每隔 means "every". It can be used as "每隔" + quantity + unit.
For example:
- every + 2 + days = 每隔+两(2)+天
I drink a cup of coffee every two days. = 我每隔2天喝一杯咖啡。
This means you consume a cup of coffee every 48 hour interval.
- every 500 meters = 每隔 + 五百(500) + 米
I stop every 500 meters. = 我每隔500米停一次。
It means you stop at 500 meters, and stop again when you reach 1000 meters.
In conclusion, as my understanding, "每隔两天" means "every 2 days"(not 3).
However, your concern is legit. It is a vague way of saying things. Thus, if it is a more serious situation (e.g. contract), instead of a day to day conversation, you should clarify.
墨西哥城大约每隔十年降一次雪。About once in ten years snow ... "汉语水平考试词典"隔[义2]时间或空间上有离距;间隔(at a distance from;after or at an interval of):~日|~夜|你~几天再去|北京和天津相~一百多公里。 If a particular repetitive eve. lasts for about one week, then "at intervals of 1 week" may mean every other week. If an event lasts about a day each time then, 每隔两天 may mean every 3 days.
– user6065 Dec 02 '17 at 21:23e.g. iciba Leap year recurs every four years.每四年闰年一次. re answer: dictionaries are hardly ever wrong
– user6065 Dec 03 '17 at 01:24