Often, when parents ask this question, they provide the child's age or height. Neither will actually help us determine your child's violin size. Instead, measure your child's arm length from the neck to the center of the palm in inches. Round to the nearest half inch. Then, use this chart to determine your child's violin size. It is better for a violin to be too small than for it to be too large. Some sizes overlap, and it is common for a child to skip sizes as they grow. Petite teens and adults sometimes choose to play a full size violin even though they may not have an arm length of at least 23 inches. However, adults with smaller hands are sometimes more comfortable with a 7/8 violin even if their arm is technically long enough for a full size.
If you are renting or purchasing a violin from a brick-and-mortar violin shop, you may want to use the chart above to estimate your child's size and call the shop in advance to check availability. However, a violin is more reliable than a tape measure when determining the proper size. So, if the violin shop makes a different recommendation after sizing your child, that should be given more weight than our chart.
Please do not buy a larger instrument hoping your child will soon grow into it. We begin in first position, which requires the hand to stretch the entire length of the neck. If your child can't reach first position comfortably, they won't be able to learn to play their instrument at all.