diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7cdd75c..babdd4b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,3 +11,4 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Port `_export_format.py` (`export_format.rs`) in [#77](https://github.com/wizard-28/wealthy/pull/77) from [@wizard-28](https://github.com/wizard-28)
 - Port `color_triplet.py` (`color_triplet.rs`) in [#78](https://github.com/wizard-28/wealthy/pull/78) from [@wizard-28](https://github.com/wizard-28)
 - Port `region.py` (`region.rs`) in [#80](https://github.com/wizard-28/wealthy/pull/80) from [@wizard-28](https://github.com/wizard-28)
+- Port `filesize.py` (`filesize.rs`) in [#83](https://github.com/wizard-28/wealthy/pull/83) from [@wizard-28](https://github.com/wizard-28)
diff --git a/Cargo.toml b/Cargo.toml
index 5e34568..7837f5c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -12,6 +12,7 @@ categories = ["command-line-interface"]
 [dependencies]
 lazy_static = "1.4.0"
 regex = "1.5.5"
+separator = "0.4.1"
 
 [dev-dependencies]
 rstest = "0.13.0"
diff --git a/src/filesize.rs b/src/filesize.rs
new file mode 100644
index 0000000..30c1d32
--- /dev/null
+++ b/src/filesize.rs
@@ -0,0 +1,123 @@
+#![allow(dead_code)]
+
+use separator::Separatable;
+
+/// Convert a filesize in to a string (powers of 1000, SI prefixes).
+///
+/// In this convention, `1000 B = 1 kB`.
+///
+/// This is typically the format used to advertise the storage
+/// capacity of USB flash drives and the like (*256 MB* meaning
+/// actually a storage capacity of more than *256 000 000 B*),
+/// or used by **Mac OS X** since v10.6 to report file sizes.
+///
+/// # Arguments
+///
+/// * `size` - A file size.
+/// * `precision` -  The number of decimal places to include (default = 1).
+/// * `separator` - The string to separate the value from the units (default = "
+///   ").
+///
+/// # Returns
+///
+/// A string containing a abbreviated file size and units.
+#[allow(clippy::cast_precision_loss)]
+#[allow(clippy::cast_possible_truncation)]
+#[allow(clippy::cast_possible_wrap)]
+pub(crate) fn decimal(size: f32, precision: Option<usize>, separator: Option<&str>) -> String {
+    let base = 1000.0;
+
+    if (size - 1.0).abs() < f32::EPSILON {
+        return String::from("1 byte");
+    } else if size < base {
+        return format!("{} bytes", size.separated_string());
+    }
+
+    let suffixes = ["kB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"];
+    let mut unit = 0.0;
+    let mut evaluated_suffix = " ";
+    for (i, suffix) in suffixes.into_iter().enumerate() {
+        unit = base.powi(i as i32 + 2);
+        evaluated_suffix = suffix;
+        if size < unit {
+            break;
+        }
+    }
+    let precision = precision.unwrap_or(1);
+    let separator = separator.unwrap_or(" ");
+
+    let magnitude = format!("{:.precision$}", base as f32 * size as f32 / unit as f32)
+        .parse::<f32>()
+        .unwrap();
+
+    let mut magnitude_separated_string = magnitude.separated_string();
+    // HACK: Workaround for `separator` crate's float truncating behavior.
+    if magnitude.fract() == 0.0 && precision != 0 {
+        magnitude_separated_string.push_str(".0");
+    }
+
+    format!(
+        "{}{separator}{evaluated_suffix}",
+        magnitude_separated_string
+    )
+}
+
+/// Pick a unit and suffix for the given size.
+#[allow(clippy::cast_possible_truncation)]
+#[allow(clippy::cast_possible_wrap)]
+fn pick_unit_and_suffix(size: u32, suffixes: &[&str], base: u32) -> (u32, String) {
+    let mut unit = 0_u32;
+    let mut evaluated_suffix = "";
+    for (i, suffix) in suffixes.iter().enumerate() {
+        unit = base.pow(i as u32);
+        evaluated_suffix = suffix;
+
+        if size < unit * base {
+            break;
+        }
+    }
+
+    (unit, String::from(evaluated_suffix))
+}
+
+#[cfg(test)]
+mod tests {
+    use rstest::rstest;
+
+    use super::*;
+
+    #[rstest]
+    #[case(0.0, None, None, "0 bytes")]
+    #[case(1.0, None, None, "1 byte")]
+    #[case(2.0, None, None, "2 bytes")]
+    #[case(1000.0, None, None, "1.0 kB")]
+    #[allow(clippy::cast_possible_truncation)]
+    #[allow(clippy::cast_sign_loss)]
+    #[allow(clippy::cast_precision_loss)]
+    #[case(1.5 * (1000 * 1000) as f32, None, None, "1.5 MB")]
+    #[case(0.0, Some(2), None, "0 bytes")]
+    #[case(1111.0, Some(0), None, "1 kB")]
+    #[case(1111.0, Some(1), None, "1.1 kB")]
+    #[case(1111.0, Some(2), None, "1.11 kB")]
+    #[case(1111.0, None, Some(""), "1.1kB")]
+    fn test_decimal(
+        #[case] size: f32,
+        #[case] precision: Option<usize>,
+        #[case] separator: Option<&str>,
+        #[case] result: &str,
+    ) {
+        assert_eq!(decimal(size, precision, separator), result);
+    }
+
+    #[rstest]
+    #[case(50, 1024, &(1, "bytes".to_owned()))]
+    #[case(2048, 1024, &(1024, "KB".to_owned()))]
+    fn test_pick_unit_and_suffix(
+        #[case] size: u32,
+        #[case] base: u32,
+        #[case] result: &(u32, String),
+    ) {
+        let suffixes = ["bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"];
+        assert_eq!(pick_unit_and_suffix(size, &suffixes, base), *result);
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 33c32cf..cd14300 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,6 +2,7 @@ mod cell_widths;
 mod emoji_codes;
 mod emoji_replace;
 mod export_format;
+mod filesize;
 mod region;
 
 pub mod color_triplet;