For quick reference:
1/4 inch = 6.35mm
1/3 inch = 8.46mm
1/2 inch = 12.7mm
1 inch = 25.4mm
Size of Freshwater Pearls
Freshwater pearls can grow up to about 11 or 12 mm in size. In common terms, while they can occasionally grow to almost half an inch in diameter, most freshwater pearls sold are between about a quarter of an inch and a third of an inch in size. Very tiny pearls are also seen.
Freshwater 6-7mm, 7-8mm, 8-9mm, 9-10mm, 10-11mm, 11-12mm
Size of Golden South Sea Pearls
Golden South Sea pearls are usually between 9mm and 20mm in size. The larger the size the harder to find very high quality pearls. The average size for South Sea pearls is about 13mm, which is just over half an inch in diameter.
Golden South Sea 9-10mm, 10-11mm, 11-12mm, 12-13mm
Size of Tahitian Pearls
Tahitian pearls are usually between 8mm and 16mm in size. That means they are larger than the classic small white pearls but don't grow quite as large as the huge South Sea pearls do. Think of pearls between one third of an inch and two thirds of an inch in diameter. They are distinguished by their natural dark color.
Tahitian 8-9mm, 9-10mm, 10-11mm, 11-12mm, 12-13mm, 13-14mm
Size of Akoya Pearls
Akoya pearls are small as they grow in a small size oyster. They can grow up to about 9mm or just over a third of an inch in size. The most popular sizes are around 7mm or 8mm. 6.0-6.5mm pearls (about a quarter of an inch) are often considered appropriate for very young or very slender ladies who like a dainty look. Slightly larger pearls tend to look better on a more mature woman who wants a more sophisticated look. If in doubt, most women would prefer the pearls a size larger rather than a size smaller. And yes, even a half a millimeter makes a difference.
Akoya 6.5-7.0mm, 7.0-7.5mm, 7.5-8.0mm, 8.0-8.5mm, 8.5-9.0mm
Size of White South Sea Pearls
White South Sea pearls grow in very large molluscs so can grow anywhere between 9mm (just over a third of an inch) and a whopping 20mm (over three quarters of an inch) in size. Prices go up steeply with size, as the largest ones are extremely rare.
White South Sea 10-11mm, 11-12mm, 12-13mm