diff --git a/CHANGELOG.md b/CHANGELOG.md
index dca0c2d4..f3aea521 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,11 @@ New features
 * [`Engine::register_custom_syntax_with_state_raw`] is added. The custom syntax parser and implementation functions take on an additional parameter that holds a user-defined custom _state_ which should substantially simplify writing some custom parsers.
 * [`Engine::register_custom_syntax_raw`] is deprecated.
 
+### Initial capacity for packages
+
+* For packages that register a large number of functions, a syntax extension for `def_package!` allows the specification of the initial _capacity_ of the package to reduce reallocation costs.
+* A `capacity` method is added to the `Package` trait.
+
 
 Version 1.10.0
 ==============
diff --git a/src/packages/arithmetic.rs b/src/packages/arithmetic.rs
index 2347f451..d1181889 100644
--- a/src/packages/arithmetic.rs
+++ b/src/packages/arithmetic.rs
@@ -190,7 +190,7 @@ macro_rules! reg_functions {
 
 def_package! {
     /// Basic arithmetic package.
-    pub ArithmeticPackage(lib) {
+    pub ArithmeticPackage(lib @ 128) {
         lib.standard = true;
 
         combine_with_exported_module!(lib, "int", int_functions);
diff --git a/src/packages/array_basic.rs b/src/packages/array_basic.rs
index 21abbde9..d4cea6b6 100644
--- a/src/packages/array_basic.rs
+++ b/src/packages/array_basic.rs
@@ -13,7 +13,7 @@ use std::{any::TypeId, cmp::Ordering, mem};
 
 def_package! {
     /// Package of basic array utilities.
-    pub BasicArrayPackage(lib) {
+    pub BasicArrayPackage(lib @ 64) {
         lib.standard = true;
 
         combine_with_exported_module!(lib, "array", array_functions);
diff --git a/src/packages/bit_field.rs b/src/packages/bit_field.rs
index 5deb09ce..ea5127b5 100644
--- a/src/packages/bit_field.rs
+++ b/src/packages/bit_field.rs
@@ -9,7 +9,7 @@ use std::prelude::v1::*;
 
 def_package! {
     /// Package of basic bit-field utilities.
-    pub BitFieldPackage(lib) {
+    pub BitFieldPackage(lib @ 16) {
         lib.standard = true;
 
         combine_with_exported_module!(lib, "bit_field", bit_field_functions);
diff --git a/src/packages/blob_basic.rs b/src/packages/blob_basic.rs
index a3bbd532..8e6a1ba1 100644
--- a/src/packages/blob_basic.rs
+++ b/src/packages/blob_basic.rs
@@ -15,7 +15,7 @@ use crate::{FLOAT, FLOAT_BYTES};
 
 def_package! {
     /// Package of basic BLOB utilities.
-    pub BasicBlobPackage(lib) {
+    pub BasicBlobPackage(lib @ 64) {
         lib.standard = true;
 
         combine_with_exported_module!(lib, "blob", blob_functions);
diff --git a/src/packages/iter_basic.rs b/src/packages/iter_basic.rs
index 6ec293cc..97a1c016 100644
--- a/src/packages/iter_basic.rs
+++ b/src/packages/iter_basic.rs
@@ -326,7 +326,7 @@ macro_rules! reg_range {
 
 def_package! {
     /// Package of basic range iterators
-    pub BasicIteratorPackage(lib) {
+    pub BasicIteratorPackage(lib @ 64) {
         lib.standard = true;
 
         reg_range!(lib | "range" => INT);
diff --git a/src/packages/logic.rs b/src/packages/logic.rs
index af494f00..da08b0d5 100644
--- a/src/packages/logic.rs
+++ b/src/packages/logic.rs
@@ -37,7 +37,7 @@ macro_rules! reg_functions {
 
 def_package! {
     /// Package of basic logic operators.
-    pub LogicPackage(lib) {
+    pub LogicPackage(lib @ 128) {
         lib.standard = true;
 
         #[cfg(not(feature = "only_i32"))]
diff --git a/src/packages/math_basic.rs b/src/packages/math_basic.rs
index f7c3c40f..595e6464 100644
--- a/src/packages/math_basic.rs
+++ b/src/packages/math_basic.rs
@@ -53,7 +53,7 @@ macro_rules! reg_functions {
 
 def_package! {
     /// Basic mathematical package.
-    pub BasicMathPackage(lib) {
+    pub BasicMathPackage(lib @ 128) {
         lib.standard = true;
 
         // Integer functions
diff --git a/src/packages/mod.rs b/src/packages/mod.rs
index ab4df033..5aa2d646 100644
--- a/src/packages/mod.rs
+++ b/src/packages/mod.rs
@@ -53,6 +53,14 @@ pub trait Package {
     #[allow(unused_variables)]
     fn init_engine(engine: &mut Engine) {}
 
+    /// Number of functions expected by the package.
+    ///
+    /// This capacity only acts as a hint. It may not be precise.
+    #[allow(unused_variables)]
+    fn capacity() -> usize {
+        16
+    }
+
     /// Register the package with an [`Engine`].
     ///
     /// # Example
@@ -112,15 +120,17 @@ pub trait Package {
 ///
 /// def_package! {
 ///     /// My super-duper package.
-///     pub MyPackage(module) {
+///     pub MyPackage(module @ 10) {
 ///         // Load a native Rust function.
 ///         module.set_native_fn("my_add", add);
+///     } |> |engine| {
+///         engine.set_custom_operator("@", 160);
 ///     }
 /// }
 /// ```
 #[macro_export]
 macro_rules! def_package {
-    ($($(#[$outer:meta])* $mod:vis $package:ident($lib:ident)
+    ($($(#[$outer:meta])* $mod:vis $package:ident($lib:ident $(@ $capacity:expr)?)
                 $( : $($(#[$base_meta:meta])* $base_pkg:ty),+ )?
                 $block:block
                 $( |> | $engine:ident | $init_engine:block )?
@@ -136,7 +146,7 @@ macro_rules! def_package {
             #[inline]
             fn init($lib: &mut $crate::Module) {
                 $($(
-                    $(#[$base_meta])* { <$base_pkg>::init($lib); }
+                    $(#[$base_meta])* <$base_pkg>::init($lib);
                 )*)*
 
                 $block
@@ -144,7 +154,7 @@ macro_rules! def_package {
             #[inline]
             fn init_engine(_engine: &mut $crate::Engine) {
                 $($(
-                    $(#[$base_meta])* { <$base_pkg>::init_engine(_engine); }
+                    $(#[$base_meta])* <$base_pkg>::init_engine(_engine);
                 )*)*
 
                 $(
@@ -152,6 +162,15 @@ macro_rules! def_package {
                     $init_engine
                 )*
             }
+            #[inline(always)]
+            fn capacity() -> usize {
+                let mut _capacity = 16;
+                $(_capacity = $capacity;)?
+                $($(
+                    $(#[$base_meta])* { _capacity += <$base_pkg>::capacity(); }
+                )*)*
+                _capacity
+            }
         }
 
         impl Default for $package {
@@ -166,7 +185,7 @@ macro_rules! def_package {
             #[inline]
             #[must_use]
             pub fn new() -> Self {
-                let mut module = $crate::Module::new();
+                let mut module = $crate::Module::with_capacity(<Self as $crate::packages::Package>::capacity());
                 <Self as $crate::packages::Package>::init(&mut module);
                 module.build_index();
                 Self(module.into())
diff --git a/src/packages/pkg_core.rs b/src/packages/pkg_core.rs
index 547274cd..815c2a56 100644
--- a/src/packages/pkg_core.rs
+++ b/src/packages/pkg_core.rs
@@ -15,7 +15,7 @@ def_package! {
     /// * [`BasicIteratorPackage`][super::BasicIteratorPackage]
     /// * [`BasicFnPackage`][super::BasicFnPackage]
     /// * [`DebuggingPackage`][super::DebuggingPackage]
-    pub CorePackage(lib) :
+    pub CorePackage(lib @ 0) :
             LanguageCorePackage,
             ArithmeticPackage,
             BasicStringPackage,
diff --git a/src/packages/pkg_std.rs b/src/packages/pkg_std.rs
index e7d4f1ff..e7878bfd 100644
--- a/src/packages/pkg_std.rs
+++ b/src/packages/pkg_std.rs
@@ -18,7 +18,7 @@ def_package! {
     /// * [`BasicMapPackage`][super::BasicMapPackage]
     /// * [`BasicTimePackage`][super::BasicTimePackage]
     /// * [`MoreStringPackage`][super::MoreStringPackage]
-    pub StandardPackage(lib) :
+    pub StandardPackage(lib @ 0) :
             CorePackage,
             BitFieldPackage,
             LogicPackage,
diff --git a/src/packages/string_basic.rs b/src/packages/string_basic.rs
index d8c3845a..41a00893 100644
--- a/src/packages/string_basic.rs
+++ b/src/packages/string_basic.rs
@@ -16,7 +16,7 @@ pub const FUNC_TO_DEBUG: &str = "to_debug";
 
 def_package! {
     /// Package of basic string utilities (e.g. printing)
-    pub BasicStringPackage(lib) {
+    pub BasicStringPackage(lib @ 128) {
         lib.standard = true;
 
         combine_with_exported_module!(lib, "print_debug", print_debug_functions);
diff --git a/src/packages/string_more.rs b/src/packages/string_more.rs
index da53a127..b40dc214 100644
--- a/src/packages/string_more.rs
+++ b/src/packages/string_more.rs
@@ -11,7 +11,7 @@ use super::string_basic::{print_with_func, FUNC_TO_STRING};
 
 def_package! {
     /// Package of additional string utilities over [`BasicStringPackage`][super::BasicStringPackage]
-    pub MoreStringPackage(lib) {
+    pub MoreStringPackage(lib @ 64) {
         lib.standard = true;
 
         combine_with_exported_module!(lib, "string", string_functions);
diff --git a/tests/packages.rs b/tests/packages.rs
index 49e9f27a..d728b4f9 100644
--- a/tests/packages.rs
+++ b/tests/packages.rs
@@ -7,7 +7,7 @@ use rhai::{def_package, Engine, EvalAltResult, Module, Scope, INT};
 fn test_packages() -> Result<(), Box<EvalAltResult>> {
     def_package! {
         /// My custom package.
-        MyPackage(m) : SSS {
+        MyPackage(m @ 32) : SSS {
             m.set_native_fn("hello", |x: INT| Ok(x + 1));
             m.set_native_fn("@", |x: INT, y: INT| Ok(x * x + y * y));
         } |> |engine| {