What run-length encoding is
Run-length encoding is a simple form of data compression in which runs of the same value - sequences where a single value repeats - are stored as one instance of the value together with a count of how many times it occurs. Instead of recording the same value over and over, it records the value once and notes how long the run is, which can save space.
The idea of a run
A run is a stretch of data where the same value appears consecutively several times. Run-length encoding focuses on these runs. Rather than writing out each repeated value individually, it captures the run compactly by pairing the value with the number of repetitions. When data contains many such runs, this approach can represent the same information using considerably less space.
How it compresses data
The compression comes from replacing long repetitions with a short description. If a value repeats many times in a row, storing it once along with a count is far more compact than storing every occurrence. The original data can be reconstructed exactly by expanding each value-and-count pair back into the full run, making run-length encoding a lossless method that loses no information.
When it works well
Run-length encoding is most effective when the data contains many long runs of repeated values. In such cases, it can shrink the data significantly. However, if the data has few repetitions and values change frequently, the method provides little benefit and may not help at all. Its usefulness depends heavily on how much repetition the data actually contains.
Where it is used
Because of its simplicity, run-length encoding is used in situations where data naturally contains runs of repeated values, such as certain kinds of simple images and other repetitive data. It is often one of the more basic compression techniques, sometimes used on its own or as part of larger compression schemes. Its straightforward idea makes it easy to understand and apply.
Why it matters
Run-length encoding is a classic and easy-to-understand compression technique. Understanding what it is - replacing repeated values with a value and a count - clarifies one of the simplest ways data can be made smaller. For anyone learning about data compression, it is a valuable starting concept that illustrates the core idea clearly.
Related on Skillo
See also: What is data compression? Explained, What is lossless vs lossy compression?.
Sources
Published date reflects the original event date (2025-06-17). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.
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