Wheeler's formula is the classic hand calculation. It models the winding as a smooth current sheet, which a real coil is not — so this shows it beside a discrete round-wire summation.
Single-layer inductance or turns, two ways.
A single-layer air-core coil is the simplest inductor — and easy to wind wrong. This shows the spread before you commit.
Wheeler models the winding as a smooth current sheet, which a real coil isn’t — the two numbers bracket reality. Wind close-spaced and measure if you can; nearby metal changes the value.
