Voltage Divider Calculator
Output voltage of a two-resistor divider (Vout = Vin·R2/(R1+R2)), the current and power in each resistor, and how a load resistance pulls the output down.
Voltage Divider Calculator: with the default inputs, output voltage (vout) is 3.8367.
Between Vin and the output. 10 kΩ = 10000.
Between the output and ground.
Volts across R2 (and the load, if any).
- Unloaded output voltage (V)
- 3.8367
- Divider ratio Vout ÷ Vin
- 0.3197
- Current through R1 (mA)
- 0.8163
- Power in R1 (mW)
- 6.664
- Power in R2 (mW)
- 3.132
- Power in load (mW)
- 0
- Total power drawn (mW)
- 9.796
- Effective bottom resistance R2 ∥ RL (Ω)
- 4,700
Assumptions
- Ideal resistors at nominal value (tolerance not modelled).
- Ideal source with zero output impedance.
- DC or low-frequency operation; no stray capacitance.
| Load (Ω) | Vout (V) | Drop vs. unloaded |
|---|---|---|
| ∞ (no load) | 3.8367 | 0% |
| 1,000,000 | 3.8245 | 0.32% |
| 100,000 | 3.7179 | 3.1% |
| 10,000 | 2.9072 | 24.23% |
| 1,000 | 0.9141 | 76.18% |
| 100 | 0.1164 | 96.97% |
The first row is the unloaded divider. A load much larger than R2 barely matters; a load comparable to R2 pulls the output well below the design value.
How this is worked out
The formula
Vout = Vin × R2 ÷ (R1 + R2) With a load RL across R2, replace R2 with R2 ∥ RL = R2·RL ÷ (R2 + RL) I = Vin ÷ (R1 + R2) P₁ = I²R1 P₂ = Vout² ÷ R2
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Input voltage (Vin)
- A number.defaults to 12
- R1 (top resistor)
- Between Vin and the output. 10 kΩ = 10000.0 or more · defaults to 10000
- R2 (bottom resistor)
- Between the output and ground.0 or more · defaults to 4700
- Load resistance(under More options)
- Resistance connected across R2 (an ADC input, a gate, a meter). Leave 0 for no load.0 or more · defaults to 0
What you get back
- Output voltage (Vout)main answer
- Volts across R2 (and the load, if any).
- Unloaded output voltage (V)
- Divider ratio Vout ÷ Vin
- Current through R1 (mA)
- Power in R1 (mW)
- Power in R2 (mW)
- Power in load (mW)
- Total power drawn (mW)
- Effective bottom resistance R2 ∥ RL (Ω)
What this assumes
- Ideal resistors at nominal value (tolerance not modelled).
- Ideal source with zero output impedance.
- DC or low-frequency operation; no stray capacitance.
About this calculator
Two resistors in series split a voltage in proportion to their resistances: the output taken between them is Vin × R2 ÷ (R1 + R2). It is the simplest circuit in electronics and one of the most used — scaling a 12 V battery down to something a 3.3 V microcontroller pin can read, biasing a transistor, turning a resistive sensor (thermistor, photoresistor, potentiometer) into a voltage. Enter Vin and the two resistors; the calculator gives the output, the current, and the power each resistor must handle.
The loading effect
The textbook formula assumes nothing is connected to the output. Anything you connect — an ADC input, a transistor base, a voltmeter — has its own resistance in parallel with R2, which lowers the effective R2 and drags the output down. Open More options, enter the load, and the calculator shows the real output next to the unloaded one, plus a table of how the output sags as the load gets heavier. The rule of thumb: keep R2 at most a tenth of the load resistance and the error stays under about 10%; a hundredth keeps it near 1%.
Choosing resistor values
The ratio sets the voltage; the absolute values set the trade-off between wasted current and stiffness. 1 kΩ + 470 Ω from 12 V burns 98 mW continuously; 100 kΩ + 47 kΩ gives the same 3.84 V at 1 mW but is easily loaded and picks up noise. Microcontroller ADC inputs usually want a source impedance under 10 kΩ, so 10 kΩ + 4.7 kΩ is a common compromise.
Common mistakes
- Swapping R1 and R2. R2 is the one the output is measured across (to ground); a bigger R2 means a bigger output.
- Using a divider as a power supply. It cannot regulate: current drawn by the load changes the output. Use a regulator for anything that draws real current.
- Ignoring tolerance. Two 5% resistors can put the ratio off by up to about 10%; use 1% parts when the voltage matters.
- Forgetting power ratings when Vin is high. The steps list P = I²R for each resistor.
Frequently asked questions
▸What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor between the output and ground. The output is the fraction of the input equal to R2's share of the total resistance.
▸Why does my divider output drop when I connect something?
The connected device's input resistance sits in parallel with R2, lowering the effective R2 and the ratio. Use smaller resistor values (so the load is at least 10× R2) or add an op-amp buffer.
▸How do I pick resistor values for a specific output?
Choose the ratio R2 ÷ (R1 + R2) = Vout ÷ Vin, then scale both to a size that gives acceptable current. For 5 V → 3.3 V the ratio is 0.66, so R2 = 2 × R1; 1 kΩ and 2 kΩ or 10 kΩ and 20 kΩ both work.
▸Can a voltage divider power a circuit?
Only very light, constant loads. Any change in load current changes the output, and the resistors waste power continuously. Use a linear or switching regulator for real supplies.
▸How much power does each resistor dissipate?
P = I²R with I = Vin ÷ (R1 + R2), or equivalently V²/R using the voltage across each resistor. The calculator lists both in milliwatts so you can check against the usual ¼ W rating.
Put this calculator on your own site
A working voltage divider, free for any site, with no ads and no sign-up. It resizes to fit wherever you paste it and updates itself as this page improves.
Paste this anywhere. It works on any site, carries no ads, never expires, and always shows the current version.
Voltage Divider Calculator by CalculateItNow
The page's own title. The clearest description of what the link leads to.
The credit line sits outside the widget on purpose, so it is a real link on your page rather than one buried in a frame. Please keep it — it is what pays for CalculateItNow staying free and ad-free. The script only resizes the widget to fit its contents; drop it and the widget still works.
Browse every calculator widget·How to add it to WordPress, Squarespace or Wix
Related calculators
The questions people ask next to a voltage divider.
Solve V = IR and P = VI for voltage, current, resistance or power from two known values, with the rearranged formula shown and all 12 Ohm's-law equations.
Power from work and time (P = W/t), force and speed (P = Fv) or voltage and current (P = VI) in watts, kW, horsepower and BTU/h, with energy use per hour and day.
Find what any appliance costs to run per day, month and year from its wattage, hours of use and your price per kWh — plus a table of common appliances at your rate.
Find what percent one number is of another, take a percentage of a number, or calculate percentage increase and decrease — with the working shown.