A loose leaf (also loose leaf paper, filler paper or refill paper) is a piece of paper of any kind that is not bound in place, or available on a continuous roll, and may be punched and organized as ring-bound (in a ring binder) or disc-bound. Loose leaf paper may be sold as free sheets, or made up into notepads, where perforations or glue allow them to be removed easily. "Leaf" in many languages refers to a sheet or page of paper, as in Folio, as in feuille de papier (French), hoja de papel (Spanish), foglio di carta (Italian), and ルーズリーフ (Japanese, /ruːzuriːfu/).

"Loose leaf" describes any kind of paper or book available in unbound single sheets. Its "leaves", or sheets, are "loose" and not bound in notebook or book form. In North America, some textbooks are sold with pre-punched holes and perforated pages, so users can remove the pages and store them in a typical 3-ring binder. This helps in that the user can then carry only that portion of book they are currently using, and need not carry the whole book.

Main paper sizes are the letter-size system mainly used in North America and the ISO system used in the rest of the world. US companies such as Staples and Office Depot manufacture and sell letter-size loose leaf products in their retail stores. For ISO-sized loose leaf systems, since Japanese companies (e.g. Kokuyo, Maruman, MUJI, King Jim) are major designers and manufacturers of ISO-size loose leaf systems, whose products are sold internationally, corresponding Japanese terms will be included in parentheses throughout this article.

There are two main types of loose leaf systems:

A loose leaf system typically contains specially designed hole punches that can be used to punch holes on any common, standard-size paper so that they can be arranged into the loose leaf system. These hole punches are sold by the companies alongside their loose leaf binder or paper products, and usually there are third-party components for sale as well.

Loose leaf systems may be “horizontal” or “vertical”. A “vertical” orientation has the punched holes along the longer side of the paper. Most loose leaf systems are vertical. Exceptions include Maruman's Mini (“ミニ”, of B7 size) binder system.

There are four common types of loose leaves: (1) ruled paper (ja: 横罫. North American sizes include wide ruled, college ruled and narrow ruled, the line height of which are approximately 11⁄32, 9⁄32 and 1⁄4 inch (8.7, 7.1 and 6.4 mm), respectively, attending to different people's needs. In ISO loose leaf system, line height of 6mm, 7mm, 8mm and 10mm are common, typically simply referred to as “6mm罫”, “7mm罫” etc.), which may have “assistive (vertical) lines” (アシストライン), aiding in alignment across lines, (2) unruled (or "blank", 無地), (3) dotted (ja: ドット方眼) and (4) graph paper (or "grid paper", 方眼罫. In North American systems, the size of a grid is typically 1⁄4 inch (6.4 mm); in ISO system, it's typically 5 mm). College ruled paper has less space between the blue lines, allowing for more rows of writing. Wide ruled paper is intended for use by grade school children and those with larger handwriting.

Companies also sell pre-printed calendar loose leaves.