diff --git a/src/uu/df/src/blocks.rs b/src/uu/df/src/blocks.rs
index e964a208a..bb6e06333 100644
--- a/src/uu/df/src/blocks.rs
+++ b/src/uu/df/src/blocks.rs
@@ -5,7 +5,7 @@
 //! Types for representing and displaying block sizes.
 use crate::OPT_BLOCKSIZE;
 use clap::ArgMatches;
-use std::fmt;
+use std::{env, fmt};
 
 use uucore::parse_size::{parse_size, ParseSizeError};
 
@@ -159,6 +159,7 @@ pub(crate) enum HumanReadable {
 /// size.
 ///
 /// The default variant is `Bytes(1024)`.
+#[derive(Debug, PartialEq)]
 pub(crate) enum BlockSize {
     /// A fixed number of bytes.
     ///
@@ -168,7 +169,11 @@ pub(crate) enum BlockSize {
 
 impl Default for BlockSize {
     fn default() -> Self {
-        Self::Bytes(1024)
+        if env::var("POSIXLY_CORRECT").is_ok() {
+            Self::Bytes(512)
+        } else {
+            Self::Bytes(1024)
+        }
     }
 }
 
@@ -195,6 +200,8 @@ impl fmt::Display for BlockSize {
 #[cfg(test)]
 mod tests {
 
+    use std::env;
+
     use crate::blocks::{to_magnitude_and_suffix, BlockSize};
 
     #[test]
@@ -252,4 +259,12 @@ mod tests {
         assert_eq!(format!("{}", BlockSize::Bytes(2 * 1024)), "2K");
         assert_eq!(format!("{}", BlockSize::Bytes(3 * 1024 * 1024)), "3M");
     }
+
+    #[test]
+    fn test_default_block_size() {
+        assert_eq!(BlockSize::Bytes(1024), BlockSize::default());
+        env::set_var("POSIXLY_CORRECT", "1");
+        assert_eq!(BlockSize::Bytes(512), BlockSize::default());
+        env::remove_var("POSIXLY_CORRECT");
+    }
 }
diff --git a/src/uu/df/src/df.rs b/src/uu/df/src/df.rs
index d9fb1be7b..192cbcadf 100644
--- a/src/uu/df/src/df.rs
+++ b/src/uu/df/src/df.rs
@@ -32,6 +32,13 @@ use crate::table::Table;
 static ABOUT: &str = "Show information about the file system on which each FILE resides,\n\
                       or all file systems by default.";
 const USAGE: &str = "{} [OPTION]... [FILE]...";
+const LONG_HELP: &str = "Display values are in units of the first available SIZE from --block-size,
+and the DF_BLOCK_SIZE, BLOCK_SIZE and BLOCKSIZE environment variables.
+Otherwise, units default to 1024 bytes (or 512 if POSIXLY_CORRECT is set).
+
+SIZE is an integer and optional unit (example: 10M is 10*1024*1024).
+Units are K, M, G, T, P, E, Z, Y (powers of 1024) or KB, MB,... (powers
+of 1000).";
 
 static OPT_HELP: &str = "help";
 static OPT_ALL: &str = "all";
@@ -427,6 +434,7 @@ pub fn uu_app<'a>() -> Command<'a> {
         .version(crate_version!())
         .about(ABOUT)
         .override_usage(format_usage(USAGE))
+        .after_help(LONG_HELP)
         .infer_long_args(true)
         .arg(
             Arg::new(OPT_HELP)
diff --git a/tests/by-util/test_df.rs b/tests/by-util/test_df.rs
index 511956ec4..81a9eef72 100644
--- a/tests/by-util/test_df.rs
+++ b/tests/by-util/test_df.rs
@@ -375,6 +375,26 @@ fn test_iuse_percentage() {
     }
 }
 
+#[test]
+fn test_default_block_size() {
+    let output = new_ucmd!()
+        .arg("--output=size")
+        .succeeds()
+        .stdout_move_str();
+    let header = output.lines().next().unwrap().to_string();
+
+    assert_eq!(header, "1K-blocks");
+
+    let output = new_ucmd!()
+        .arg("--output=size")
+        .env("POSIXLY_CORRECT", "1")
+        .succeeds()
+        .stdout_move_str();
+    let header = output.lines().next().unwrap().to_string();
+
+    assert_eq!(header, "512B-blocks");
+}
+
 #[test]
 fn test_block_size_1024() {
     fn get_header(block_size: u64) -> String {