The aim with this paper is to discuss play as an arena for mathematical learning and children’s everyday mathematics in preschool. The study focuses on what the children are doing in the play activities and how they express their everyday mathematics. Play can be a mathematical activity (Bishop, 1992) and children’s mathematical formation in play shows their everyday mathematics (Ginsburg, 2006). This paper discusses play and everyday mathematics from a sociocultural perspective (Vygotskij, 1990) and in relation to curriculum (National Agency, 2010, 2012). The empirical data is drawn from a larger study in Sweden (Bäckman, 2015) and video observation is used as a method to explore 4 year old children’s play and everyday mathematics. The examples used in this paper were chosen because they are common play situations in the participating preschools. The ethical considerations follow the rules from The Swedish Research Council (2011). The parents have given permission for the video observations and research to be conducted. The results of the analysis of children’s mathematical formation show two main categories: Exploring mathematics through play and children comparing mathematical experiences. Exploring mathematics through play consists of five sub-categories: experiencing volume, exploring geometrical shapes, discerning weight, discerning quantity and acting for positioning. The category children comparing mathematical experiences consists of four sub-categories: experiencing and comparing size, creating and comparing patterns, comparing proportions and counting and pair production. Implications for practice is that the preschool teachers should have knowledge to design a learning environment that support children’s mathematical exploring and learning through play.