Phred Quality Score Table
A Phred quality score converts a sequencing base-call error probability into a compact Q score: Q = -10 x log10(P_error). The table below shows the common FASTQ quality scores, their expected error rate, base-call accuracy and ASCII characters for Phred+33 and Phred+64 encodings.
| Q score | Error rate | P(error) | Accuracy | Phred+33 | Phred+64 |
|---|---|---|---|---|---|
| Q10 | 1 in 10 | 10% | 90% | + | J |
| Q20 | 1 in 100 | 1% | 99% | 5 | T |
| Q25 | 1 in 316 | 0.316% | 99.684% | : | Y |
| Q30 | 1 in 1,000 | 0.1% | 99.9% | ? | ^ |
| Q35 | 1 in 3,162 | 0.0316% | 99.968% | D | c |
| Q40 | 1 in 10,000 | 0.01% | 99.99% | I | h |
| Q50 | 1 in 100,000 | 0.001% | 99.999% | S | r |
| Q60 | 1 in 1,000,000 | 0.0001% | 99.9999% | ] | | |
Common quality bands
| Range | Label | Interpretation |
|---|---|---|
| Q10-Q19 | Low | Often trimmed or treated cautiously. |
| Q20-Q29 | Usable | Common minimum for rough variant or read filtering. |
| Q30-Q39 | High | A widely used benchmark for high-quality Illumina bases. |
| Q40+ | Very high | Very low expected base-call error rate. |
How FASTQ stores quality scores
A FASTQ record stores one quality character for every base. To decode a Phred+33 file, subtract 33 from each ASCII code; for Phred+64, subtract 64. Choosing the wrong encoding shifts every quality score by 31, which is why old Illumina FASTQ files need special care.
Frequently asked questions
- What does a Phred quality score mean?
- A Phred quality score Q is a logarithmic estimate of base-call error probability: Q = -10 x log10(P_error). Higher Q means a lower chance that the base call is wrong.
- What does Q30 mean?
- Q30 means the estimated error probability is 1 in 1,000, or 0.1%. Put another way, the expected base-call accuracy is 99.9%.
- What is the difference between Phred+33 and Phred+64?
- They are ASCII encodings for storing Q scores as characters in FASTQ files. Modern Sanger / Illumina 1.8+ FASTQ uses Phred+33. Older Illumina 1.3-1.7 data used Phred+64.
- How do I know which FASTQ quality encoding I have?
- Most current FASTQ files are Phred+33. If very low-quality characters such as !, " or # appear, the file must be Phred+33. Phred+64 is mainly relevant for old Illumina datasets.
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Related tools and references
Use these related pages when this table raises a practical calculation or workflow question.