A sinking fund is a savings category for a known but irregular future expense. Instead of being caught off-guard when your car insurance renewal arrives, you set aside a small amount each month so the payment arrives as a planned, budgeted event rather than a financial emergency.
The Core Idea
Every large, periodic expense can be broken into monthly increments:
| Annual expense | Monthly sinking fund |
|---|---|
| Car insurance ($1,200/yr) | $100/mo |
| Christmas gifts ($600) | $50/mo |
| Holiday / vacation ($2,400/yr) | $200/mo |
| Annual subscriptions ($300/yr) | $25/mo |
| Car maintenance ($800/yr) | $67/mo |
| Total | $442/mo |
Instead of these expenses hitting your budget as surprise shocks, they are planned costs that you fund incrementally.
YNAB’s Implementation
YNAB’s “true expenses” principle (Rule 2 of their four rules) is built around sinking funds. YNAB’s interface lets you create any number of categories with target amounts and target dates. It calculates the monthly funding needed automatically.
This is one of YNAB’s strongest features. The app handles the math; you just fund the categories each month when you assign your income.
Other Apps
Monarch Money ($99.99/yr) has a goals feature that functions similarly — set a target amount and date, and Monarch calculates the monthly contribution needed.
Goodbudget ($10/mo) uses an envelope system that naturally supports sinking funds as named envelopes.
EveryDollar (free tier with manual entry, $79.99/yr for bank sync) supports savings categories that function as sinking funds within the ZBB framework.
Why “Sinking Fund”
The term comes from corporate accounting. Companies establish sinking funds to retire debt obligations over time — setting aside money periodically so the final payment does not require a large lump sum. The household version applies the same principle to predictable irregular expenses.
The anti-pattern is the “surprise bill” — an annual or semi-annual expense that was predictable in advance but not budgeted for. Sinking funds eliminate surprise bills. They do not eliminate the expenses; they eliminate the surprise.