True Interest Cost vs Net Interest Cost

By Ryan O'Connell, CFA, FRM Published November 14, 2026

True interest cost vs net interest cost is a comparison of two ways to measure an issuer’s borrowing cost. When a municipality receives competing bond bids, the lowest coupon does not necessarily provide the cheapest financing. The purchase price and the timing of principal and interest payments also matter.

Net interest cost (NIC) spreads premium-adjusted interest over principal-dollar years. True interest cost (TIC) discounts the actual debt-service schedule to the issuer’s defined purchase proceeds. This U.S. municipal-finance guide shows a reproducible example in which NIC ties two bids while TIC distinguishes them.

What Do NIC and TIC Measure?

A coupon determines contractual interest on principal; it does not by itself describe how much cash the issuer receives. A premium purchase price provides more than par, while a discount provides less. For the borrower context, see municipal bonds. Investor bond pricing and yield to maturity use related discounting mathematics, but the investor’s purchase price and the issuer’s proceeds can differ.

True Interest Cost vs Net Interest Cost: Comparison

NIC incorporates how long principal stays outstanding. Its limitation is that it does not discount the individual interest payments. TIC applies the time value of money to the whole payment schedule. ^[2]

The Net Interest Cost Formula

Bond-year dollars equal the sum of each principal amount multiplied by its years outstanding. A $500,000 maturity one year away contributes $500,000 dollar-years; the same amount two years away contributes $1,000,000 dollar-years. The denominator is not simply total principal multiplied by the final maturity when principal retires earlier. ^{[1], [2]}

For a simple issue without accrued interest, total interest plus par minus issuer purchase proceeds gives the same numerator. A premium reduces the net interest burden; a discount increases it. If underwriting compensation has already reduced those proceeds, do not deduct it again. Underwriter spread is the difference between issuer purchase price and initial public offering prices, which is a separate concept from discount below par. ^[1]

The True Interest Cost Formula

For regular semiannual payments, solve for the nominal annual rate r:

The rate is found numerically. As a check, substitute it back into the equation: discounted debt service minus proceeds should be approximately zero. Mandatory sinking-fund principal belongs on its retirement dates, rather than entirely at the final term-bond maturity. ^[1]

Irregular first periods require the sale’s prescribed date and stub-period treatment. An annual cash-flow calculation cannot silently substitute for semiannual TIC. Also identify whether proceeds include accrued interest: the CDIAC glossary includes it, while a particular notice may prescribe another basis. ^{[1], [4]}

Worked Bid Comparison: Same NIC, Different TIC

Consider a hypothetical $1,000,000 serial issue settling June 1, 2027. Half the principal matures June 1, 2028; the remainder matures June 1, 2029. Both bids provide $1,000,000 at closing. Interest is paid December 1 and June 1, using four full half-years under a 30/360 convention. Ignore fees, accrued interest, calls and business-day adjustments.

A’s first half-year interest is ($500,000 × 6% + $500,000 × 3%) / 2 = $22,500. After the first maturity retires, it falls to $7,500 per half-year. B starts at $17,500 per half-year and falls to $12,500. Add principal on the two maturity dates to obtain debt service:

Calculate the NIC tie

A’s interest total is 2 × $22,500 + 2 × $7,500 = $60,000. B’s is 2 × $17,500 + 2 × $12,500 = $60,000. Neither bid has a premium or discount.

Solve TIC from the payments

For A, solve $1,000,000 = $22,500/(1+r/2) + $522,500/(1+r/2)² + $7,500/(1+r/2)³ + $507,500/(1+r/2)⁴. The nominal annual TIC is approximately 4.013156%.

Using B’s four payments in the same equation gives approximately 3.986929%. Substituting each rate into its own payment schedule returns a present value of approximately $1,000,000, so NPV is approximately zero. B’s TIC is lower by about 2.62 basis points, even though NIC ties.

Why TIC breaks the tie

Bid A pays $5,000 more than B at each of the first two dates, then $5,000 less at each of the last two. Their proceeds and principal payments are identical; only interest timing changes. The extra early payments exactly offset the later reductions in undiscounted dollars.

At a positive discount rate, those earlier extra payments have greater present-value weight. A therefore requires a higher rate to discount its payments to the same proceeds. This is the timing distinction NIC misses.

How Premium and Discount Change Borrowing Cost

Now hold A’s debt-service schedule unchanged and change only the purchase proceeds. These are separate hypothetical price sensitivities, not bids for the same net project funding requirement.

The premium NIC is ($60,000 − $20,000) / $1,500,000; the discount NIC is ($60,000 + $20,000) / $1,500,000. TIC instead solves the same four payments against each new proceeds amount. More cash received for identical future payments lowers borrowing cost. A higher coupon can similarly be offset by a higher purchase price, so coupon-only rankings are unreliable.

Bid TIC vs All-In TIC

Bid TIC uses the price and debt service specified for bid comparison. Underwriting compensation may already be reflected in that issuer purchase price. All-in TIC extends the cash-flow analysis to additional financing expenses, such as legal, advisory, rating or insurance costs, with their actual timing and stated treatment. CDIAC’s September 2026 teaching materials distinguish these measures. ^[3]

Suppose B’s issuer purchase proceeds are $1,000,000 and a separate $10,000 issuance bill is paid at closing. Available proceeds fall to $990,000. With unchanged debt service, an all-in calculation against that smaller amount produces a higher rate. Subtract each expense once; identify future charges on their payment dates. A label alone does not establish which costs were included.

How to Evaluate Actual Financing Bids

1. Read the award rule. Identify the required cost measure, admissibility conditions, permitted coupons and prices, adjustment provisions and tie rule.
2. Reconcile the proceeds. Separate par, premium/discount, underwriting compensation, accrued interest and additional expenses.
3. Rebuild dated payments. Include every interest date and principal retirement, including mandatory sinking-fund installments.
4. Match conventions. Use the required reference date, day count, first-period treatment and annualization.
5. Check the result. Reconcile NIC’s numerator and denominator; substitute TIC back into present value. Then assess funding adequacy and budget payments.

A historical example shows why the first step matters. Monmouth County’s December 2020 note notice ranked legally acceptable proposals by TIC, required a price at least equal to par, and specified doubling the semiannual discount rate. It also set a timestamp tie rule and reserved rejection rights. Our discounted sensitivity above would fail that notice’s price floor. These were that sale’s terms, not a nationwide rule. ^[4]

Nominal annual vs effective annual

In our example, B’s half-year rate is approximately 1.993464%. Doubling gives nominal annual TIC of 3.986929%; compounding gives (1 + 0.039869286672/2)² − 1 = 4.026668% effective annually. Compare rates using the same convention. Monmouth’s note paid interest at maturity, illustrating that a sale’s discounting convention also needs to be read separately from its coupon-payment terms. ^[4]

Common Mistakes

• Choosing the lowest coupon. Compare purchase proceeds and the complete debt service.
• Using only final maturity in NIC. Weight each principal retirement by its own years outstanding.
• Reversing premium and discount. Premium reduces NIC’s interest numerator; discount increases it.
• Double-counting underwriting cost. Check whether it already reduced the purchase price.
• Comparing different cost scopes. Bid TIC, all-in TIC and investor yield answer different questions.

Limitations of NIC and TIC

Compare like funding requirements and document assumptions when principal amounts or financing structures change. Different call rights, fees and repayment concentrations may justify further analysis. Bond duration addresses price sensitivity to yields, a separate question from the issuer’s borrowing-rate calculation.

Sources

Primary sources accessed November 13, 2026 (U.S. Central time). All bid calculations are hypothetical; the issuer notice is historical.

1. California Debt and Investment Advisory Commission: Debt Financing Guide, Appendix E glossary, entries Net Interest Cost, True Interest Cost and Underwriter’s Gross Spread; undated online entries.
2. Oregon State Treasury: Bond Computations, page 4, NIC formula and TIC definition; undated.
3. CDIAC: 2026 Municipal Debt Essentials, Session Three: Bond Math, Definitions, PDF page 22; September 15–17, 2026 program.
4. County of Monmouth, New Jersey: Series 2020 Reclamation Center Utility Note official statement, dated December 17, 2020, and Appendix C, C-1–C-2, Notice of Sale.