Price an 18-month CD against the terms either side.
Rate is quoted as
1 year 6 mo
Common terms
Compounding frequency
$20,000.00 in a 1 year 6 mo CD at 4.05% APY, compounded daily, grows to $21,227.22 — that’s $1,227.22 of interest, averaging $68.18 per month.
Balance over the term
| Month | Principal | Interest | Balance |
|---|---|---|---|
| At opening | $20,000.00 | $0.00 | $20,000.00 |
| 4mo | $20,000.00 | $266.43 | $20,266.43 |
| 7mo | $20,000.00 | $468.59 | $20,468.59 |
| 11mo | $20,000.00 | $741.26 | $20,741.26 |
| 14mo | $20,000.00 | $948.15 | $20,948.15 |
| 18mo | $20,000.00 | $1,227.22 | $21,227.22 |
Calculated in your browser using A = P(1 + r/n)nt. Nothing is sent to a server. Figures are before tax; confirm exact terms with your bank.
Formulas and content last reviewed . Bank rates change frequently — confirm current rates directly with your institution.
The short answer
An 18-month CD sits in the gap between the one- and two-year terms, and it exists mainly because banks use odd terms to price promotions without repricing their whole rate sheet. A $20,000 deposit at 4.05% APY earns $1,227.22 over the term and matures at $21,227.22. Because the term is a year and a half, the quoted APY overstates what you get in the first twelve months and understates the total: you receive roughly 6.14% of the deposit across the full period. The comparison worth running is against a 12-month CD followed by a 6-month one. On $20,000, twelve months at 4.25% then six at 4.40% produces $21,303.76 — $76.54 more than the single 18-month term, while giving you a decision point after a year. The 18-month CD wins only when its rate premium is large enough to pay for that lost flexibility, which is exactly what the odd term is designed to test.
Formula & method
A = P(1 + r/n)^(nt), t = 1.5
Eighteen months is 1.5 years. Odd terms like 13, 14, 15 and 18 months are handled the same way — set t to months ÷ 12.
Divide the term in months by twelve to get t. Eighteen months is 1.5.
Convert the rate to a decimal and divide by n.
Raise (1 + r/n) to the power n × t.
Multiply by the deposit for the maturity value.
To test the odd term, price a 12-month CD and a 6-month renewal separately and compare.
Guide
Type the amount or drag the slider. Odd-term promotional CDs sometimes attach a minimum well above the bank's standard tier.
The slider takes any number of months, so 13, 14 and 15-month promotional terms can be priced here too — they behave identically.
Odd terms usually exist to carry a promotion. Use the rate quoted for that exact term, and check whether it reverts at renewal.
Price a 12-month CD and then a 6-month one at today's rates. If the split wins, the promotional premium is not paying for the extra lock-up.
Examples
Inputs
Result
6.14% over the term annualises to the 4.05% quoted. Savers comparing an 18-month CD against a 12-month one on total interest alone will always pick the longer term — annualise first, then compare.
Inputs
Result
On an inverted curve the split wins on both counts — more money and a decision point after a year. The 18-month CD is only the better trade when its rate is above the shorter terms, which is precisely when banks are trying to lengthen their deposit book.
Methodology
Eighteen months is treated as exactly 1.5 years. Banks count actual days, which shifts the result by a few dollars either way.
The split comparison assumes the 6-month rate is still available in a year. It is the assumption doing all the work, and it is unknowable.
The rate is fixed and interest compounds inside the CD.
Figures are gross of tax. An 18-month CD generates 1099-INT interest in two separate tax years.
Conventions follow Regulation DD (12 CFR 1030), which governs how US institutions disclose APY on deposit accounts. Deposits at insured institutions are FDIC-protected up to $250,000 per depositor, per bank, per ownership category.
Primary sources
Details
Odd terms — 13, 14, 15, 18 months — are a pricing tool. They let a bank run a promotion without moving its published 12- and 24-month rates, so the promotional rate is genuinely competitive but almost never survives renewal.
An 18-month CD straddles two tax years. Interest is taxable in the year it is credited, so you will report part of it before the CD matures and can access the money.
The renewal term is often not 18 months. Many odd-term CDs roll into the nearest standard term — usually 12 or 24 months — at the standard rate. The disclosure states which.
Early withdrawal is typically penalised at 180 days of interest on terms over a year, double the 90 days common on shorter CDs. On $20,000 at 4.05% that is $399.45.
If your horizon is flexible, the odd term is often worth taking. If it is fixed at exactly one or two years, an odd term means either breaking the CD early or leaving money idle after it matures.
How it connects
Applications
If your bank is pushing a 13, 15 or 18-month CD, this page prices whether the premium is real. Compare it against the standard terms either side before accepting.
Eighteen months suits money you will not need for at least a year and probably not for two. The odd term captures a promotional rate for the whole of that window.
An 18-month rung sits neatly between the 12- and 24-month rungs of a ladder, smoothing the reinvestment schedule to every six months rather than every year.
Related tools
FAQs
At 4.05% APY, a $20,000 deposit earns $1,227.22 and matures at $21,227.22 — a 6.14% return across the full term. The APY is annualised, so the total is roughly one and a half times the annual figure.
Security & privacy
Uses the same compound interest and APY conventions as banks under Regulation DD.
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Price your 18-month offer and compare it against splitting the money across two shorter CDs.