What is the equivalent of fullpage in geometry package?

The only difference between

\documentclass[a4paper]{article}
\usepackage[
  margin=1.5cm,
  includefoot,
  footskip=30pt,
]{geometry}
\usepackage{layout}

\begin{document}
\layout
\end{document}

and

\documentclass[a4paper]{article}
\usepackage[cm]{fullpage}
\usepackage{layout}

\begin{document}
\layout
\end{document}

seems to be that the head height and the head separation in fullpage are zero.

For the in default option, change margin=1.5cm into margin=1in.

---

Just for experimenting, I compiled the same document with both preambles; the one with geometry with red text. I converted the DVI files to PNG with transparent background with dvipng. Next I superimposed the two PNG files in a single document. Here's the picture showing no essential difference between the two.

I have found that loading geometry with the [pass]option gives you access to all the features of the geometry package (at least the ones I use), without changing the extant page dimensions. Thus you can get the dimensions of the fullpage package, with the features of the geometry package available.

The pass option disables most of the options (not features), except verbose and showframe.

\documentclass{article}
\usepackage{fullpage}
\usepackage[pass, showframe]{geometry}
\usepackage{lipsum}
\begin{document}
\lipsum[1-7]
\newgeometry{margin=2in}
\lipsum[8-10]
\end{document}

Page 1 has the default fullpage dimensions (with showframe), but that has been reset on page 2 with a \newgeometry command, courtesy of the geometry package.