diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9eb4084..072640e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -41,6 +41,7 @@ All notable changes to eww will be listed here, starting at changes since versio
 - Add `:lines` and `:wrap-mode` properties to label widget (By: vaporii)
 - Add `value-pos` to scale widget (By: ipsvn)
 - Add `floor` and `ceil` function calls to simplexpr (By: wsbankenstein)
+- Add `formatbytes` function calls to simplexpr (By: topongo)
 
 ## [0.6.0] (21.04.2024)
 
diff --git a/Cargo.lock b/Cargo.lock
index 158aff4..f479f98 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -400,6 +400,12 @@ version = "1.7.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "428d9aa8fbc0670b7b8d6030a7fadd0f86151cae55e4dbbece15f3780a3dfaf3"
 
+[[package]]
+name = "bytesize"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a3c8f83209414aacf0eeae3cf730b18d6981697fba62f200fcfb92b9f082acba"
+
 [[package]]
 name = "cached"
 version = "0.53.1"
@@ -2683,6 +2689,7 @@ dependencies = [
 name = "simplexpr"
 version = "0.1.0"
 dependencies = [
+ "bytesize",
  "cached",
  "chrono",
  "chrono-tz",
diff --git a/Cargo.toml b/Cargo.toml
index 398a4c7..08f50fc 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -11,6 +11,7 @@ notifier_host = { version = "0.1.0", path = "crates/notifier_host" }
 
 anyhow = "1.0.86"
 bincode = "1.3.3"
+bytesize = "2.0.1"
 cached = "0.53.1"
 chrono = "0.4.38"
 chrono-tz = "0.10.0"
diff --git a/crates/simplexpr/Cargo.toml b/crates/simplexpr/Cargo.toml
index 2b94ae9..dc5d7fd 100644
--- a/crates/simplexpr/Cargo.toml
+++ b/crates/simplexpr/Cargo.toml
@@ -14,6 +14,7 @@ build = "build.rs"
 [dependencies]
 eww_shared_util.workspace = true
 
+bytesize.workspace = true
 cached.workspace = true
 chrono-tz.workspace = true
 chrono = { workspace = true, features = ["unstable-locales"] }
diff --git a/crates/simplexpr/src/eval.rs b/crates/simplexpr/src/eval.rs
index 556c72a..4c137a0 100644
--- a/crates/simplexpr/src/eval.rs
+++ b/crates/simplexpr/src/eval.rs
@@ -1,3 +1,4 @@
+use bytesize::ByteSize;
 use cached::proc_macro::cached;
 use chrono::{Local, LocalResult, TimeZone};
 use itertools::Itertools;
@@ -61,6 +62,9 @@ pub enum EvalError {
     #[error("Error parsing date: {0}")]
     ChronoError(String),
 
+    #[error("Error parsing byte format mode: {0}")]
+    ByteFormatModeError(String),
+
     #[error("{1}")]
     Spanned(Span, Box<EvalError>),
 }
@@ -534,6 +538,34 @@ fn call_expr_function(name: &str, args: Vec<DynVal>) -> Result<DynVal, EvalError
             }
             _ => Err(EvalError::WrongArgCount(name.to_string())),
         },
+        "formatbytes" => {
+            let (bytes, short, mode) = match args.as_slice() {
+                [bytes] => (bytes.as_i64()?, false, "iec".to_owned()),
+                [bytes, short] => (bytes.as_i64()?, short.as_bool()?, "iec".to_owned()),
+                [bytes, short, mode] => (bytes.as_i64()?, short.as_bool()?, mode.as_string()?),
+                _ => return Err(EvalError::WrongArgCount(name.to_string())),
+            };
+            let neg = bytes < 0;
+            let disp = ByteSize(bytes.abs() as u64).display();
+            let disp = match mode.as_str() {
+                "iec" => {
+                    if short {
+                        disp.iec_short()
+                    } else {
+                        disp.iec()
+                    }
+                }
+                "si" => {
+                    if short {
+                        disp.si_short()
+                    } else {
+                        disp.si()
+                    }
+                }
+                _ => return Err(EvalError::ByteFormatModeError(mode)),
+            };
+            Ok(DynVal::from(if neg { format!("-{disp}") } else { disp.to_string() }))
+        }
 
         _ => Err(EvalError::UnknownFunction(name.to_string())),
     }
diff --git a/docs/src/expression_language.md b/docs/src/expression_language.md
index 480b2d5..867b641 100644
--- a/docs/src/expression_language.md
+++ b/docs/src/expression_language.md
@@ -70,3 +70,8 @@ Supported currently are the following features:
      Same as other `formattime`, but does not accept timezone. Instead, it uses system's local timezone.
      Check [chrono's documentation](https://docs.rs/chrono/latest/chrono/format/strftime/index.html) for more
      information about format string.
+  - `formatbytes(bytes, short, format_mode)`: Display bytes in a human-readable format.
+    Arguments:
+    - `bytes`: `i64` of bytes, supports negative sizes.
+    - `short`: set true for a compact version (default: false)
+    - `format_mode`: set to either to "iec" (eg. `1.0 GiB`) or "si" (eg. `1.2 GB`) (default: "iec")