按月收費計算正確的實際利率

Question

我有一個用python編寫的簡單工作年金貸款計算器，與在線計算器相比，它提供了正確的結果。也就是說，每月的金額（什麼部分是利息，什麼是首付金額等）和實際利率（EIR）。它使用兩個numpy的函數，PPMT和IPMT

loanAmount  = 100000 
monthlyIntRate = 2.5/12 
effectiveIntRate = 100 * ((1 + monthlyIntRate/100.)**12 - 1) 

然而，當我加月租費的支付，我的EIR的變化，但確實不再等於通過在線貸款計算器給出的答案。

monthlyFee = -5 
monthlyIntToBePaid = np.ipmt(rate, per, nPer, loanAmount) 
monthDownPay = np.ppmt(rate, per, nPer, loanAmount) 
amountDue = monthlyInt + monthDownPay + monthlyFee 

其他一切，仍然完全一致。我認爲我的公式有點不錯，但我想知道更好的方法來做到這一點！

effectiveIntRate = 100 * ((1+ monthlyIntRate/100.)**12 - 1) 
effectiveIntRate += 100 * monthlyFee*12*2./loanAmount # <-- this line! 

Answer 1

試試這個（IRR使用查找速率費後）：

nPer=12 
rate=monthlyIntRate/100. 
Monthpay=np.pmt(rate, nPer, loanAmount, fv=0) 
amountDue = Monthpay + monthlyFee 

effectiveIntRate = 100 * ((1+ monthlyIntRate/100.)**12 - 1) 
#effectiveIntRate += 100 * monthlyFee*12*2./loanAmount # <-- this line! 

monthpays = [-amountDue] * nPer 

monthpaysf=[-loanAmount] + monthpays 


efratem=np.irr(monthpaysf) 

effectiveIntRateF = 100 * ((1 + efratem)**12 - 1) 

print(efratem*100,effectiveIntRateF) 

(0.21749271256861213, 2.6413600327578557)