Electronics Calculators & Engineering Tools
Six free calculators covering the arithmetic that comes up most often at the bench and in schematic capture: reading a resistor off its color bands, sizing a voltage divider, combining resistors, checking Ohm's law, picking an LED series resistor, and decoding capacitor markings.
Every tool is a single page with no login, no ads between you and the answer, and no external dependencies — open it, use it, close it.
All Six Tools
Resistor Color Code Calculator
Read 4-band, 5-band and 6-band resistors in both directions — bands to value, or value to bands. Includes the temperature coefficient band.
Most used — 22.4K searches/month
Series & Parallel Resistor Calculator
Combine any number of resistors in series or parallel, and see the nearest E12/E24 standard values.
11.6K searches/month
Voltage Divider Calculator
Find Vout from R1 and R2, or work backwards to find the standard resistor pair for a target output — with optional load current.
6.6K searches/month
Capacitor Conversion Calculator
Convert between pF, nF, µF and F, decode three-digit capacitor markings, and total up series or parallel banks.
5.2K searches/month
Ohm's Law Calculator
Enter any two of voltage, current, resistance or power and get the other two, with automatic SI prefixes.
3.7K searches/month
LED Series Resistor Calculator
Size the series resistor and its wattage from supply voltage, LED forward voltage and forward current.
2.4K searches/month
Which Tool Do I Need?
| If you are… | Use |
|---|---|
| Holding a resistor and want its value | Resistor Color Code Calculator |
| Designing a divider to drop 5 V to 3.3 V | Voltage Divider Calculator |
| Missing a value and combining two parts you have | Series & Parallel Resistor Calculator |
| Checking whether a part will overheat | Ohm's Law Calculator |
| Driving an indicator LED from a GPIO pin | LED Series Resistor Calculator |
| Reading “104” printed on a ceramic cap | Capacitor Conversion Calculator |
How These Tools Handle Standard Values
Answers that land on arbitrary numbers are not much use at the bench, because you can only buy standard values. Where a result has to be a real part, these calculators snap the answer to the E12 or E24 series defined by IEC 60063 and report how far the real output lands from your target.
Two practical notes that catch people out:
- Series values are not evenly spaced. The gap between 4.7 kΩ and 5.1 kΩ is not the same as between 8.2 kΩ and 9.1 kΩ. Tolerance bands are set so that adjacent values just overlap, which is why E24 exists for 5% parts and E12 is enough for 10%.
- A divider is not a supply. Its output collapses under load unless the divider current is much larger than the load current. The voltage divider calculator takes an optional load current and picks values that hold up under it — but for anything drawing real current, a regulator is the right answer, not a divider.
Units and Notation
Results use SI prefixes throughout, so you will see 4.7 kΩ rather than 4700 Ω, and 100 nF rather than 0.0000001 F. The capacitor tool also accepts the three-digit code printed on ceramic and film capacitors, where the first two digits are significant figures and the third is the number of zeros in picofarads.