The Workbook
Kids savings growth calculator
Updated 27 April 2026
The single most powerful factor in building a child's savings is time. A $50 monthly contribution starting at birth grows to over $12,500 by age 18 at 3.5% APY. Starting the same $50 at age 5 produces only about $9,200. Those five extra years of compound interest add over $3,300 without contributing a single extra dollar. Use the calculator below or read the reference tables.
The Family Calculator
Project your child's savings
Slide the dials to see what your contributions become with compound interest. The ledger updates instantly.
Target age
Balance at 18
$12,659
with compound interest
Contributions
$9,500
money you put in
Interest earned
$3,159
free growth from APY
Piggy bank only
$9,500
no interest paid
Compound advantage: choosing a savings account over a piggy bank earns your child an extra $3,159 by age 18, a 33.3% return on contributions.
Reference table
$50 per month, no initial deposit
Projected balance contributing $50 per month with no initial deposit at common APY rates.
| Years | 3.0% APY | 3.5% APY | 4.0% APY | 4.5% APY | 5.0% APY | No interest |
|---|---|---|---|---|---|---|
| 5 years | $3,232 | $3,273 | $3,315 | $3,357 | $3,400 | $3,000 |
| 10 years | $6,987 | $7,172 | $7,362 | $7,560 | $7,764 | $6,000 |
| 15 years | $11,349 | $11,814 | $12,305 | $12,821 | $13,364 | $9,000 |
| 18 years | $14,297 | $15,015 | $15,780 | $16,593 | $17,460 | $10,800 |
At $50 per month over 18 years, total contributions are $10,800. At 3.0% APY, compound interest adds $1,363, bringing the total to $12,163. At 5.0% APY, interest adds $2,744, for a total of $13,544. Compound interest accelerates: in the first 5 years, interest at 3.5% is just $159. In the last 5 years (years 13 to 18), the same contribution rate generates over $500 in interest because the base balance is much larger.
Reference table
$100 per month, no initial deposit
Doubling the contribution does not just double the final balance. It more than doubles it because compound interest works on the larger base throughout.
| Years | 3.0% APY | 3.5% APY | 4.0% APY | 4.5% APY | 5.0% APY | No interest |
|---|---|---|---|---|---|---|
| 5 years | $6,465 | $6,547 | $6,630 | $6,715 | $6,801 | $6,000 |
| 10 years | $13,974 | $14,343 | $14,725 | $15,120 | $15,528 | $12,000 |
| 15 years | $22,697 | $23,629 | $24,609 | $25,641 | $26,729 | $18,000 |
| 18 years | $28,594 | $30,031 | $31,559 | $33,187 | $34,920 | $21,600 |
At $100/month over 18 years, contributions are $21,600. At 3.5% APY, interest adds $3,490. At 5.0% APY, interest adds $5,487. The extra $5,487 is money your child earns without doing anything. That is the lesson compound interest teaches: money grows faster when it has more time.
A side-by-side scenario
The power of starting early
Two families both contribute $75 per month at 3.5% APY. Family A starts at birth. Family B starts when their child is 8. Both stop on the child's 18th birthday.
Family A: from birth
$22,523
18 years of contributions
Contributions: $16,200
Interest earned: $2,618
Family B: from age 8
$10,757
10 years of contributions
Contributions: $9,000
Interest earned: $763
Family A contributed $7,200 more than Family B but ended with about $8,855 more. The extra $1,855 beyond the additional contributions is purely compound interest having more time to work. A $25 monthly contribution started at birth beats a $50 contribution started at age 10 in total interest earned.
The mechanics
Compound interest in plain English
Compound interest means you earn interest on your interest. When a savings account pays 3.5% APY, the bank calculates interest on the full balance, including previously earned interest. In month one, interest is on the deposits. In month two, interest is on deposits plus month one's interest. By year 10, a significant portion of each month's interest is earned on accumulated interest rather than original contributions.
Concrete example: deposit $1,000 at 4% APY (compounded monthly). After one year, $1,040.74. After two years, $1,082.86 (not $1,080, because year two's 4% is calculated on $1,040.74). After 10 years, $1,490.83. After 18 years, $2,044.41. Your $1,000 has more than doubled, with $1,044 of growth coming entirely from interest on interest.
APY (Annual Percentage Yield) already accounts for compounding frequency, so a 4.00% APY account always yields 4% per year regardless of whether the bank compounds daily, monthly, or quarterly. When comparing accounts, APY is the number that matters. APR (Annual Percentage Rate) does not account for compounding and may understate the actual return.
Three common goals
Goal-based scenarios at 3.5% APY
Different goals require different timelines and contributions. Three scenarios most families recognise:
College fund
$25,000 by 18
From birth: about $100/month with a $500 initial. From age 5: roughly $145/month. From age 10: about $235/month. Later starts demand higher contributions because compounding has less time.
Average in-state tuition is roughly $11,000/year. $25,000 covers about two years.
First car
$5,000 by 16
From birth: roughly $22/month. From age 8: about $50/month. From age 12: roughly $100/month. A $5,000 fund also doubles as an emergency fund through the teen years.
Very achievable with small consistent contributions started early.
Emergency fund
$2,000 by 18
At $10/month from birth at 3.5% APY, the balance reaches about $2,750 by 18. One of the most overlooked but impactful goals. Many young adults face unexpected expenses (car repair, medical bill, deposit) within their first year of independence.
$10/month provides meaningful financial security for a new adult.
You do not need to choose one. A $50 monthly contribution can be split: $25 toward college, $15 toward a first car, $10 toward an emergency fund. The calculator above lets you model each scenario.