Sinking Fund Calculator
Inputs
| Mode | Forecast — how much will I accumulate? |
|---|---|
| Periodic Payment | 200 $ |
| Target Amount | 10,000 $ |
| Annual Interest Rate | 4 % |
| Savings Duration | 5 yr |
| Contribution Frequency | Monthly |
Visualization
Sinking Fund Calculator
Calculate the periodic payment needed to reach a savings goal, or project how much your regular deposits will accumulate over time — with interest.
Inputs
Savings Plan
Rate & Duration
Results
Enter a value to see results.
Details
The sinking fund explained
A sinking fund is a pool of money built up through equal, regular deposits into a dedicated account to meet a specific future expense — a car replacement, a home renovation, a vacation, or any other planned one-time cost. Unlike a general savings account that grows indefinitely, a sinking fund has a target balance and an end date, and the entire balance is spent when the goal is reached. The term originates in corporate-bond finance, where a company sets aside money each year into a fund that "sinks" a debt by accumulating enough to redeem the bonds at maturity; in personal finance the same mechanism applies to any purpose-specific savings goal.
How a sinking fund accumulates
A sinking fund has three core inputs: a target amount (or a fixed payment), an interest rate, and a time horizon. Each deposit earns interest, and because the deposits repeat at fixed intervals, the balance compounds across every period. Each deposit can be treated as an independent investment that grows at the per-period rate for the remaining number of periods; summing those independent investments gives a geometric series that reduces to a single closed-form expression — the future value of an ordinary annuity. The "ordinary" annuity assumes deposits arrive at the end of each period; deposits made at the start of each period accumulate fractionally more, but the difference is rarely meaningful for practical sinking funds.
Formula
The future value of an ordinary annuity is
FV=PMT×i(1+i)n−1where PMT is the periodic payment, i is the interest rate per period (r ÷ m for annual rate r and m periods per year), and n is the total number of deposits (years × m). Solving backwards for PMT gives the periodic deposit required to reach a specific FV:
PMT=FV×(1+i)n−1iWhen the interest rate is zero, both expressions collapse to the simple split of the target across the deposits: .
Worked example
Suppose the goal is $8,500 for a car down payment in three years, held in a high-yield savings account earning 4 % per year, compounded monthly.
- Annual rate r = 0.04; monthly rate i = 0.04 ÷ 12 ≈ 0.003333
- Duration: n = 3 × 12 = 36 months
- Required monthly payment: approximately $222.23 per month
Substituting into the payment formula:
PMT=8,500×(1.003333)36−10.003333=8,500×0.127160.003333≈$222.23 per monthAfter 36 months the out-of-pocket deposits total $222.23 × 36 = $8,000.28; the remaining $499.72 comes from interest. Compared with saving the same nominal amount at zero interest ($8,500 ÷ 36 ≈ $236/mo), interest covers about $14 per month of the goal.
Contribution frequency
For the same annual total, more frequent deposits compound more often and produce a slightly larger final balance, though the effect is smaller than it might appear at typical savings rates. The table below compares $3,600 saved per year at 4 % over five years:
| Frequency | Deposit amount | Total saved | Final balance (5 yr) |
|---|---|---|---|
| Annual | $3,600/yr | $18,000 | ~$19,491 |
| Monthly | $300/mo | $18,000 | ~$19,860 |
| Quarterly | $900/qtr | $18,000 | ~$19,817 |
The monthly schedule produces about 2 % more than annual at 4 %, because each month's deposit has slightly more time to compound. The gap widens at higher rates or longer terms.
Sinking fund vs. savings account
The distinction between a sinking fund and a regular savings account is one of purpose and horizon. A regular savings account accumulates indefinitely and may begin with an initial lump sum already in the account. A sinking fund always starts from zero — built up purely through contributions — and has a defined end date at which the entire balance is spent.
For a goal that already has some money set aside, one option is to reduce the target by the amount saved and enter the net remaining goal here. The alternative is Savings Goal & Growth Calculator, which accepts both an initial deposit and regular contributions.
Choosing an interest rate
Sinking funds are typically short-to-medium horizon (1–7 years), so the savings vehicle should match that horizon:
- High-yield savings accounts: 3–5 % in a typical rising-rate environment. Liquid and insured.
- Money-market funds: 3–5 %. Slightly higher but less liquidity for very short-term needs.
- Short-term CDs (certificates of deposit): 4–5.5 %. Better rates but lock up the money.
- Bond funds: 3–5 % longer-run average. Appropriate only if the horizon is flexible.
Equity-like returns (7–10 %) are not appropriate for a sinking fund with a fixed end date: equities can drop 30–50 % in a year, and the money may be needed exactly when the market is down. The rate entered should reflect the actual account where the fund is held.
Frequently Asked Questions (FAQ)
What is a sinking fund and how does it work?
A sinking fund is a dedicated savings pool you build by making regular, equal deposits over a fixed period to meet a future one-time expense or obligation. The term originates in corporate bond finance, where companies set aside money each year to retire debt at maturity, but in personal finance it simply means a purpose-specific account you fill up with consistent contributions.
Each deposit earns interest, and because those deposits happen repeatedly over time, the math follows the ordinary-annuity formula. At maturity you have the sum of all deposits plus all the compound interest those deposits earned.
What is the difference between a sinking fund and a regular savings account?
A regular savings account typically assumes you start with an existing lump sum and keep adding to it indefinitely. A sinking fund starts from zero, has a defined end date, and has a specific target — the point is to accumulate exactly enough for one goal, then spend the whole balance.
Think of a savings account as a reservoir that keeps growing; a sinking fund is a bucket you fill to a planned level and then drain. The math differs accordingly: the sinking fund formula ignores an initial balance and focuses purely on the periodic-deposit component.
Does it matter whether I contribute monthly or annually?
Yes, though for typical rates the difference is modest. Monthly deposits compound more frequently than annual ones, so for the same annual total a monthly schedule accumulates slightly more.
Example: $1,200 deposited annually at 4% for 5 years grows to about $6,497; the same $1,200 as $100/month grows to about $6,631 — roughly 2% more because each month's deposit has a little more time to earn interest. The gap widens at higher rates or longer terms.
What if I already have some money saved toward my goal?
This calculator intentionally starts the fund balance at zero — that is the defining feature of a pure sinking fund. If you already have savings to apply to the goal, use the Savings & Investment Calculator instead, which lets you enter an initial deposit alongside regular contributions. Alternatively, reduce your target amount by the amount already saved and enter the net remaining goal here.
What interest rate should I enter for a sinking fund?
Use the actual rate of the account where you plan to hold the money. Sinking funds are typically kept in low-risk, liquid accounts because the money will be spent within a fixed horizon:
- High-yield savings: 3–5%
- Money-market funds: 3–5%
- Short-term certificates of deposit: 3–5.5%
Avoid projecting equity-like returns (7–10%) — equity markets can drop 30–50% just when you need to spend.
Disclaimer
This calculator assumes constant equal deposits, a fixed nominal interest rate, and end-of-period contributions. Real-world savings rates fluctuate, fees reduce effective returns, and irregular contributions are not modelled. The results are illustrative estimates, not financial advice. Consult a licensed financial adviser for goal-specific guidance.
Recommended Next
Savings Goal & Growth Calculator
Project the future balance of a savings plan, or work backward to the monthly contribution needed to hit a target — with a growth curve and inflation-adjusted purchasing power in today's money.