Insulin syringes come in different sizes depending on the dose of insulin needed1:
0.3 mL syringes are for insulin doses under 30 units of insulin and are numbered at 1/2-unit or 1-unit intervals.
0.5 mL syringes are for 30 to 50 units of insulin and are numbered at 1-unit intervals.
1.0 mL are for doses more than 50 units of insulin and are numbered at 2 units per interval.