Use three months, not one
Export three months of transactions and average them. One month always contains something unusual, and three months smooths out both the quiet and the expensive weeks.
Convert annual costs to a monthly figure
Add every yearly and quarterly cost — insurance, licensing, memberships, school-related expenses, gifts and festive spending — and divide the total by twelve. This single step corrects most underestimates.
Categorise in two passes
- First pass: split fixed versus variable.
- Second pass: within each, group into no more than eight categories.
Do not forget these
- Bank charges and card fees.
- Data top-ups outside your main contract.
- Medical co-payments and pharmacy spending.
- Home and vehicle maintenance.
- Family support and gifts.
- Pet costs.
Worked example
Fixed costs of 9,200, three-month variable average of 5,400, and annual costs of 24,000 which becomes 2,000 a month. The real monthly figure is 16,600 — not the 14,600 that a fixed-plus-variable view would have suggested.
Sanity-check the total
Compare your calculated expenses to your take-home income. If the difference does not roughly match what your savings actually grew by, something is missing from the list.