Background: Teaching vocabulary may be an effective way to address poor reading comprehension. However, it is unclear how many words or word families a school aged child with poor vocabulary would need to learn to meaningfully impact comprehension. This study provides estimates of vocabulary size for children with differing vocabulary achievement in middle elementary grades. Methods: English speaking children (n = 41) completed an adapted version of the Vocabulary Size Test (VST-a) , which estimates number of word families known. VST-a scores were significantly related to raw scores on the Peabody Picture Vocabulary Test-4 (PPVT-4) , F(1,40)=71.6, p < 0.0001, R[superscript 2] of 0.642. The regression equation defining the relationship between VST-a and PPVT-4 raw scores and normative data from the PPVT-4 were used to estimate vocabulary size for children (fourth-eighth grades) with typical, high and low scores on the PPVT-4. These were considered in terms of word families required to support comprehension of a range of texts (Nation, 2006). Results: Typical vocabulary size for fourth-eighth grades was estimated to be approximately 7,300-9,100 word families, increasing at approximately 460 word families annually. Within grades, there is a gap of approximately 1,300 word families between individuals with PPVT-4 scores at the mean and 1 SD below the mean. Conclusions: Direct teaching of vocabulary may make a meaningful contribution to typical rates of vocabulary growth. Long-term vocabulary instruction would be required for those with low vocabulary knowledge, if 'typical' vocabulary breadth is the target. Vocabulary gains required to reach a threshold of 8,000 word families may be smaller. Results could help to identify students most likely to have improved reading comprehension as a result of vocabulary instruction. Theoretically, these estimates help specify the nature of vocabulary as a comprehension 'pressure point'.