The volume of a Moderna booster dose is .25mL. Click here to read the full Moderna Booster Guidance Fact Sheet! Booster Vaccination: One 0.25 mL dose. A booster dose may be administered intramuscularly at least 6 months after completing a primary series with the Moderna COVID-19 Vaccine to individuals: 65 years of age and older
https://vax.phila.gov/index.php/notices/moderna-covid-19-vaccine-booster-dose-guidance/