31 CFR § 359.19 - How is interest calculated?

---
identifier: "/us/cfr/t31/s359.19"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "31 CFR § 359.19 - How is interest calculated?"
title_number: 31
title_name: "Money and Finance: Treasury"
section_number: "359.19"
section_name: "How is interest calculated?"
chapter_name: "FISCAL SERVICE, DEPARTMENT OF THE TREASURY"
subchapter_number: "A"
subchapter_name: "BUREAU OF THE FISCAL SERVICE"
part_number: "359"
part_name: "OFFERING OF UNITED STATES SAVINGS BONDS, SERIES I"
positive_law: false
currency: "2026-04-05"
last_updated: "2026-04-05"
format_version: "1.1.0"
generator: "[email protected]"
authority: "5 U.S.C. 301; 12 U.S.C. 391; 31 U.S.C. 3105."
regulatory_source: "67 FR 64278, Oct. 17, 2002, unless otherwise noted."
cfr_part: "359"
---


# 359.19 How is interest calculated?

We base all calculations of interest on a $25 unit. We use the value of this unit to determine the value of bonds in higher denominations. The effect of rounding off the value of the $25 unit increases at higher denominations. This can work to your slight advantage or disadvantage, depending on whether we round the value up or down. 
<sup>3</sup>
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<sup>3</sup> For example: A composite rate of 2.57% will result in a newly purchased $25 unit increasing in value after six months to $25.32, when rounded to the nearest cent. Thus, a $5,000 bond purchased at the same time as the $25 unit will be worth $5,064 after six months ([$5,000 divided by $25] × $25.32 = $5,064.) In contrast, if it applied directly to a $5,000 bond, the rate would render a value of $5,064.25 after six months, a difference of 25 cents. (This example does not include any discussion of the three-month interest penalty that applies if you redeem a bond less than five years after its issue date.)