Raspberry Pi Power Calculator
Power supply sizing from Pi model and peripherals.
About this calculator
What this engine actually computes is a resistive voltage divider — the two-resistor circuit at the heart of level-shifting a sensor's output down to a safe input range, a very real Raspberry Pi use case since Pi GPIO pins are not 5V-tolerant. From your input voltage and the two resistor values it computes the unloaded output as Vout = Vin × R2/(R1+R2), plus the divider ratio and the small quiescent current that flows through R1 and R2 continuously, even with nothing connected downstream. It then models what happens when you attach a real load: the load resistance goes electrically in parallel with R2, pulling the effective bottom resistance down and dragging the output voltage below the unloaded ideal — the calculator reports this loaded output directly alongside a "load voltage error" percentage so you can see how much the load is sagging your reference voltage.
It also reports the Thevenin output impedance (R1 parallel R2), the key number for judging whether this divider can actually drive your downstream circuit without unacceptable sag — a good rule of thumb is keeping the load resistance at least 10x the output impedance. Power dissipated as heat in both resistors is reported too, worth checking on lower resistor values where continuous current can add up meaningfully. With load resistance left at zero, only the unloaded case applies, since a true open-circuit load draws no current at all.
Inputs
Results
Output Voltage (unloaded, V)
2.5
Output Voltage (loaded, V)
2.5
How to Use This Calculator
- Enter your supply voltage and the resistor values R1 (top) and R2 (bottom) in the divider network.
- Set the load resistance to model the circuit under load.
- Review unloaded and loaded output voltage, divider ratio, and quiescent current (mA).
- Check total current draw to ensure your power supply can handle the combined load.
How the result changes with Input Voltage (V)
| Input Voltage (V) | Output Voltage (unloaded, V) | Output Voltage (loaded, V) |
|---|---|---|
| 2.5 | 1.25 | 1.25 |
| 3.75 | 1.88 | 1.88 |
| 7.5 | 3.75 | 3.75 |
| 13 | 6.5 | 6.5 |
What each input means
- Input Voltage (V)
- Source voltage applied to the divider (e.g. 5V, 12V).
- R1 - Top Resistor (Ω)
- Resistance of top resistor (connected to Vin).
- R2 - Bottom Resistor (Ω)
- Resistance of bottom resistor (connected to ground).
- Load Resistance (Ω)
- Resistance of load connected to output (0 = no load / open circuit).
What each result means
- Output Voltage (unloaded, V)
- Vout = Vin x R2 / (R1 + R2) with no load.
- Output Voltage (loaded, V)
- Output voltage with load resistance in parallel with R2.
- Divider Ratio
- Fraction of input voltage appearing at output (R2 / (R1+R2)).
- Quiescent Current (mA)
- Current flowing through divider with no load.
- Total Current (mA)
- Total current from supply including load.
- Output Impedance (Ω)
- Thevenin output impedance (R1 || R2). Lower is better for driving loads.
- Power Dissipated (mW)
- Total power wasted as heat in the resistors.
- Load Voltage Error (%)
- Percent deviation from ideal due to load. Keep under 1% for accurate readings.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersInput Voltage (V) = 5, R1 - Top Resistor (Ω) = 10000, R2 - Bottom Resistor (Ω) = 10000, Load Resistance (Ω) = 0 = 4 input(s) provided
- Calculate Output VoltageOutput Voltage = inputVoltage * r2 / (r1 + r2)2.5 = 2.5
- Calculate Output Voltage2.5 = 2.5
- Calculate Divider RatioDivider Ratio = r2 / (r1 + r2)0.5 = 0.5
- Calculate Quiescent CurrentQuiescent Current = (inputVoltage / (r1 + r2)) * 10000.25 = 0.25
Engine last updated . Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does adding a load resistance change my output voltage?
The moment you connect a real load, it forms an electrical path from the output node to ground in parallel with R2, which pulls the effective bottom resistance down below R2's own value (calculated as R2 × loadResistance / (R2 + loadResistance)). A smaller effective bottom resistance shifts the divider ratio, dragging the loaded output voltage below the unloaded ideal — the 'Load Voltage Error (%)' output quantifies exactly how much that sag is, which is why leaving Load Resistance at 0 (open circuit) is the only way to see the pure unloaded math.
What does the output impedance number tell me, and why does it matter for driving a load?
Output impedance is the Thevenin equivalent resistance of the divider as seen from its output, calculated as R1 and R2 in parallel. A good rule of thumb is keeping your actual load resistance at least 10 times this output impedance — if the load is comparable to or smaller than the output impedance, the loaded-output-voltage sag reported elsewhere becomes significant, which is exactly the scenario this calculator's loaded-vs-unloaded comparison is built to reveal.
Why is this called a Raspberry Pi calculator when it's really a generic voltage divider?
It's framed around the Pi because level-shifting is a very typical reason hobbyists reach for a voltage divider on that platform: Pi GPIO input pins tolerate a maximum of about 3.3V, but many sensors output signals up to 5V, and this two-resistor divider is the standard way to bring that voltage down safely. The underlying math (Vout = Vin × R2/(R1+R2)) is the same for any voltage-divider application, not just Pi-specific circuits.
Why does the calculator report both a Quiescent Current and a Total Current?
Quiescent Current is the current that flows through R1 and R2 continuously, purely from the divider itself, even with nothing connected to the output — it never goes to zero as long as the input voltage is applied. Total Current adds in whatever additional current the load resistance draws once connected, computed from the input voltage divided by R1 plus the parallel combination of R2 and the load. Comparing the two shows you how much extra current your downstream circuit is pulling from the supply.
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