scaling a signal for measurement, setting a reference level, or biasing a circuit input. It is not automatically a suitable replacement for a regulated power supply: a load connected to the output changes the effective resistance and can pull the voltage down.

When a load resistance R_L connects from the output to ground, it is in parallel with R₂. Calculate the effective lower resistance as R_effective = R₂ × R_L / (R₂ + R_L), then use that value in place of R₂ in the divider formula. If the load is not much larger than R₂, the unloaded calculator result may be a poor estimate. For a substantial or changing load, consider a regulator or a buffer circuit instead.

Choosing Resistor Values and Checking Power

The voltage ratio determines the divider output, but the absolute resistance values determine how much current the divider consumes and how strongly it can hold its output under load. Lower values draw more current and waste more power; higher values reduce that current but make the output more sensitive to load and leakage.

For the unloaded two-resistor divider, the power dissipated by each resistor can be checked with P = I²R, where P is in watts, I is in amperes, and R is in ohms. The supply delivers total divider power P_total = V_s × I. Compare these calculations with the resistor’s rated power and allow appropriate margin for operating conditions and component tolerances. The calculator gives ideal values; it does not account for resistor tolerance, supply variation, temperature effects, or a connected load.

Common Mistakes and Limits

• Reversing the resistor positions: R₁ is the upper resistor, from supply to output; R₂ is the lower resistor, from output to ground. Swapping them changes the result.
• Mixing units: Enter both resistances in ohms or both in kilohms. For example, 10 kΩ is 10,000 Ω. Mixing 10 and 10,000 as though they used the same unit gives a misleading ratio.
• Assuming a loaded output stays fixed: The formula assumes the output is not loaded. A connected device can alter the divider ratio.
• Relying on rounded values: Displayed results are rounded. Use unrounded values for further calculations when precision matters.
• Using the divider for a power load: A divider is generally a signal or reference arrangement, not an efficient way to supply a load that draws significant current.

This calculation assumes a steady DC supply, two ideal resistors in series, and a known ground reference. For AC circuits, reactive components, transient behavior, or circuits with additional branches, the simple divider equation may not describe the node voltage.

Frequently Asked Questions

How do I calculate voltage after one resistor?
For a two-resistor series divider, the voltage at the junction relative to ground is V_s × R₂ / (R₁ + R₂). The answer depends on both resistors, not just the resistor immediately before the node.

Is voltage after a resistor the same as voltage across it?
Not necessarily. Voltage after a resistor usually means the node voltage relative to a reference such as ground. Voltage across a resistor is the difference between the voltages at its two terminals.

What happens if I connect a load to the divider output?
The load usually lowers the output because it acts in parallel with the lower resistor. Use the parallel equivalent of the load and lower resistor in the divider formula, or use a circuit designed to regulate the required load.

Can a resistor divider reduce 12 V to 5 V?
Yes, an unloaded divider can produce a 5 V output from 12 V if R₂ / (R₁ + R₂) equals 5/12. The actual output may differ under load, with resistor tolerances, or if the supply is not exactly 12 V.

Does a resistor reduce voltage by a fixed amount?
No. The voltage drop depends on the current and resistance, according to V = IR. In a divider, the output also depends on the other resistor and any connected load.

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in vs | Supply voltage | V | 12 | 0
in r1 | Upper resistor | ohm | 10000 | 0
in r2 | Lower resistor | ohm | 10000 | 0
out vout | Voltage after lower resistor | V | vs * r2 / (r1 + r2) | 2
out vdrop1 | Voltage drop across upper resistor | V | vs * r1 / (r1 + r2) | 2
out current | Divider current | A | vs / (r1 + r2) | 4