Phred quality score converter
Convert between a Phred quality score (Q) and base-call accuracy / error probability, both directions. Q30 means 99.9% accuracy.
How it works
Formula
error probability P = 10^(−Q/10); accuracy = 1 − P; and inversely Q = −10 × log₁₀(P).
Worked example
Q30: P = 10^(−30/10) = 0.001, so accuracy = 1 − 0.001 = 99.9% — one wrong base in 1,000.
When to use it
To read the %≥Q30 metric from a run, set a quality threshold for trimming or filtering, or turn a measured error rate back into a Phred score.
Sensible defaults
The default is Q30 → 99.9% accuracy, the common per-base quality benchmark for short-read data.
FAQ
- What does Q30 mean?
- A 1 in 1,000 chance that a base call is wrong — 99.9% accuracy. Q20 is 1 in 100 (99%); Q40 is 1 in 10,000 (99.99%).
- Why must error probability be between 0 and 1?
- It is a probability. Exactly 0 (perfect) or 1 (always wrong) have no finite Phred value, so they are rejected.