Sinking Fund Calculator

What this calculator does

A sinking fund calculator answers a simple planning question: to have a specific amount by a specific date, how much do I need to put aside each month? Enter the target amount, an expected annual return on the savings, and the number of years available.

It returns the required monthly contribution, the target balance, and how much of that balance comes from interest rather than your own deposits. On short horizons the interest portion is usually modest — the point of a sinking fund is not to grow money but to make a large expense arrive as a series of small, survivable ones.

When to use it

Use it for any cost you can see coming. Property taxes due annually, a car you expect to replace in four years, a roof with maybe eight years left, a wedding, next year's vacation, or the deductible on your homeowners policy. All of these are predictable in timing and rough size.

It is also the tool for the reverse decision. If the required monthly figure is clearly unaffordable, that is useful information now rather than in three years — early enough to extend the timeline, lower the target, or decide the purchase is not happening.

Understanding the inputs

Target amount should be the full cost including tax, delivery, and installation, since these are routinely underestimated. For a car, include registration and the first insurance payment. Add a contingency of ten percent to anything involving construction work.

Annual return should match where the money will actually sit — 4 to 5 percent for a high-yield savings account in the current environment, close to zero for a checking account. Do not assume investment returns for money with a fixed date. Years to save accepts decimals, so eighteen months is 1.5.

How is this calculated?

Monthly Payment = FV × r / ((1+r)^n − 1) where r = monthly rate, n = months.

A worked example

Suppose you need $12,000 in three years to replace a car, and the money will sit in a savings account paying 4 percent. The monthly rate is 0.3333 percent and there are 36 contributions.

The required monthly payment is $314.29. Across 36 months you contribute $11,314 of your own money, and interest supplies the remaining $686. If the same $12,000 had sat in a checking account paying nothing, you would need $333.33 a month — the interest saves you about $19 a month, which is worth having but is not the point.

Limitations and assumptions

The calculation assumes a constant interest rate and contributions made faithfully at the end of every month. Savings rates move, and a rate that looks good today may not hold for three years. It also assumes the target is right, and targets for construction and vehicles are frequently understated.

It does not model inflation on the cost itself, which matters for longer horizons — a roof quoted at $18,000 today may cost $22,000 in six years. Nor does it account for tax on interest earned. For anything more than five years out, revisit the target annually rather than setting and forgetting.