Free calculators for matching amplifiers to speakers, sizing speaker cable, working out impedance loads and predicting how loud a system will play. Every one shows its working.
Most audio calculators on the web give you a number and no way to check it. Several of them are quietly wrong — the two most common faults being forgetting that speaker cable carries current there and back, and treating a 2.83 V sensitivity rating as though it were dB per watt on a 4 ohm speaker, which understates the power needed by half.
Every tool here has a Show the working panel listing each term that went into the answer, and says plainly which numbers are physics and which are a judgement call. Where a figure is contested — how fast sound falls off in a real room, for instance — it is a control you can change rather than a constant we picked for you.