Calculators
Every calculator here draws the circuit it solves, so you can sanity-check the answer before you pick up the iron.
Ohm's law calculator
Volts, amps, ohms and watts on a live circuit. Type any two, read the rest.
Parallel resistor calculator
Two resistors or ten. The combined value, and how far off the nearest standard part is.
Voltage divider calculator
Vin, R1 and R2 in, Vout on the diagram. Works backwards from a target voltage too.
Resistor color code
Set the bands you see, read the value. 4-band and 5-band.
LED series resistor
Supply, forward voltage and current in, the resistor that keeps the smoke inside out.
Wire gauge calculator
Current and run length in, the AWG that stays cool and the voltage you lose along the way.
555 timer calculator
Astable and monostable. Pick the parts, read the frequency, duty cycle and pulse width.
Capacitor code converter
From 104 printed on the case to 100 nF, and back the other way.
SMD resistor code
Three digits, four digits or EIA-96 on a part the size of a grain of rice.
Battery life calculator
Capacity against draw, with a real-world efficiency factor, in hours and days.
RC time constant
Resistor and capacitor in, time constant and filter cutoff frequency out.
Ohm's law wheel
All twelve formulas on one wheel. Tap the quantity you want, see the three ways to get it.
Why these and not a hundred more
These twelve cover what actually comes up at a hobby bench: sizing a resistor for an LED, splitting a voltage for a sensor, decoding the part you just dropped on the carpet, and checking whether the wire in your drawer can carry the current you're about to ask of it. Each one shows its work on a diagram, because a number without a picture is easy to misread by a factor of a thousand.
If you don't know which calculator you need, start with the component identifier or the Ohm's law wheel, and the charts cover the values that don't need computing at all.