diff --git a/src/bin/installer.rs b/src/bin/installer.rs
index 732e9bee14..4872c43163 100644
--- a/src/bin/installer.rs
+++ b/src/bin/installer.rs
@@ -157,7 +157,23 @@ fn main() {
     }
 
     if parser.found("filesystem-size") {
-        println!("{}", config.general.filesystem_size.unwrap_or(0));
+        // An explicit `filesystem_size` in the config is an override and is
+        // honored as-is; otherwise the size is COMPUTED from the actual package
+        // set (see redox_installer::compute_filesystem_size_mb) — never guessed.
+        let size = match config.general.filesystem_size {
+            Some(size) => u64::from(size),
+            None => {
+                let cookbook = parser.get_opt("cookbook").unwrap_or_else(|| ".".to_string());
+                match redox_installer::compute_filesystem_size_mb(&config, &cookbook) {
+                    Ok(mb) => mb,
+                    Err(err) => {
+                        eprintln!("installer: filesystem-size: {err}");
+                        process::exit(1);
+                    }
+                }
+            }
+        };
+        println!("{size}");
     } else if parser.found("list-packages") {
         // List the packages that should be fetched or built by the cookbook
         for (packagename, package) in &config.packages {
diff --git a/src/installer.rs b/src/installer.rs
index dec5d1a6ad..f513593b71 100644
--- a/src/installer.rs
+++ b/src/installer.rs
@@ -130,6 +130,104 @@ fn install_packages(config: &Config, dest: &Path, cookbook: Option<&str>) -> any
     Ok(())
 }
 
+/// Compute the RedoxFS image size (in MiB) needed to hold this config's package
+/// set, derived from the ACTUAL package contents — never a hardcoded guess. It
+/// walks the transitive package closure (config `[packages]` plus each pkgar's
+/// `depends`), does per-file RedoxFS block accounting (`ceil(size/BLOCK_SIZE)`
+/// data blocks + one node block per file/dir), adds the record-tree/header
+/// structural overhead, and a free-space margin so the resulting live/writable
+/// filesystem is functional (a 100%-full RedoxFS cannot be created or used).
+///
+/// Target-agnostic: identical logic sizes mini, full, bare, grub, or any config.
+/// Reads pkgars from `<cookbook>/repo/<target>/` using the repo's public key.
+pub fn compute_filesystem_size_mb(config: &Config, cookbook: &str) -> Result<u64> {
+    use pkgar::PackageFile;
+    use pkgar_core::PackageSrc;
+
+    let target = get_target();
+    let repo = Path::new(cookbook).join("repo").join(&target);
+    let pubkey_path = Path::new(cookbook).join("build").join("id_ed25519.pub.toml");
+    let pkey = pkgar_keys::PublicKeyFile::open(&pubkey_path)
+        .map_err(|e| anyhow::anyhow!("opening public key {}: {:?}", pubkey_path.display(), e))?
+        .pkey;
+
+    // Transitive closure: explicitly-requested packages + their pkgar `depends`.
+    let mut stack: Vec<String> = config
+        .packages
+        .iter()
+        .filter_map(|(name, pkg)| match pkg {
+            PackageConfig::Build(rule) if rule == "ignore" => None,
+            _ => Some(name.clone()),
+        })
+        .collect();
+    let mut seen: BTreeMap<String, ()> = BTreeMap::new();
+
+    let block = BLOCK_SIZE; // redoxfs block size (4096)
+    let mut data_blocks: u64 = 0;
+    let mut nodes: u64 = 0;
+    let mut missing: Vec<String> = Vec::new();
+
+    while let Some(name) = stack.pop() {
+        if seen.insert(name.clone(), ()).is_some() {
+            continue;
+        }
+        // Follow dependencies recorded in the published repo metadata.
+        let toml_path = repo.join(format!("{name}.toml"));
+        if let Ok(text) = fs::read_to_string(&toml_path) {
+            if let Ok(value) = text.parse::<toml::Value>() {
+                if let Some(deps) = value.get("depends").and_then(|d| d.as_array()) {
+                    for dep in deps.iter().filter_map(|d| d.as_str()) {
+                        stack.push(dep.to_string());
+                    }
+                }
+            }
+        }
+        // Per-file block accounting from the pkgar itself.
+        let pkgar_path = repo.join(format!("{name}.pkgar"));
+        if !pkgar_path.is_file() {
+            missing.push(name.clone());
+            continue;
+        }
+        let mut package = PackageFile::new(&pkgar_path, &pkey)
+            .map_err(|e| anyhow::anyhow!("opening {}: {:?}", pkgar_path.display(), e))?;
+        let entries = package
+            .read_entries()
+            .map_err(|e| anyhow::anyhow!("reading entries of {}: {:?}", pkgar_path.display(), e))?;
+        for entry in &entries {
+            data_blocks += entry.size().div_ceil(block);
+            nodes += 1; // one RedoxFS node per file/dir
+        }
+    }
+
+    // Config `[[files]]` payloads (init.d services, filesystem.toml, os-release …).
+    for file in &config.files {
+        data_blocks += (file.data.len() as u64).div_ceil(block);
+        nodes += 1;
+    }
+
+    if !missing.is_empty() {
+        eprintln!(
+            "installer: filesystem-size: warning: {} package(s) missing a pkgar in {} \
+             (size may be under-counted): {}",
+            missing.len(),
+            repo.display(),
+            missing.join(", "),
+        );
+    }
+
+    // RedoxFS structural overhead: one node block per entry is already counted in
+    // `nodes`; the record tree holding those nodes needs ~1 block per 64 entries,
+    // plus a fixed header/root/allocator allowance (~1 MiB).
+    let tree_blocks = nodes / 64 + 256;
+    let content_bytes = (data_blocks + nodes + tree_blocks).saturating_mul(block);
+
+    // Free-space margin so the live/writable image is usable, not 100% full.
+    let margin = std::cmp::max(content_bytes / 4, 64 * 1024 * 1024); // +25%, min +64 MiB
+    let total_bytes = content_bytes.saturating_add(margin);
+
+    Ok((total_bytes.div_ceil(1024 * 1024)).max(64))
+}
+
 pub fn install_dir(
     config: Config,
     output_dir: impl AsRef<Path>,