Free electronics calculators

8 calculators for the sums you meet most often when you build circuits. Each one is free, runs in your browser, accepts values like 4.7k, 100n and 20m, and explains the formula, the letters in it and a worked example.

• LED Resistor Calculator

  Which resistor keeps an LED safe? Gives the exact value, the E12 value to buy and its power rating.

• Ohm's Law Calculator

  Know any two of voltage, current, resistance and power? Get the other two.

• Voltage Divider Calculator

  Output voltage of two resistors, the effect of a load, or the resistors for a target voltage.

• RC Time Constant Calculator

  How long a capacitor takes to charge or discharge through a resistor.

• RC Filter Calculator

  Cutoff frequency, gain and phase of a low-pass or high-pass RC filter.

• Resistors in Series and Parallel Calculator

  Total resistance of two to ten resistors wired in series or in parallel.

• 555 Timer Calculator

  Frequency and duty cycle of a 555 astable, or the pulse length of a 555 monostable.

• PCB Trace Width Calculator

  How wide a copper track must be for a current, using the IPC-2221 formula.

Want to see why the formulas work? Every calculator links to a live lesson where you can change the same circuit and watch it respond. Browse the lessons.