How to calculate the total interest paid on a loan
✓ Verified in LibreOffice 25.8.7.3 ✓ Verified in Google Sheets (2026-08-30)The eye-opening number: everything you'll pay the bank beyond the principal.
The formula
| App | Formula | Notes |
|---|---|---|
| Excel (desktop) | =-PMT(A2/12,B2*12,C2)*B2*12-C2 | A2=annual rate, B2=years, C2=principal: payment x number of payments, minus what you borrowed. |
| Google Sheets | =-PMT(A2/12,B2*12,C2)*B2*12-C2 | Identical. |
| LibreOffice Calc | =-PMT(A2/12,B2*12,C2)*B2*12-C2 | Identical. |
How it works
Every payment is the same, so lifetime cost is simply payment × payment-count; subtracting the principal leaves pure interest. A $300,000 mortgage at 6% for 30 years: $1,798.65 × 360 − $300,000 ≈ $347,515 — more than the house. The formula's real use is comparing scenarios: rerun it at 15 years ($155,683) or at 5.5% to see what rate shopping and shorter terms are actually worth. CUMIPMT(rate,periods,principal,1,periods,0) gives the same total with more ceremony.
Verified, not just documented
We ran =ROUND(-PMT(A2/12,B2*12,C2)*B2*12-C2,2) in LibreOffice 25.8.7.3 (headless, with forced recalculation) and it returned 347514.57 — exactly the expected result. We then ran the same formulas in Google Sheets, executed 2026-08-30: a formula-only workbook goes into Google Drive, which converts it to a Sheet and recalculates every formula with Google’s own engine, and comes back out as .xlsx carrying the values Google computed. It returned 347514.57 for the worked example, the same value LibreOffice produced. Both engines’ numbers on this page are executed results. The Excel formula follows Microsoft’s official documented syntax — we do not run desktop Excel.
Functions used
PMT — see full Excel, Google Sheets & LibreOffice compatibility for each.
Related recipes
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- How to calculate the monthly savings needed for a goal
- How to calculate the remaining balance on a loan