Electronics Calculators
Calculators for the numbers a circuit actually runs on — reading a resistor off the bench, then working out the current, voltage and resistance around it.
Start with the part in your hand
Most hobby electronics problems begin with a component whose value you cannot read. The Resistor Colour Code decodes the bands into ohms and tolerance, and flags when the value it produces is not one that is actually manufactured — which is the usual sign the bands are being read from the wrong end. Once you know the value, the Ohm's Law turns it into the current and voltage the rest of the circuit will see, and the LED Resistor works the problem the other way round — given the supply and the LED, it picks the series resistor you should fit.
Series and parallel change the answer, not the law
Ohm’s law applies to a single resistance, so the first job in any real circuit is reducing a network to one figure. The Resistance Calculator does that for resistors in series and in parallel. Two habits catch most mistakes: parallel resistance is always lower than the smallest resistor in the group, and a network that produces a total higher than its largest member has been wired, or entered, as series when it is parallel.
The bands say nothing about power
A resistor’s colour code carries its value and tolerance and nothing else. Power rating comes from physical size, and it is the specification hobbyists most often overlook — a quarter-watt part asked to dissipate half a watt will discolour, drift and eventually fail open. Work out the dissipation with the Power Calculator before choosing a part, and derate generously in an enclosure.
Units are the other half of the problem
Electronics spans nine orders of magnitude, from milliohms in a current-sense shunt to megohms in a bias network, and prefix errors are the single most common source of a plausible but wrong answer. The Unit Converter handles the arithmetic when a datasheet and a schematic disagree on prefix. Every formula behind these tools is listed with its symbols on the engineering formulas reference reference page.
Frequently asked questions
Which end of a resistor do I start reading from?＋
From the end whose first band sits closest to the lead. The tolerance band is at the other end and is usually separated by a wider gap; on cheap parts it is gold or silver, and neither colour is ever a significant digit. If the value you decode belongs to no E-series at all, you are almost certainly reading the resistor backwards.
Do these tools cover mains voltage work?＋
No. Everything here assumes low-voltage DC hobby and study work. Mains wiring is governed by national wiring regulations, carries a real risk of death, and the calculations that matter there — cable sizing, protective device selection, earth fault loop impedance — are not what these tools do. Use a qualified electrician.
Why does my multimeter disagree with the calculated value?＋
Within tolerance, disagreement is expected: a 10% part may legitimately measure 10% either side of its marking. Larger gaps almost always mean you are measuring the resistor in circuit, so your meter is reading it in parallel with everything around it. Desolder one leg and measure again.
Are these accurate enough for coursework?＋
Yes, and they show the substitution as well as the answer, which is what method marks are awarded for. What they cannot do is decide which components are in series and which are in parallel — reducing the diagram is the part that carries the marks.