Normally interest is calculated daily. The key point is that as you pay more earlier, you cut down the principal earlier too, which the next set of interest calculated would be based on the new principal. Where as if you pay lump sum say in 6 months time, the first 5 months' principal is higher in comparison thus higher interest.
This is where monthly repayment (or bi weekly, etc) trumps lump sum with all things being equal (i.e. total amount you pay for a set period of time). The exception is if you front load the overpayment (i.e. overpay the 6K in Month 1), which is not really realistic in real life.
For illustration, lets say you got a mortgage of 100K at 5% interest rate for 10 years (For ease of nicely rounded number), your monthly repayment is 1.06K.
Scenario 1: If you pay 2K every month (i.e. overpay by 1K). Your first 6 months will be calculated as follows:
Month | Interest | Principal | Total Payment | LTV | Closing Balance |
Jan | 416.67 | 1,643.99 | 2,060.66 | 98.4 | 98,356.01 |
Feb | 409.82 | 1,650.84 | 2,060.66 | 96.7 | 96,705.17 |
Mar | 402.94 | 1,657.72 | 2,060.66 | 95.0 | 95,047.46 |
Apr | 396.03 | 1,664.62 | 2,060.66 | 93.4 | 93,382.83 |
May | 389.10 | 1,671.56 | 2,060.66 | 91.7 | 91,711.27 |
Jun | 382.13 | 1,678.52 | 2,060.66 | 90.0 | 90,032.75 |
Scenario 2: If you pay 1.06K monthly and a single 6K lump sum in June, your first 6 months will be calculated as follows:
Month | Interest | Principal | Total Payment | LTV | Closing Balance |
Jan | 416.67 | 643.99 | 1,060.66 | 99.4 | 99,356.01 |
Feb | 413.98 | 646.67 | 1,060.66 | 98.7 | 98,709.34 |
Mar | 411.29 | 649.37 | 1,060.66 | 98.1 | 98,059.97 |
Apr | 408.58 | 652.07 | 1,060.66 | 97.4 | 97,407.90 |
May | 405.87 | 654.79 | 1,060.66 | 96.8 | 96,753.11 |
Jun | 403.14 | 6,657.52 | 7,060.66 | 90.1 | 90,095.60 |
As you can see there is a difference of about 60 euros over 6 months at the closing balance.
Over the space of 5 years and 5 months (assuming you keep paying the same overpayment in the above scenario, that is when your overpayment would kill off the 10 year), that is 340 euros. The longer your mortgage is, the bigger the saving.
The saving is not massive but if you are certain that you will overpay in 6 months time (or whatever period) and you have the money now, paying it now regularly to the same equivalent amount will always result in slightly more saving