Please use this chart as a guide to obtain the approximate size you require.
Remember that these sizes are for an 'average' sized child which many are not. A good way to get an idea of where your child sits in the scale is to look at their school class pictures. How many of their classmates are taller or shorter than them? If they tend to be smaller than the majority then you might consider buying the size below their current age. Vice versa if they're one of the taller kids in the class you may as well buy a size up. They're always growing so it's not a bad idea to buy larger anyway.
We hope this helps with your decision and look forward to seeing any snaps of our threads in action!