hi.....i will try my best to show you! sometimes they have the same meaning :"where",for example:where is my watch?(我的手表在哪里？or 我的手表在哪儿？） but sometimes 哪里 has another meaning: some kinds of words about courtesy ! for example: anna said----wow, your chinese is really good,Alex! Alex said---哪里哪里。

the north people often like to say"儿"，so they often say"哪儿" but the south people cant say "儿"，it is difficult for most of them ,so they often say "那里"。you can test your chinese friends,ask them what's mean of "where are you?",if he or she is from the north,he or she would often say"你在哪？"，otherwise he or she would often say"你在哪里？" manderin is from the north,so they speak very naturely,but the south people said a little stiff. if someone said,you are very good,you can say'哪里哪里'or '哪儿啊'，"哪里"is a little stiff,so 哪里哪里'is a little formal than '哪儿啊'.anyway,i dont mean the north people cant say "哪里"and the south people cant say"哪儿"，i just say they would often say like that.and "哪里"is little more formal and polite than '哪儿',for example ,where are you from,in chinese"你是哪里人？"is more formal and polite than "你是哪儿人啊？"so "哪里"is more often in the books than "哪儿"，and in spoken language, '哪儿' (one syllable)is little more easier than '哪里'（two syllable）.so the people (mainly the north )like using "儿" all the time.

hello Harry Generally the two words "哪儿" and "哪里" are interchangeable,but "哪儿" sounds more colloquial.Actually,in this word "哪里",it's not that change "里" to "儿"，but that add "儿" after "哪",you can add "儿" after some words,in Chinese it's called "儿化"(rhoticized),this kind of words are called "儿化词"，for example: 小孩-小孩儿，花篮-花篮儿，鸟-鸟儿，花-花儿，玩-玩儿， etc. So it's not right if you think we change "里" to "儿"，there're also some other words like this,"这里" "那里"，you can say "这儿" "那儿"，we add "儿" after "这" and "那"。 the grammar about "儿化" is a difficult point in Chinese,hope my answer could help you a little~! ^^

Both of them mean "where".The difference between them is :哪里 is the written language ，but 哪儿is the spoken language.

they are the same...没有difference...