Purpose Estimating the bladder capacity is highly important in diagnosing voiding or bladder dysfunction in children. The convenient formula, the Koff formula (bladder capacity (in ounces) = age (in years) + 2), has been established as gold standard. Despite its relevance, it shows certain weaknesses, especially in young children. It is often associated with larger bladder volumes and higher daily fluid intake. The aim of this study was to develop a formula which puts bladder capacity in association with age and micturition frequency. Material and Methods We examined prospectively the bladder diary in 60 children with no voiding or bladder dysfunction, which were seen for other urological problems. We differentiated three age groups of children. Group 1: little children aged 5-7, (n = 20) Group 2: small school children aged 8-10 (n = 20) Group 3: big school children aged 11-15 (n = 20) After computerized evaluation, an equation got developed out of these micturition data. Results Relevant parameters for estimating age-related bladder capacity are micturition frequency and maximum voided volume. This equation got developed the ABC formula: Age Micturition frequency Micturition vol. (ml) (acc. ABC-formula) Daily amount of urine (ml) 5 6-7x/d 75 450-525 6 90 540-630 7 105 630-735 8 5-6x/d 120 600-720 9 135 675-810 10 150 750-900 11 4-5x/d 165 660-825 12 180 720-900 13 195 780-975 14 210 840-1050 15 225 900-1125 (Age (A) x 30)/2 The results are represented in table 1. Table 1: parameter marks for healthy children in relation to the ABC-formula Conclusions The formula presented shows its relevance based on the co-parameter age, micturition frequency and bladder capacity. It is thought to be a useful guide for diagnosing voiding and bladder dysfunctions. With the ABC-formula [(Age x 30)/2], bladder capacity in healthy children can be determined exactly.