diff --git a/spec/std/array_spec.cr b/spec/std/array_spec.cr
index 0f44cfb6774e..9f052415c4d5 100644
--- a/spec/std/array_spec.cr
+++ b/spec/std/array_spec.cr
@@ -2204,6 +2204,66 @@ describe "Array" do
     end
   end
 
+  describe "#ensure_capacity" do
+    it "does nothing if it fits" do
+      ary = [1, 2, 3]
+      capacity1 = ary.remaining_capacity
+      ary.ensure_capacity(3)
+      ary.should eq([1, 2, 3])
+      ary.remaining_capacity.should eq(capacity1)
+    end
+
+    it "grows the array if it doesn't fit" do
+      ary = [1, 2, 3]
+      ary.ensure_capacity(4).should eq([1, 2, 3])
+      ary.remaining_capacity.should eq(4)
+    end
+
+    it "doesn't rewinds the array" do
+      ary = [1, 2, 3]
+      ary.shift
+      ary.ensure_capacity(3).should eq([2, 3])
+      ary.remaining_capacity.should eq(2)
+    end
+
+    it "does nothing if there's not enough capacity" do
+      ary = [1, 2, 3]
+      ary.ensure_capacity(2).should eq([1, 2, 3])
+      ary.remaining_capacity.should eq(3)
+    end
+  end
+
+  describe "trim_to_size" do
+    it "trims" do
+      a = Array(Int32).new(10)
+      a << 1 << 2 << 3
+      a.trim_to_size.should eq([1, 2, 3])
+      a.remaining_capacity.should eq(3)
+    end
+
+    it "trims with extra capacity" do
+      a = Array(Int32).new(10)
+      a << 1 << 2 << 3
+      a.trim_to_size(extra: 2).should eq([1, 2, 3])
+      a.remaining_capacity.should eq(5)
+    end
+
+    it "rewinds the array" do
+      a = Array(Int32).new(10)
+      a << 1 << 2 << 3
+      a.shift
+      a.trim_to_size.should eq([2, 3])
+      a.remaining_capacity.should eq(2)
+    end
+
+    it "raises on negative extra capacity" do
+      expect_raises(ArgumentError, "Negative extra capacity: -1") do
+        a = [1, 2, 3]
+        a.trim_to_size(extra: -1)
+      end
+    end
+  end
+
   describe "capacity re-sizing" do
     it "initializes an array capacity to INITIAL_CAPACITY" do
       a = [] of Int32
diff --git a/src/array.cr b/src/array.cr
index 30425c6869e3..247a6ded7a4c 100644
--- a/src/array.cr
+++ b/src/array.cr
@@ -2115,6 +2115,27 @@ class Array(T)
     @capacity - @offset_to_buffer
   end
 
+  private def rewind
+    root_buffer.copy_from(@buffer, @size)
+    shift_buffer_by -@offset_to_buffer
+  end
+
+  # Ensures that the internal buffer has at least `capacity` elements.
+  def ensure_capacity(capacity : Int32) : self
+    resize_to_capacity capacity if capacity >= @size
+    self
+  end
+
+  # Reduces the internal buffer to exactly fit the number of elements in the
+  # array, plus `extra` elements.
+  def trim_to_size(*, extra : Int32 = 0) : self
+    raise ArgumentError.new("Negative extra capacity: #{extra}") if extra < 0
+
+    rewind
+    resize_to_capacity(@size + extra)
+    self
+  end
+
   # behaves like `calculate_new_capacity(@capacity + 1)`
   private def calculate_new_capacity
     return INITIAL_CAPACITY if @capacity == 0