A few years ago I queried my repayments as the ECB rate was tumbling and PTSB gave me a formula which calculates my monthly repayment. Maybe some posters might find it interesting, especially PTSB mortgage holders.
(R X P)/(F X (1-((R/F) +1)^-N))
R = Rate/100 eg 0.0318
F = payments in a year ie 12
P = balance outstanding eg 321,912.67
N = number of repayments left eg 331 months
(0.0318 x 321,912.67)/(12 x (1-((0.0318/12) +1)^-331))
= 1,461.85
less TRS of 170.52 = repayment 1,291.33
(R X P)/(F X (1-((R/F) +1)^-N))
R = Rate/100 eg 0.0318
F = payments in a year ie 12
P = balance outstanding eg 321,912.67
N = number of repayments left eg 331 months
(0.0318 x 321,912.67)/(12 x (1-((0.0318/12) +1)^-331))
= 1,461.85
less TRS of 170.52 = repayment 1,291.33