那如果卖房子需要还回cpf的钱的利息也是按月息计？而不是说简单的算做你需要还回的本金*2.5%？
是的。
因为CPF 就是这样给你利息的（按月计复利）。
这一点请参考 http://ask-us.cpf.gov.sg/efa/answer/index.htm?faqName=EFA_001377
第一句话就是：
CPF interest is computed monthly, then compounded and credited annually to your respective accounts.
这一点请参考 http://ask-us.cpf.gov.sg/efa/answer/index.htm?faqName=EFA_001377
第一句话就是：
CPF interest is computed monthly, then compounded and credited annually to your respective accounts.
Huasing Association 1999 - 2013