The mechanisms underlying simple arithmetic difficulties in dyscalculia are unknown. Evidence suggests deficiencies may arise from basic processes supporting magnitude perception and arithmetic proficiency. We focused on ‘groupitizing’, the ability to decompose and recompose quantities into a whole, examining whether adults with dyscalculia could benefit from visuospatial grouping cues when asked to estimate the quantity of a given set of different number ranges. Results showed adults with dyscalculia tended to focus on individual items rather than grouping them, resulting in low estimation abilities in both small and large quantity ranges. However, the results highlight the importance of differentiating between large and small quantities in the groupitizing process: only where arithmetic was easy (small but not large quantities), were adults with dyscalculia able to demonstrate significant improvement when given grouping cues. Our findings suggest weaker groupitizing abilities play a significant role in magnitude perception and arithmetic proficiency levels in dyscalculia. People with dyscalculia have difficulties in basic arithmetic that impact their life in many ways, from simple tasks such as calculating change, to life-changing events such as entering higher education or making financial decisions. They use immature counting strategies to solve simple arithmetic questions despite otherwise intact intellectual abilities. The reasons for this are unclear. We argued there may be a link between simple arithmetic proficiency and quantity perception processes. Our study investigated the mechanisms underlying quantity perception that play a role in dyscalculia deficiencies. We focused on ‘groupitizing,’ a process that enables a person to estimate a given quantity more efficiently by recomposing subgroups rather than simply using a counting strategy. The findings suggested a disadvantage in basic groupitizing in people with dyscalculia. Moreover, counting procedures interfered with groupitizing processes and hindered arithmetic proficiency. •Adults with dyscalculia show poorer quantity estimation than typically developed peers, even with very small quantities (4-6 dots).•Given grouping cues, adults with dyscalculia can estimate small quantities, but this ability is restricted to small amounts.•Arranging quantities into small subgroups ("groupitizing") is crucial for achieving proficiency in arithmetic.