Resistor Divider Calculator

Calculator Inputs

Mode

Use Analyze to evaluate an existing divider. Use Design to size R1 and R2.

Vin (V)

Tolerance (%)

E-Series

Used when rounding to common resistor values.

Round to standard values

Enable rounding (Design mode)

R1 (Ω)

Top resistor from Vin to Vout.

R2 (Ω)

Bottom resistor from Vout to ground.

Load RL (Ω) (optional)

If set, Vout is computed with R2 ∥ RL.

Target Vout (V)

Used in Design mode only.

Design preference

Total resistance R1+R2 (Ω)

Higher values reduce current, but increase loading sensitivity.

Divider current target (mA)

Lower values save power, but can be inaccurate with loads.

Example Data Table

Use case	Vin (V)	R1 (Ω)	R2 (Ω)	RL (Ω)	Expected Vout (V)
5 V from 12 V sensor bias	12	6800	3300	none	~3.93
3.3 V logic from 5 V rail	5	5100	10000	100000	~3.22
ADC reference with load	24	47000	10000	20000	~3.69

Values are illustrative. Always verify loading and power.

Formula Used

No-load output voltage

Vout = Vin × (R2 / (R1 + R2))

Assumes the output is unloaded.

Loaded output voltage

Rbottom = R2 ∥ RL

Vout_loaded = Vin × (Rbottom / (R1 + Rbottom))

Load reduces the effective bottom leg.

Divider current and power

Iin = Vin / (R1 + Rbottom)

P(R1) = Iin² × R1

P(R2) = Vout_loaded² / R2

Check resistor wattage ratings.

Thevenin equivalent at Vout

Vth = Vin × (R2 / (R1 + R2))

Rth = R1 ∥ R2

Useful for quick loading estimates.

How to Use This Calculator

Select Analyze Divider to evaluate existing resistors.
Enter Vin, R1, and R2.
Optionally set RL to model your load.
Press Calculate to see results above the form.
Select Design Divider to size resistors for target Vout.
Pick total resistance or divider current as your basis.
Enable rounding to standard values and review the error.
Download CSV or PDF to save your work.

FAQs

1) Why does Vout drop when I connect a load?

The load is in parallel with R2. This lowers the bottom resistance and changes the ratio. Increase divider current or buffer the output to reduce the drop.

2) How do I choose a good divider current?

Keep divider current at least ten times your load current. This reduces ratio error from loading. Higher current wastes more power, so balance accuracy and efficiency.

3) What is the Thevenin resistance used for?

Rth shows how strongly the divider can drive a load. Lower Rth means less sag and better noise immunity. It also increases the divider’s standby power.

4) Can I use a divider to power a device?

Not recommended for variable loads. The output voltage changes with load current. Use a regulator, reference, or buffer if the circuit draws meaningful current.

5) How does tolerance affect output accuracy?

Tolerance changes the resistor ratio. This tool estimates a simple worst-case Vout band using ±tolerance on both resistors. For better accuracy, use tighter tolerance parts or calibration.

6) What does rounding to an E-series do?

It selects the nearest preferred resistor values found in common series like E12 or E24. After rounding, check the achieved Vout and percentage error before building.

7) What wattage rating should I select?

Pick a rating comfortably above calculated dissipation. Many designs use at least a 2× margin, and increase it for high temperature or limited airflow.