Free online electronics calculators
Resistor calculators, Ohm's law solvers, LED resistor pickers, and other electronics utilities.
Four calculators cover the arithmetic that comes up at a workbench, and each does the full job rather than one direction of it. Resistor Calculator runs in three modes. Colour to value decodes 4, 5 and 6 band resistors, with the sixth band read as a temperature coefficient — brown 100 ppm per kelvin, red 50, yellow 25, orange 15, blue 10, violet 5 — and shows the tolerance from the band you picked, down to 0.05 percent for grey. Value to colour reverses it, so you can find the bands for a value you need to pull from a drawer. SMD mode decodes the printed codes on surface-mount parts, both the three-digit form and EIA-96, where the two digits index a three-significant-figure value — 01 is 100 ohms — and the letter sets the multiplier, with R and Y both meaning 0.01 and A meaning 1. A full E12 table sits under the calculator with the E24 series one click away, because the value your maths produces is rarely a value anyone manufactures. Ohm's Law Calculator takes any two of voltage, current, resistance and power and solves the other two, showing the formula it used — P equals V times I, V equals the square root of P times R, and the rest — with unit pickers running from microvolts to kilovolts and nanoamps to kiloamps so you can enter what is printed on the datasheet without converting first. LED Resistor Calculator starts from a supply voltage and an LED, with forward-voltage presets for the common colours: 2.0 V red, 2.1 V orange and yellow, 2.2 V green, 3.3 V blue, white and UV, 3.2 V pink and 1.5 V infrared, all at 20 mA. It handles several LEDs in series or parallel, rounds the calculated resistance up to the nearest E24 value so the current never exceeds the target, and reports the power the resistor will dissipate along with the rating you need — an eighth, a quarter, a half watt or more — which is the step beginners skip until something discolours. PCB Trace Calculator implements IPC-2221 in both directions: the width needed for a current at a given temperature rise, and the current a width can carry, for external layers with k of 0.048 and internal layers with 0.024. Copper weights from half an ounce to six ounces are covered, results snap to standard widths from 4 to 200 mil, and trace resistance, voltage drop and power loss come from copper resistivity at 1.724e-5 ohm-millimetres. Everything runs in the page, so the calculators keep working on a bench with no connection.
4 tools · always free · no account needed
Resistor Calculator
Visual resistor color code calculator with reverse lookup
Ohm's Law Calculator
Calculate voltage, current, resistance, and power
LED Resistor Calculator
Calculate series resistor for LEDs with circuit diagram
PCB Trace Calculator
Calculate PCB trace width using IPC-2221 standard
How to choose the right tool
Reach for Resistor Calculator when a part is in your hand and the question is what it is — colour to value for through-hole bands, SMD mode for the codes on a reel — or when the question is the reverse and you need to recognise a value on a shelf. Ohm's Law Calculator is the everyday one: you know a supply voltage and a resistance and want the current, or you know a current and a resistance and want to check that the part is not about to dissipate more than it is rated for. LED Resistor Calculator is the specialised case of that, and worth using instead of doing it by hand because it deals with the parts you actually have: it rounds up to an E24 value, warns when several LEDs in series exceed the supply voltage, and tells you the resistor wattage rather than leaving you to discover it. PCB Trace Calculator belongs at layout time, before the board is ordered, when a power trace has to carry a few amps and you need to know whether 20 mil on an external layer is enough or whether the copper weight has to go up.
Frequently asked questions
What does the sixth band on a resistor mean?
It is the temperature coefficient, in parts per million per kelvin, and it tells you how much the resistance drifts as the part heats up. The calculator reads brown as 100, red as 50, yellow as 25, orange as 15, blue as 10 and violet as 5 ppm per kelvin. On a 4 or 5 band part there is no such band and the last band is tolerance. Six-band parts are usually precision resistors where drift matters more than the initial tolerance.
Why does the LED calculator round the resistance up?
Because rounding down would push more current through the LED than you asked for. The exact value from Ohm's law almost never exists as a part, so the result is snapped up to the next E24 value, which lands slightly under the target current — safe for the LED and invisible in brightness. The calculator also shows the actual current at that standard value, so you can see how far from 20 mA you ended up.
Do I really need to check the resistor's power rating?
Yes, and it is the most common omission. The resistor dissipates the voltage dropped across it times the current through it, so a 12 V supply driving a 2 V LED at 20 mA burns 200 mW in the resistor — beyond a standard eighth-watt part and close to the limit of a quarter-watt one. The calculator prints the dissipation and the minimum rating, and warns whenever it passes a quarter of a watt.
How conservative is the IPC-2221 trace width formula?
It is an empirical curve fit from the standard, not a physics simulation, and it assumes a bare trace in still air on a board that is otherwise unloaded. Internal layers use half the external constant because they shed heat far worse. Real boards differ: dense copper pours, stacked current-carrying traces, high ambient temperature or a conformal coating all reduce the real capacity. Treat the number as a starting point and add margin for anything continuous.
What are E12 and E24 values, and why show them?
They are the preferred number series resistors are actually manufactured in — twelve values per decade for 10 percent parts, twenty-four for 5 percent. A calculation that returns 137 ohms is not something you can order, so the resistor page keeps both series on screen as a lookup table and the LED calculator snaps its answer to E24. Designing around those values from the start is what keeps a bill of materials short and a parts drawer useful.
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