For read ahead of digests in similarity based data deduplication in a data deduplication system using a processor device in a computing environment, input data is partitioned into data chunks and digest values are calculated for each of the data chunks. The positions and sizes of similar data intervals in a repository of data are found for each of the data chunks. The positions and the sizes of read ahead intervals are calculated based on the similar data intervals. The read ahead digests of the read ahead intervals are located and loaded into memory in a background read ahead process.