10 across the inside top opening (shown in pictures). 12 1/2 across the top outside diameter (with side tabs). 4 inches deep (base only). Lid adds another 1 ½ inches (with handle height). See pictures since I try to capture the condition and sizes (with a measuring tape) of the iron.
This great historic piece has a smooth cooking surface and like great cast iron it just keeps getting better with age and loving care. And of course, please reach out if you have any questions.