diff --git a/README.md b/README.md
index 1efaa90..0450138 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@ That means:
 ### Parsing a string
 ```ruby
 Filesize.from("1 GiB")
-# => #<Filesize:0x93c06c8 @bytes=1073741824, @type={:regexp=>/^([\d,.]+)?\s?(?:([kmgtpezy])i)?b$/i, :multiplier=>1024, :presuffix=>"i"}>
+# => #<Filesize:0x00007fd71f103328 @bytes=1073741824, @type={:regexp=>/^(-?)([\d,.]+)?[[:space:]]?(?:([kmgtpezy])i)?b?$/i, :multiplier=>1024, :prefixes=>["Ki", "Mi", "Gi", "Ti", "Pi", "Ei", "Zi", "Yi"], :presuffix=>"i"}>
 ```
 
 ### Converting filesizes
diff --git a/lib/filesize.rb b/lib/filesize.rb
index 745ab06..c72aa1f 100644
--- a/lib/filesize.rb
+++ b/lib/filesize.rb
@@ -13,14 +13,14 @@ class Filesize
 
   # Set of rules describing file sizes according to SI units.
   SI = {
-    :regexp => /^([\d,.]+)?[[:space:]]?([kmgtpezy]?)b?$/i,
+    :regexp => /^(-?)([\d,.]+)?[[:space:]]?([kmgtpezy]?)b?$/i,
     :multiplier => 1000,
     :prefixes => TYPE_PREFIXES[:SI],
     :presuffix => '' # deprecated
   }
   # Set of rules describing file sizes according to binary units.
   BINARY = {
-    :regexp => /^([\d,.]+)?[[:space:]]?(?:([kmgtpezy])i)?b?$/i,
+    :regexp => /^(-?)([\d,.]+)?[[:space:]]?(?:([kmgtpezy])i)?b?$/i,
     :multiplier => 1024,
     :prefixes => TYPE_PREFIXES[:BINARY],
     :presuffix => 'i' # deprecated
@@ -71,14 +71,14 @@ def to_s(unit = 'B')
   # @return [String]
   # @see #to_s
   def pretty
-    size = @bytes
+    size = @bytes.abs
     if size < @type[:multiplier]
-      unit = "B"
+      unit = 'B'
     else
       pos = (Math.log(size) / Math.log(@type[:multiplier])).floor
       pos = @type[:prefixes].size-1 if pos > @type[:prefixes].size - 1
 
-      unit = @type[:prefixes][pos-1] + "B"
+      unit = @type[:prefixes][pos-1] + 'B'
     end
 
     to_s(unit)
@@ -151,10 +151,11 @@ def parse(string)
         end
       }
 
-      prefix = $2 || ''
-      size   = ($1 || 0).to_f
+      sign   = $1 == '' ? 1 : -1
+      prefix = $3 || ''
+      size   = ($2 || 0).to_f * sign
 
-      return { :prefix => prefix, :size => size, :type => type}
+      return { :prefix => prefix, :size => size, :type => type }
     end
   end
 
diff --git a/spec/lib/filesize_spec.rb b/spec/lib/filesize_spec.rb
index 711bd8f..54178a6 100644
--- a/spec/lib/filesize_spec.rb
+++ b/spec/lib/filesize_spec.rb
@@ -42,6 +42,18 @@
       it 'parses megabytes without b suffix' do
         expect(Filesize.from('1 M').to).to eq 1000 * 1000
       end
+
+      it 'parses negative kilobytes' do
+        expect(Filesize.from('-1 kB').to).to eq -1000
+      end
+
+      it 'parses negative megabytes' do
+        expect(Filesize.from('-1 MB').to).to eq -1000 * 1000
+      end
+
+      it 'parses negative megabytes without b suffix' do
+        expect(Filesize.from('-1 M').to).to eq -1000 * 1000
+      end
     end
 
     context 'BINARY units' do
@@ -56,6 +68,18 @@
       it 'parses mebibytes without b suffix' do
         expect(Filesize.from('1 Mi').to).to eq 1024 * 1024
       end
+
+      it 'parses negative kilobytes' do
+        expect(Filesize.from('-1 KiB').to).to eq -1024
+      end
+
+      it 'parses negative megabytes' do
+        expect(Filesize.from('-1 MiB').to).to eq -1024 * 1024
+      end
+
+      it 'parses negative mebibytes without b suffix' do
+        expect(Filesize.from('-1 Mi').to).to eq -1024 * 1024
+      end
     end
   end
 
@@ -105,12 +129,24 @@
   end
 
   describe '#pretty' do
-    it 'returns the number of the most matching prefix with its unit (BINARY default)' do
-      expect(Filesize.new(1024).pretty).to eq '1.00 KiB'
+    context 'BINARY units (default)' do
+      it 'returns the number of the most matching prefix with its unit' do
+        expect(Filesize.new(1024).pretty).to eq '1.00 KiB'
+      end
+
+      it 'returns the negative number of the most matching prefix with its unit' do
+        expect(Filesize.new(-1024).pretty).to eq '-1.00 KiB'
+      end
     end
 
-    it 'returns the number of the most matching prefix with its unit (SI)' do
-      expect(Filesize.new(1000, Filesize::SI).pretty).to eq '1.00 kB'
+    context 'SI units' do
+      it 'returns the number of the most matching prefix with its unit' do
+        expect(Filesize.new(1000, Filesize::SI).pretty).to eq '1.00 kB'
+      end
+
+      it 'returns the negative number of the most matching prefix with its unit' do
+        expect(Filesize.new(-1000, Filesize::SI).pretty).to eq '-1.00 kB'
+      end
     end
   end