Audio tape length and thickness

Since the widespread adoption of reel-to-reel audio tape recording in the 1950s, audio tapes and tape cassettes have been available in many formats. This article describes the length, tape thickness and playing times of some of the most common ones.

All tape thicknesses here refer to the total tape thickness unless otherwise specified, including the base, the oxide coating and any back coating. In the USA, tape thickness is often expressed as the thickness of the base alone. However, this varies from manufacturer to manufacturer and also between tape formulations from the same manufacturer. Outside of the USA, the overall thickness is more often quoted, and is the more relevant measurement when relating the thickness to the length that can be fit onto a reel or into a cassette.

The tape decks of the 1950s were mainly designed to use tape ¼" wide and to accept one of two reel formats:

In each case the shaft or hub had three splines. In machines designed to allow for vertical mounting, the upper part of the shaft or hub could commonly be rotated by 60° so the upper splines locked the reel in place. Some tape decks could accommodate either format by using removable hubs for the larger reel size. When in use these hubs were locked onto the cine spindles by the same mechanism used to secure the smaller reels.

Reel capacity is affected by both the reel diameter and the reel hub diameter. The standard ten and a half inch reel has approximately twice the capacity of the seven inch reel, which in turn has twice the capacity of the five inch. Some (not all) reels described as three inches are in fact three and a quarter inches in diameter, in order to have half the capacity of a five-inch reel.

The first commonly available increase in tape length resulted from a reduction in thickness to 35 µm, which allowed 3600', 1800' and 900' tapes to fit on ten-and-a-half, seven-, and five-inch reels respectively. These were known as long play tapes.

Confusingly, for a short time some equipment manufacturers also referred to 3¾-ips tape speed as long-play, but this usage did not persist.

A further reduction to 25 µm resulted in double-play tapes of 2400' on a seven-inch reel. This and thinner tapes were not commonly used on ten-and-a-half-inch reels, as the tape was too fragile for the angular momentum of the larger reels, particularly when rewinding.

Even thinner tapes fitting 3600' on a seven-inch reel and 1800' on a five-inch reel were known as triple-play tapes. Triple-play tape was too fragile for many tape decks to safely rewind even on a full seven-inch reel, and was more commonly used on five-inch- and smaller reels. However 3600' tapes on seven-inch reels were commercially available for those who wanted them.

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