CD Calculator

What the calculator computes

A certificate of deposit has a lean model: one deposit, one rate that holds for the whole term, no additions, no withdrawals. The engine therefore reduces it to the general compound-growth form documented on the methodology page:

Maturity value = P · (1 + r/m)^(m·t)

P is the deposit, r the annual rate as a decimal, m the number of compounding periods per year from the dropdown (annually 1, quarterly 4, monthly 12, daily 365), and t the term in years. From that single value the other two outputs follow directly: interest earned is the maturity value minus the deposit, and the effective APY is (1 + r/m)^m − 1 — the growth the rate produces over exactly one year once compounding inside that year is counted. The Compound Interest Calculator runs the identical formula for any lump sum; this page applies it to the certificate's vocabulary — deposit, term, maturity — and adds the APY line so that offers stated with different compounding schedules can be put on one footing. Where a certificate sits within a retirement plan — inside a taxable account, a traditional IRA, or a Roth — is a question about the account rather than the certificate, and the companion guide 401(k), Roth IRA, and Taxable Accounts: How They Fit Together lays out those mechanics.

The worked deposit

The reference example throughout this page is $10,000 in a 3-year certificate at an assumed 4.5% rate, compounded monthly. The engine converts the rate to a per-period rate of 4.5% ÷ 12, counts 36 periods, and applies the per-period factor that many times. The deposit matures at $11,442.48: $1,442.48 of interest and an effective APY of 4.594%. Each month's interest is computed on the running balance rather than on the original deposit, so the balance the second month's rate applies to is slightly larger than the first month's, and so on through every period. That is the whole mechanism; everything else on this page is a consequence of it. Every figure here is computed at build time by the same tested engine as the calculator above, which is why the copy cannot drift from the tool.

How compounding frequency changes the APY

The dropdown changes only m. Holding the reference deposit, rate, and term fixed and stepping through the four schedules the form offers gives:

Two things stand out. First, the direction: at any positive rate, more frequent compounding produces a larger maturity value, because interest posted earlier begins earning interest sooner. Second, the size of the steps shrinks. Moving from annual to quarterly adds $25.08 over the three years; quarterly to monthly adds $5.73; monthly to daily adds only $2.79. Each additional split divides an already small per-period rate into smaller pieces applied more often, and the product of those pieces converges toward a ceiling rather than growing without limit. The practical reading is that the rate and the term move the maturity value far more than the posting schedule does — and that the APY column, which already contains the schedule, is the like-for-like figure when two offers compound differently.

Why the APY exceeds the stated rate

With annual compounding there is nothing to fold in: the factor (1 + r) is applied once a year, and the APY equals the rate — 4.5% in the table. With any more frequent schedule the per-period rate r/m is applied m times within the year, and the interest posted in the early periods is part of the balance for the later ones, so the year-end factor (1 + r/m)^m is larger than 1 + r. The gap between that compounded factor and the plain rate is the difference between APY and rate. At the assumed 4.5%, the gap is 0.094 percentage points with monthly compounding and 0.102 with daily.

Because the APY formula contains only r and m, the APY does not depend on how much is deposited or for how long. A $1,000 certificate for one year and a $250,000 certificate for ten years, both at the assumed 4.5% compounded monthly, report 4.594% and 4.594% — identical to the reference deposit's. What the deposit and term change is the dollar interest and the total percentage return over the whole term, which is a different quantity from the APY: over the reference three years the deposit grows by 14.425% in total, more than three times the APY (3 × 4.594% = 13.782%), because the second and third years compound on the first year's interest as well as on the deposit.

Three more deposits: one year, six months, five years

A one-year term. $25,000 at an assumed 4% rate for exactly one year, compounded monthly, matures at $26,018.54, earning $1,018.54. Over a single year the total return and the APY are the same number by construction — 4.074% earned against an APY of 4.074% — because (1 + r/m)^(m·1) is the APY factor itself. A one-year certificate is therefore the one case where multiplying the deposit by the APY gives the interest, to the rounding of the displayed APY.

A six-month term. $5,000 at an assumed 5% rate for 0.5 years, compounded daily, matures at $5,126.57 with $126.57 of interest. The APY is still 5.127% — an annualized figure, unchanged by the short term — while the interest earned is slightly less than half of the $256.34 a full year at the same terms would produce, because the second half-year would compound on the first half's interest. Note how the engine counts the periods: 365 × 0.5 = 182.5 daily periods, and the per-period factor is raised to that fractional power. The model does not know the calendar — it treats six months as exactly half of a year, and whatever day count a real certificate's terms use is not modeled.

A five-year term. $50,000 at an assumed 4% rate for five years, compounded quarterly, matures at $61,009.50, earning $11,009.50. The same deposit for one year at the same rate and schedule earns $2,030.20; five repetitions of that first year, with no interest carried forward, would total $10,151.00. The five-year certificate earns $858.50 more than that, and every dollar of the difference is interest earned on earlier years' interest. The longer the term, the larger that share becomes — the compounding curve the Compound Interest Calculator is built to show.

How to read the output

The result panel shows three lines and updates as the fields change. The first, "at maturity", is the deposit plus all interest, in nominal dollars, shown to the cent. The second, Interest earned, is the maturity value minus the deposit, rounded to the nearest dollar for readability — so the maturity value minus the deposit can differ from the interest line by rounding alone. The third, Effective APY, is the annualized yield implied by the rate and the compounding schedule; it does not change when the deposit or the term changes, and it is not the total percentage return over a multi-year term. The panel shows the APY to two decimal places, while this page quotes it to three so that the small gaps between compounding schedules stay visible — the same engine value, displayed at different precision. "Copy result" places a one-line summary — deposit, rate, term, maturity value, and APY — on the clipboard. All of it is computed in the browser from the four inputs; nothing is transmitted.

Common misreadings

Comparing rates across different schedules. A rate compounded daily and the same rate compounded annually are not the same yield; the table above shows how far apart they land. The APY is the figure that already accounts for the schedule. Multiplying the APY by the years. That gives 13.782% for the reference deposit; the engine reports 14.425%, because compounding continues across years. The APY is a one-year figure by definition. Expecting daily compounding to transform the result. On the reference deposit, daily instead of monthly adds $2.79 over three years — real, and small. Reading the maturity value as spendable. It is a pre-tax, nominal figure; taxes on the interest, the terms for withdrawing early, and inflation are all outside the model. Treating the term as a day count. The model uses years and fractions of years; the number of days in a real term, and how interest accrues across them, is set by the certificate's terms and is not modeled.

What the model does not include

Early-withdrawal terms. The model assumes the deposit stays in place from the first period to the last. Whatever terms apply to withdrawing before maturity are not modeled, and this page quotes no figures for them. Taxes on the interest. Not modeled. The maturity value and the interest earned are pre-tax figures. Which account holds the certificate — a taxable account, a traditional IRA, or a Roth — determines when any tax on what the certificate earns lands, and the guide 401(k), Roth IRA, and Taxable Accounts: How They Fit Together lays out the three timings. Rate changes, renewals, and special features. One rate for one term is the entire model. What happens at maturity — a renewal at a new rate, a payout — is a fresh run with fresh inputs, and callable or brokered features, additional deposits, and partial withdrawals are not represented at all. A projection with ongoing deposits belongs on the Investment Calculator, which layers a contribution stream on the same compounding. Inflation. The maturity value is in nominal dollars. The Inflation Calculator restates it under an assumed rate: at an assumed 3% inflation, the reference maturity value of $11,442.48 three years out has the purchasing power of about $10,471.49 in today's dollars. That is an illustration built on a chosen assumption, not a forecast. Day-count conventions and deposit insurance. Neither is modeled; both are matters of the specific institution and certificate.

Related tools and guides

The Compound Interest Calculator runs the same formula on any lump sum and adds a semiannual option; the Investment Calculator adds monthly contributions; the Inflation Calculator converts nominal figures to purchasing power. The guides index collects the site's long-form explainers, including the account-type guide linked above and the drawdown material in The 4% Rule.

Frequently asked questions

Not financial advice: a general educational estimate. It excludes early withdrawal penalties, taxes on interest, and callable/brokered CD features. Values are processed locally in your browser and never transmitted. See the methodology page.