Savings Calculator: Growth and Time to Goal

How the savings calculator works

It simulates your account month by month. Each month the balance earns one month of interest, then your deposit is added at month end. Because banks quote APY, the effective yearly rate that already includes compounding, the calculator converts it to the matching monthly rate: (1 + APY)^(1/12) − 1. At 4 percent APY that is about 0.327 percent a month, and twelve months of it compounds back to exactly 4 percent.

Future balance is where you land after the number of years you enter. Interest earned is that balance minus every dollar you deposited. Time to goal runs the same simulation until the balance reaches your target and reports years and months. If deposits are zero and the balance is zero, or the goal is more than 100 years away, it says so rather than spinning forever.

Worked example: $250 a month for 5 years

Start with $1,000, add $250 a month, and earn 4 percent APY. After 5 years you have about $17,761. Your deposits total $16,000 ($1,000 + 60 × $250), so interest contributed about $1,761.

Same plan at 0.4 percent APY, roughly what many large banks pay on standard savings: about $16,168. The rate difference is worth nearly $1,600 over five years on modest deposits. On a larger balance it gets serious: $30,000 sitting at 4 percent earns $1,200 a year versus $120 at 0.4 percent.

With a $20,000 goal and the default inputs, the calculator says 5 years 8 months. Push the deposit to $350 and it drops to about 4 years 2 months. Saving more moves the date far more than chasing a slightly higher rate.

Picking the right place for savings

High-yield savings accounts at online banks pay many times the national average and let you withdraw anytime. The FDIC publishes a national average savings rate every month, and it has sat well under 1 percent for years, while competitive online accounts have paid around 4 percent. Rates float, and they tend to fall when the Federal Reserve cuts.

Certificates of deposit lock a rate for a set term, useful if you know you will not touch the money for 6 to 24 months. Money market accounts behave like savings with check-writing. Treasury bills pay interest that is exempt from state and local income tax, which matters in high-tax states like California and New York.

Stocks are not a savings account. Money you need within about 3 to 5 years, such as an emergency fund or a house down payment, belongs somewhere its value cannot drop 20 percent the month you need it.

Real returns: taxes and inflation

Interest is taxed as ordinary income. In the 22 percent federal bracket, a 4 percent APY nets about 3.12 percent before state tax. If inflation runs 3 percent, your purchasing power grows by roughly 0.1 percent a year after tax, and it shrinks at 0.4 percent APY.

That is fine. The job of savings is safety and access, not growth. Keep your emergency fund and short-term goals here, and invest longer-term money where it can outpace inflation.

One exception worth knowing: interest on Treasury bills and I bonds is exempt from state and local income tax, and I bond interest can be deferred until you cash them. In a state with a 5 to 10 percent income tax, a Treasury yielding slightly less than a savings account can leave you with more after tax.

Common savings mistakes

Entering an APR as an APY or the other way around. The gap is small at low rates but real: 5 percent APR compounded daily is about 5.13 percent APY.

Leaving an old account at 0.01 percent out of inertia. Switching takes an afternoon and costs nothing.

Forgetting teaser rates. Some accounts pay a promotional APY for three to six months, then drop. Run the calculator at the ongoing rate, not the headline one.

Saving without automation. A fixed transfer scheduled for payday beats good intentions. Set the monthly deposit in this calculator to the number you will automate, not the one you hope to manage.