diff --git a/Makefile b/Makefile
index 46fd88e..f181e20 100644
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@ CC=cc
 RUSTC=rustc
 
 .PHONY: all
-all: dj false fop intcmp rpn scrut str strcmp true
+all: dj false fop hru intcmp rpn scrut str strcmp true
 
 build:
 	# keep build/include until bindgen(1) has stdin support
@@ -74,6 +74,13 @@ build/bin/fop: src/fop.rs build build/o/libgetopt.rlib build/o/libsysexits.rlib
 		--extern sysexits=build/o/libsysexits.rlib \
 		-o $@ src/fop.rs
 
+.PHONY: hru
+hru: build/bin/hru
+build/bin/hru: src/hru.rs build build/o/libgetopt.rlib build/o/libsysexits.rlib
+	$(RUSTC) $(RUSTFLAGS) --extern getopt=build/o/libgetopt.rlib \
+		--extern sysexits=build/o/libsysexits.rlib \
+		-o $@ src/hru.rs
+
 .PHONY: intcmp
 intcmp: build/bin/intcmp
 build/bin/intcmp: src/intcmp.c build
diff --git a/docs/hru.1 b/docs/hru.1
new file mode 100644
index 0000000..c75cb73
--- /dev/null
+++ b/docs/hru.1
@@ -0,0 +1,57 @@
+.\" Copyright (c) 2024 Emma Tebibyte <emma@tebibyte.media>
+.\"
+.\" This work is licensed under CC BY-SA 4.0. To see a copy of this license,
+.\" visit <http://creativecommons.org/licenses/by-sa/4.0/>.
+
+.TH rpn 1
+
+.SH NAME
+
+hru \(en human readable units
+
+.SH SYNOPSIS
+
+hru
+
+.SH DESCRIPTION
+
+Hru reads byte counts in the form of whole numbers from the standard input and
+writes to the standard output the same number converted one of the units of data
+defined by the International System of Units.
+
+The program will convert the byte count to the highest unit possible where the
+value is greater than one.
+
+.SH DIAGNOSTICS
+
+If encountering non-integer characters in the standard input, hru will exit with
+the appropriate error code as defined by sysexits.h(3) and print an error
+message.
+
+.SH RATIONALE
+
+The GNU project’s ls(1) implementation contains a human-readable option (-h)
+that, when specified, makes the tool print size information in a format more
+immediately readable. This functionality is useful not only in the context of
+ls(1) so the decision was made to split it into a new tool. The original
+functionality in GNU’s ls(1) can be emulated with fop(1) combined with this
+program.
+
+.SH STANDARDS
+
+Hru follows the standard unit prefixes as specified by the Bureau International
+des Poids et Mesures (BIPM) in the ninth edition of The International System of
+Units (SI).
+
+.SH AUTHOR
+
+Written by Emma Tebibyte <emma@tebibyte.media>.
+
+.SH COPYRIGHT
+
+Copyright (c) 2024 Emma Tebibyte. License AGPLv3+: GNU AGPL version 3 or later
+<https://gnu.org/licenses/agpl.html>.
+
+.SH SEE ALSO
+
+GNU ls(1), The International System of Units (SI) 9th Edition
diff --git a/src/hru.rs b/src/hru.rs
new file mode 100644
index 0000000..0e0b25d
--- /dev/null
+++ b/src/hru.rs
@@ -0,0 +1,107 @@
+/*
+ * Copyright (c) 2023–2024 Emma Tebibyte <emma@tebibyte.media>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ *
+ * This program is free software: you can redistribute it and/or modify it under
+ * the terms of the GNU Affero General Public License as published by the Free
+ * Software Foundation, either version 3 of the License, or (at your option) any
+ * later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see https://www.gnu.org/licenses/.
+ */
+
+use std::{
+	cmp::Ordering,
+	env::args,
+	io::{ stdin, stdout, Write },
+	process::{ ExitCode, exit },
+};
+
+extern crate sysexits;
+
+use sysexits::{ EX_DATAERR, EX_IOERR, EX_SOFTWARE };
+
+const LIST: [(u32, &str); 10] = [
+	(3, "k"),
+	(6, "M"),
+	(9, "G"),
+	(12, "T"),
+	(15, "P"),
+	(18, "E"),
+	(21, "Z"),
+	(24, "Y"),
+	(27, "R"),
+	(30, "Q")
+];
+
+fn convert(input: u128) -> Result<(f64, (u32, &'static str)), String> {
+
+	let mut out = (input as f64, (0_u32, ""));
+	if input < 1000 { return Ok(out); }
+
+	for (n, p) in LIST {
+		let c = match 10_u128.checked_pow(n) {
+			Some(c) => c,
+			None => {
+				return Err(format!("10^{}: Integer overflow.", n.to_string()));
+			},
+		};
+
+		match c.cmp(&input) {
+			Ordering::Less => {
+				out = (input as f64 / c as f64, (n, p));
+			},
+			Ordering::Equal => {
+				return Ok((input as f64 / c as f64, (n, p)));
+			},
+			Ordering::Greater => {},
+		};
+	}
+
+	Ok(out)
+}
+
+fn main() -> ExitCode {
+	let argv = args().collect::<Vec<String>>();
+	let mut buf = String::new();
+	while let Ok(_) = stdin().read_line(&mut buf) {
+		if buf.is_empty() { return ExitCode::SUCCESS; }
+
+		let n: u128 = match buf.trim().parse() {
+			Ok(f) => {
+				buf.clear();
+				f
+			},
+			Err(err) => {
+				eprintln!("{}: {}.", argv[0], err);
+				return ExitCode::from(EX_DATAERR as u8);
+			},
+		};
+
+		let (number, prefix) = match convert(n) {
+			Ok(x) => x,
+			Err(err) => {
+				eprintln!("{}: {}.", argv[0], err);
+				return ExitCode::from(EX_SOFTWARE as u8);
+			},
+		};
+
+		let si_prefix = format!("{}B", prefix.1);
+
+		let out = ((number * 10.0).round() / 10.0).to_string();
+
+		stdout().write_all(format!("{} {}\n", out, si_prefix).as_bytes())
+			.unwrap_or_else(|e| {
+				eprintln!("{}: {}.", argv[0], e);
+				exit(EX_IOERR);
+			});
+	}
+
+	ExitCode::SUCCESS
+}