The meaning, but theoretically not the same. 但是usually use with 虽然（it means although). 可是usually use in speaking. 但是use in the middle of the sentence 可是can use in the first or middle of the sentence . Eg： I’m ready to place an order with you, but only one condition is that the goods are confined to Finland. 我准备向你们订货,但是唯一的条件是,货物只限卖给芬兰的公司｡ Bill was going to ask Lisa out on date, but he chickened out at the last minute. 比尔本来要约丽莎出去,但是最后却不敢开口。 I gave it a shot , but I can't find any way to help the dude.我已经尽力了 但是我实在找不出办法 帮那个伙计 This time round though, there was an instant, troubling discord between the impressions I with me and what I found. 可是这一次,我眼前所见竟然大异于我脑海中的印象,这颇为恼人 t, when I look at my desk now, I feel no desire to deface it as it means nothing to me at all. 前这张桌子时,却毫无在上面涂鸦的兴致,因为它对我来说毫无意义。

但是、可是都是转折连词，只出现在后半句，上半句都可跟：虽然。 但是比可是意思强烈; 他想睡一会， 但是 （=可是） 睡不着。 他虽然七十多了，但是 （= 可是）精力仍然很健旺。 句中的“但是”可以用 “可是”替换。 可是也表示转折，语气没有 ”但是"强烈。前半句表示已定的事实或存在的理由，后半句转折表达立场或思想，不能用“但是”替换。 大家虽然很累，可是都很愉快。（不能用 “但是”替换） 她是个贤惠的媳妇，可是百里挑一 ）（不可用 “但是” 替换）

“但是” and “可是” almost have the same meaning,but still have very little difference. “但是”use in article more,but can still use in speaking,turn weak tone Example：我本来想帮你，但是你不领情。（I was trying to help you, but you do not appreciate.） “可是”use in speaking more,but can still use in article,the very strong tone than “但是” Example：我本来想帮你，可是你居然不领情，气死我了！！（I was trying to help you, but you do not even appreciate, shiver my soul!!）

感觉在口语都差不多，但又注意细节的时候，人们在后面的语境是反面或者贬义的时候用可是还多些，但是的转折感觉更中性点。