About
About Ohms Law Wheel
A focused reference for the V-I-R-P formula wheel and the twelve electrical calculators built around it.
What This Site Is
Ohms Law Wheel is a focused reference for the four quantities that describe a resistive electrical circuit: voltage, current, resistance and power. The wheel arranges the twelve equations linking them into four groups of three, so that whichever pair of values you happen to know, the formula for the ones you need is easy to find.
Around that central reference sit twelve calculators covering the problems that come up most often in practical work, from sizing an LED resistor to estimating voltage drop along a cable run. Each one has its own page explaining the formula it uses, a worked example, and an honest account of where the underlying model stops being accurate.
How the Calculators Work
Everything runs in your browser using plain JavaScript. There is no account, no upload step and no server-side processing: the numbers you type are handled by code already loaded on the page, and the results appear without anything being sent anywhere.
Internally, every entry is converted to SI base units before any arithmetic happens. A value typed as 4.7 kΩ becomes 4700 Ω, and 20 mA becomes 0.02 A. Calculations run at full floating-point precision throughout, and rounding is applied only at the point of display. That is why you can enter values in whichever units your notes or datasheet use without worrying about conversion errors creeping in.
Where a calculation cannot be completed, the tools say why. Asking for resistance from a current of zero, or for an LED resistor when the supply is below the forward voltage, produces an explanation rather than an error value or a misleading number.
Who It Is For
Students and Apprentices
Every result is shown alongside the formula used and the same formula with your numbers substituted in, so the tools can be used to check working rather than replace it.
Electricians and Technicians
Quick answers on site for drop, conductor resistance and load current, with the assumptions behind each model stated plainly.
Hobbyists and Makers
Practical component sizing for LEDs, dividers, resistor networks and 555 timing circuits, with standard values suggested where they apply.
Engineers
A fast sanity check for first-pass figures, with the limitations of each simplified model listed rather than buried.
Our Approach to Accuracy
The formulas used here are standard and well established, and the constants are the conventional published values, stated on each page so you can see exactly what has been applied. Copper resistivity, for example, is taken as 1.724 × 10⁻⁸ Ω·m at 20 °C, and the figure appears in the results rather than being hidden in the code.
What we do not do is claim more than the models support. Ohms Law describes linear, resistive behaviour at constant temperature. Real circuits involve tolerance, temperature drift, non-linear components and, in AC systems, reactance and power factor. Every calculator page carries a limitations section that sets out where its model stops applying, because knowing that boundary is part of using the result well.
These tools are intended for study, prototyping and planning. They are not a substitute for local electrical codes, manufacturer datasheets, or the judgement of a qualified person.
Content and Independence
All explanations, worked examples and calculator logic on this site are written specifically for it. The site is not affiliated with any component manufacturer, distributor or certification body, and no calculator recommends a particular brand of part.
The review cards shown on the homepage are clearly labelled demonstration content used to illustrate the layout. They are not verified customer testimonials, and we do not publish aggregate ratings.
Get in Touch
If you find an error in a formula, a constant, or an explanation, we want to hear about it. Corrections are the most useful message we receive. The contact page explains how to reach us and what to include so an issue can be reproduced quickly.
Back to the Wheel
Solve for voltage, current, resistance or power from any two known values.