Electronics Design Calculators
These calculators cover the small design decisions in every circuit: the resistor that keeps an LED alive, the divider that scales a voltage for an ADC, the combined value of parts in series and parallel, what the number printed on a capacitor means, where an RC filter cuts off, what frequency a 555 timer runs at, the impedance of an RLC network, the gain of an op-amp stage, how wide a PCB trace must be for a current and how many decibels a ratio is.
10 free tools · no sign-up · no ads
All Electronics (10)
LED Resistor Calculator
Series resistor for one or more LEDs from supply voltage and currentVoltage Divider Calculator
Output voltage or resistor values for a two-resistor dividerSeries and Parallel Calculator
Total resistance, capacitance or inductance in series and parallelCapacitor Code Calculator
Capacitance and tolerance from 3-digit capacitor codes, and backRC Filter Calculator
Cutoff frequency and time constant of RC and RL filters555 Timer Calculator
Frequency and duty cycle for 555 astable and monostable circuitsReactance and Impedance Calculator
Capacitive and inductive reactance, RLC impedance and resonanceOp-Amp Gain Calculator
Gain and output of inverting, non-inverting and differential op-amp circuitsPCB Trace Width Calculator
Trace width for a current and temperature rise per IPC-2221Decibel Calculator
Convert power and voltage ratios to dB, dBm and dBW and back
Which tool do I need?
Choose the resistor for an LED on 5 V
Enter the supply, LED forward voltage and current to get the resistor, its power rating and the nearest standard value.
Set a 555 timer to blink at 1 Hz
Enter R1, R2 and C, or the frequency you want, to get the timing, duty cycle and standard part values.
Read the code on a ceramic capacitor
Type the three digits and tolerance letter to get the value in pF, nF and µF.
Size a PCB trace for 3 A
Enter the current, copper weight and allowed temperature rise to get the internal and external trace widths.
About Electronics
Values snap to standard E-series parts where it helps, so you see both the ideal value and what a real resistor or capacitor gives. PCB trace widths follow the IPC-2221 equations, and every tool accepts SI prefixes such as 4k7, 100n and 2.2µ.
They are design aids for prototypes and hobby builds as well as professional work. Real parts have tolerances, temperature drift and parasitics that these first-order formulas leave out, so check critical values on the bench.
Guides
- LED Current-Limiting Resistors: The Formula and the JudgmentWhy an LED needs a resistor, how R = (Vs − n·Vf) / I comes from the diode curve, how much headroom to leave, and how to wire series and parallel strings safely.
- Voltage Divider Design: Ratios, Loading and Picking Real ResistorsDerive Vout = Vin·R2/(R1+R2), see why a load pulls the output down, use the Thevenin view to size values, and choose standard resistor pairs for ADC inputs.
- Series and Parallel Circuits: Resistors, Capacitors and InductorsWhy resistors add in series but capacitors add in parallel, how to reduce a mixed network step by step, and the power, voltage and tolerance traps in each.
- Reading Capacitor Codes: 104, 2A104J, X7R and C0G ExplainedHow three-digit capacitor codes work, what tolerance letters and EIA voltage codes mean, and why the dielectric code matters as much as the value.
Frequently asked questions
Why does an LED need a series resistor?
An LED's current rises steeply with a small change in voltage, so without something to limit it the current runs away and destroys the LED. The resistor sets the current from the voltage left over after the LED's forward drop.
What does the 104 on a capacitor mean?
The first two digits are the value and the third is the number of zeros, in picofarads. 104 is 10 followed by four zeros, which is 100,000 pF or 100 nF.
Why are internal PCB traces wider than external ones for the same current?
Inner layers are surrounded by board material and cannot shed heat into the air, so IPC-2221 requires roughly twice the cross-section for the same temperature rise.
What is the difference between dB and dBm?
dB is a ratio between two levels. dBm is an absolute power level referenced to one milliwatt, so 0 dBm is 1 mW and 30 dBm is 1 W.