BACKGROUND: Summer is a period of accelerated body mass index (BMI) gain for elementary school-aged children. Summer day camps may provide a structured environment, which has been shown to mitigate accelerated summer BMI gain. Many of these programs have a fee-for-service structure, creating a financial barrier for families with low-income. Providing vouchers to pay for these programs may be an effective strategy for addressing this barrier and mitigating accelerated summer BMI gain but requires further investigation on the optimal dose - the minimum exposure needed to see meaningful results while not overextending resources. METHODS: This study will use a multi-arm randomized controlled trial with three treatment levels. Children (n = 360) ages 5-12 years from participating schools (n = 4) will be randomly assigned to either summer as usual (comparison group) or to receive a voucher to attend an existing summer day camp for 4-, 6-, or 8-, weeks. BMI will be objectively measured at baseline (i.e., ∼May), 3-months (i.e., ∼August), and 12-months (i.e., ∼May of following school year). Obesogenic behaviors (e.g., physical activity, diet, screen time, and sleep) will be assessed in spring (i.e., late May) and summer (i.e., late June and July). Implementation factors, such as content, attendance frequency, duration, and coverage, relationship with children's summer BMI gain and obesogenic behaviors will also be evaluated. The study will also evaluate the cost-effectiveness of each duration. DISCUSSION: The study's findings will identify the optimal dose of summer programming to mitigate excess summer BMI gain, informing effective public health initiatives to combat childhood obesity. TRIAL REGISTRATION: NCT06158594 https://clinicaltrials.gov/study/NCT06158594?titles=determining%20optimal%20amount%20of%20structured%20environments&rank=1.