A shredder is a machine that tears objects into smaller pieces. The most common types are paper shredders and garden waste shredders. There are also shredders capable of shredding optical discs. See: data remanence for disposal of magnetic media such as magnetic tape and hard drives.

Paper shredders

Paper shredders are used to cut paper into very fine strips or tiny paper chips. This is usually done by individuals or organizations to destroy confidential documents in such a manner that they are impossible to read. Privacy experts often recommend that individuals shred bills, account statements, and other documents to keep those who would use the items to commit fraud or identity theft.

However, careless users of cheaper shredders can have their privacy violated despite this precaution. For instance, if a person simply shreds the documents and does nothing else beyond putting it in the garbage, all the shredded pieces can be collected by an investigator. Once collected, they can be reassembled in an attempt to discover the content of the documents. For maximum security, documents should be shredded so that the words of the document go through the shredder horizontally. Many of the documents in the Enron accountancy scandal were fed through the shredder the wrong way, making them easier to reassemble.

Most paper shredders come with a waste basket of their own. Those that don't are built to fit over another waste basket. They range in size and price from small and inexpensive units meant for home use, to larger units used in business settings that cost several hundred dollars.

These machines are classified according to the size and shape of the waste they produce.

• Strip-cut shredders use rotating knives to cut narrow strips as long as the original sheet of paper. These strips can be reassembled by a determined investigator, so this type of shredder is the least secure. It also creates the highest volume of waste.
• Cross-cut shredders use two contra-rotating drums to cut rectangular or parallelogram-shaped shreds.
• Particle-cut shredders create tiny square or circular pieces.
• Disintegrators and granulators repeatedly cut the paper at random until the particles are small enough to pass through a mesh.
• Hammermills and pulverizers reduce the paper to dust using a combination of rotating and stationary blades.

There are numerous standards for the security levels of paper shredders, including:

• DIN 32757
  • Level 1 = 12 mm strips
  • Level 2 = 6 mm strips
  • Level 3 = 2 mm strips (Confidential)
  • Level 4 = 2 × 15 mm particles (Commercially Sensitive)
  • Level 5 = 0.8 × 12 mm particles (Top Secret or Classified)
• United States Department of Defense (DoD)
  • Top Secret = 0.8 x 11.1 mm (1/32" × 7/16") (= Federal Specification F-S-001169 ?)
• United States NSA/CSS 02-01 = 1 × 4 mm

Garden waste shredders

Also called wood chippers or tree chippers. This type of shredder turns pieces of wood into smaller pieces, or chips. These smaller pieces can then become more rapidly bio-degradable as more surface area is exposed. Garden shredders can also be used to compact material such as dried leaves.

Shredders in films

• Fargo (1996) "Wood chipper. Unauthorized use" - The four-word film review