Ohm's Law Calculator

Ohm’s Law is the foundational equation of electronics. It describes the relationship between voltage, current, and resistance in a circuit. Choose any two values, and the calculator solves for the third — with a live circuit animation showing what’s happening.

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The Equations

$V = I \times R$

$I = \frac{V}{R}$

$R = \frac{V}{I}$

$P = V \times I = I^2 \times R = \frac{V^2}{R}$

Where:

• V = Voltage in volts (V)
• I = Current in amperes (A)
• R = Resistance in ohms (Ω)
• P = Power in watts (W)

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The Water Analogy

The circuit animation uses the water pipe analogy — the classic way to build intuition for Ohm’s Law:

Electricity	Water
Voltage (V)	Water pressure (height of reservoir)
Current (I)	Flow rate (litres per second)
Resistance (R)	Pipe narrowness / restriction
Power (P)	Work done (turning a wheel)

Increasing voltage with the same resistance = more current (more pressure, faster flow). Increasing resistance with the same voltage = less current (narrower pipe, slower flow). The animated electrons in the circuit speed up and slow down accordingly.

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Practical Examples

Choosing an LED Resistor

A standard red LED has a forward voltage of ~2.0 V and needs ~20 mA. Running from 3.3 V (tinyCore GPIO):

$R = \frac{V_{supply} - V_{LED}}{I_{LED}} = \frac{3.3 - 2.0}{0.020} = \frac{1.3}{0.020} = 65\ \Omega$

Use the next standard value: 68 Ω or 100 Ω for a slightly dimmer but safer LED.

I2C Pull-up Resistors

I2C lines need pull-up resistors. For tinyCore at 3.3 V running I2C at 400 kHz (fast mode), 4.7 kΩ is typical:

$I_{pullup} = \frac{3.3}{4700} \approx 0.7\ mA$

This is well within GPIO drive capability and keeps bus capacitance manageable.

Voltage Divider

To measure a 5 V signal with a 3.3 V ADC, use a voltage divider:

$V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$

For R1 = 10 kΩ, R2 = 22 kΩ: $V_{out} = 5 \times \frac{22}{32} \approx 3.4\ V$ — just within range.

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Power Dissipation Warning

Always check how much power a resistor will dissipate. Standard resistors are rated at 1/4 W (250 mW). If your calculation exceeds this, use a higher-rated resistor.

$P = I^2 \times R$

Example: 100 mA through a 10 Ω resistor: $P = (0.1)^2 \times 10 = 0.1\ W$

That’s 100 mW — within spec for a 1/4 W resistor, but getting warm.