diff --git a/src/uu/sort/src/numeric_str_cmp.rs b/src/uu/sort/src/numeric_str_cmp.rs
index 681ca2f789d..fe9a16d6479 100644
--- a/src/uu/sort/src/numeric_str_cmp.rs
+++ b/src/uu/sort/src/numeric_str_cmp.rs
@@ -31,6 +31,8 @@ pub struct NumInfo {
 #[derive(Debug, PartialEq, Eq, Clone)]
 pub struct NumInfoParseSettings {
     pub accept_si_units: bool,
+    /// Also accept lowercase SI units, as with `-f`.
+    pub fold_si_units: bool,
     pub thousands_separator: Option<u8>,
     pub decimal_pt: Option<u8>,
 }
@@ -39,6 +41,7 @@ impl Default for NumInfoParseSettings {
     fn default() -> Self {
         Self {
             accept_si_units: false,
+            fold_si_units: false,
             thousands_separator: None,
             decimal_pt: Some(b'.'),
         }
@@ -81,8 +84,13 @@ impl NumInfo {
 
             if Self::is_invalid_char(char, &mut had_decimal_pt, parse_settings) {
                 return if let Some(start) = start {
+                    let unit = if parse_settings.fold_si_units {
+                        char.to_ascii_uppercase()
+                    } else {
+                        char
+                    };
                     let has_si_unit =
-                        parse_settings.accept_si_units && b"kKMGTPEZYRQ".contains(&char);
+                        parse_settings.accept_si_units && b"kKMGTPEZYRQ".contains(&unit);
                     (
                         Self { exponent, sign },
                         start..if has_si_unit { idx + 1 } else { idx },
@@ -165,14 +173,21 @@ impl NumInfo {
     }
 }
 
+/// A lowercase unit other than `k` is only part of the range with `-f`, so it is
+/// read as the uppercase one.
 fn get_unit(unit: Option<u8>) -> u8 {
     match unit {
-        Some(b'k') => 1,
-        Some(u) => b"KMGTPEZYRQ"
-            .iter()
-            .position(|&c| c == u)
-            .map_or(0, |pos| (pos + 1) as u8),
-        None => 0,
+        Some(b'k' | b'K') => 1,
+        Some(b'M' | b'm') => 2,
+        Some(b'G' | b'g') => 3,
+        Some(b'T' | b't') => 4,
+        Some(b'P' | b'p') => 5,
+        Some(b'E' | b'e') => 6,
+        Some(b'Z' | b'z') => 7,
+        Some(b'Y' | b'y') => 8,
+        Some(b'R' | b'r') => 9,
+        Some(b'Q' | b'q') => 10,
+        _ => 0,
     }
 }
 
diff --git a/src/uu/sort/src/sort.rs b/src/uu/sort/src/sort.rs
index 94edeaa8ca2..992267333c4 100644
--- a/src/uu/sort/src/sort.rs
+++ b/src/uu/sort/src/sort.rs
@@ -336,9 +336,10 @@ impl Default for NumericLocaleSettings {
 }
 
 impl NumericLocaleSettings {
-    fn num_info_settings(self, accept_si_units: bool) -> NumInfoParseSettings {
+    fn num_info_settings(self, accept_si_units: bool, fold_si_units: bool) -> NumInfoParseSettings {
         NumInfoParseSettings {
             accept_si_units,
+            fold_si_units,
             thousands_separator: self.thousands_sep,
             decimal_pt: self.decimal_pt,
         }
@@ -788,9 +789,10 @@ impl<'a> Line<'a> {
             match selector.settings.mode {
                 SortMode::Numeric | SortMode::HumanNumeric => {
                     // find out which range is used for numeric comparisons
+                    // GNU marks a lowercase unit as not part of the key, even with -f.
                     let mut parse_settings = settings
                         .numeric_locale
-                        .num_info_settings(selector.settings.mode == SortMode::HumanNumeric);
+                        .num_info_settings(selector.settings.mode == SortMode::HumanNumeric, false);
                     // Debug annotations should ignore thousands separators to match GNU output.
                     parse_settings.thousands_separator = None;
                     let (_, num_range) =
@@ -1369,7 +1371,10 @@ impl FieldSelector {
             // Parse NumInfo for this number.
             let (info, num_range) = NumInfo::parse(
                 range_str,
-                &numeric_locale.num_info_settings(self.settings.mode == SortMode::HumanNumeric),
+                &numeric_locale.num_info_settings(
+                    self.settings.mode == SortMode::HumanNumeric,
+                    self.settings.ignore_case,
+                ),
             );
             // Shorten the range to what we need to pass to numeric_str_cmp later.
             range_str = &range_str[num_range];
diff --git a/tests/by-util/test_sort.rs b/tests/by-util/test_sort.rs
index 23dd109dd12..daead3a1baa 100644
--- a/tests/by-util/test_sort.rs
+++ b/tests/by-util/test_sort.rs
@@ -592,6 +592,44 @@ fn test_human_block_sizes2() {
     }
 }
 
+#[test]
+fn test_human_numeric_ignore_case() {
+    // With -f, lowercase units other than k are read as the uppercase ones
+    for args in [&["-h", "-f"][..], &["-hf"], &["-k1,1hf"]] {
+        new_ucmd!()
+            .args(args)
+            .pipe_in("2m\n1e\n3g\n1K\n-1m\n")
+            .succeeds()
+            .stdout_only("-1m\n1K\n2m\n3g\n1e\n");
+    }
+    // Without -f (a key with its own options does not inherit the global
+    // one) they are not units
+    for args in [&["-h"][..], &["-f", "-k1h"]] {
+        new_ucmd!()
+            .args(args)
+            .pipe_in("2m\n1e\n3g\n1K\n-1m\n")
+            .succeeds()
+            .stdout_only("-1m\n1e\n2m\n3g\n1K\n");
+    }
+}
+
+#[test]
+fn test_human_numeric_ignore_case_debug() {
+    // GNU sorts by the folded unit but does not mark a lowercase unit as part of the key
+    new_ucmd!()
+        .env("LC_ALL", "C")
+        .args(&["--debug", "-h", "-f"])
+        .pipe_in("2m\n1e\n3g\n1K\n-1m\n")
+        .succeeds()
+        .stdout_is(concat!(
+            "-1m\n__\n___\n",
+            "1K\n__\n__\n",
+            "2m\n_\n__\n",
+            "3g\n_\n__\n",
+            "1e\n_\n__\n",
+        ));
+}
+
 #[test]
 fn test_human_numeric_zero_stable() {
     let input = "0M\n0K\n-0K\n-P\n-0M\n";