We know Earth’s diameter is approximately 7,900 miles (12,640 kilometers).
Half of that is Earth’s radius, the distance to the center.
The crust runs approximately 55 miles (88 kilometers) deep.
The mantle is about 1,800 miles (2,880 kilometers) deep; the outer core, approximately 1,400 miles (2,240 kilometers); and the inner core about 750 miles (1,200 kilometers).
Added together, that makes the distance to the center of Earth about 3,900 miles (6,240 kilometers).
That’s just short of our 7,900-mile (12,640-kilometer) diameter (twice the radius) but, remember, these are approximations.