BLEU Score Calculator
Calculate the BLEU score to evaluate machine translation or text generation quality.
Inputs
BLEU Score
0.4635
Brevity Penalty
0.8869
Geometric Mean Precision
0.5226
Step by step
Brevity Penalty
exp(1 − 28/25)
= 0.886920
Geometric mean of precisions
exp((ln0.85 + ln0.65 + ln0.45 + ln0.30)/4)
= 0.522597
BLEU = BP × geometric mean
0.8869 × 0.5226
= 0.463502
How it works
BLEU (Bilingual Evaluation Understudy) measures machine translation quality by computing the geometric mean of n-gram precisions (1 through 4), multiplied by a brevity penalty to prevent artificially short translations from scoring high. Scores range from 0 to 1, with higher being better.
Formula
BLEU-4
BLEU = BP * exp((1/4) * sum(ln(p_n)))
- p_n
- Modified n-gram precision for n=1..4
- BP
- Brevity penalty: min(1, exp(1 - ref_len/cand_len))
Frequently Asked Questions
What is a good BLEU score?
BLEU scores above 0.3 are generally considered decent for machine translation, above 0.5 is very good, and scores above 0.6 are near human-level for many language pairs. The absolute number depends heavily on language pair and domain.
Why use geometric mean instead of arithmetic mean?
Geometric mean is more sensitive to low individual precisions — if any n-gram precision is zero, the entire BLEU score drops to zero, enforcing that good translations must match at all n-gram levels.