Clean Price vs. Dirty Price Distinctions

Every bond trade involves two prices—the one you see quoted and the one you actually pay. The difference is accrued interest, and misunderstanding it leads to settlement surprises, miscalculated yields, and confused comparisons between bonds. MSRB Rule G-15 requires confirmations to break out accrued interest as a separate line item precisely because the gap between quoted price and settlement amount trips up investors regularly (MSRB Rule G-15). The practical antidote is straightforward: learn the one formula that connects clean price to dirty price, and verify it on every trade confirmation you receive.

TL;DR: The clean price is what dealers quote. The dirty price is what you pay. The difference is accrued interest—interest the seller earned but hasn't collected yet. One formula connects them: dirty price = clean price + accrued interest.

Why Bond Markets Quote Clean Prices (And Charge You Dirty Prices)

Bond markets worldwide quote clean prices—the price of a bond excluding any interest that has accumulated since the last coupon payment. This is also called the flat price. When you see a corporate bond quoted at 98.50, that means $985.00 per $1,000 of face value, with no interest included.

The price you actually pay at settlement is the dirty price (also called the full price, invoice price, or settlement price). This equals the clean price plus accrued interest. On that same bond quoted at 98.50, your actual cost might be $993.33 depending on when you settle.

Why the split? Because accrued interest changes every single day. If bond markets quoted dirty prices, every bond's quote would tick upward daily as interest accumulated—then drop sharply on coupon payment dates. That sawtooth pattern would make it impossible to tell whether a bond's value actually changed or whether you're just seeing the normal daily interest buildup. The point is: clean prices isolate genuine price movement from predictable interest accrual, making bond comparison and analysis far more practical.

FINRA's TRACE system, which reports over-the-counter fixed-income transactions, uses clean prices as the reporting convention for corporate and agency debt (FINRA TRACE). Prices are reported with precision up to six decimal places, and over 80% of trades are reported within five minutes of execution.

How Accrued Interest Builds Between Coupons

Most U.S. bonds pay interest semiannually—twice per year, creating six-month coupon periods. On each coupon payment date, the bondholder receives the full semiannual coupon and accrued interest resets to $0.00.

Between coupon dates, interest accumulates daily. The buyer at settlement compensates the seller for the interest the seller earned during the days they held the bond (but won't receive, since the next full coupon goes to whoever holds the bond on the record date).

The formula:

Accrued Interest = (Annual Coupon Rate ÷ Periods Per Year) × (Days Since Last Coupon ÷ Days in Coupon Period)

And then:

Dirty Price = Clean Price + Accrued Interest

Why this matters: accrued interest isn't a fee or a surcharge. It's a reimbursement. You're paying the seller for interest they earned but will never collect. When you receive the next full coupon, part of it effectively reimburses what you paid at settlement. The net interest you keep reflects only the days you actually held the bond.

The maximum accrued interest in any semiannual period approaches (but never equals) the full semiannual coupon. For a 5% coupon on a $1,000 bond, that ceiling is just under $25.00 per period. On a coupon payment date, accrued interest is exactly zero—and the clean price equals the dirty price.

Worked Example: Buying a 5% Corporate Bond (Step by Step)

Here's a concrete trade calculation using a standard U.S. corporate bond. Corporate bonds use the 30/360 day-count convention (every month counts as 30 days, every year as 360 days).

The bond:

• Face value: $1,000
• Annual coupon rate: 5% (semiannual payments)
• Semiannual coupon: $25.00
• Day-count convention: 30/360
• Last coupon payment: assume it was paid, and you're settling 60 days into the current 180-day coupon period

Phase 1: The Setup. You find a corporate bond quoted at a clean price of 100.000 (par). The screen shows $1,000.00. You place the order.

Phase 2: The Calculation. Settlement falls 60 days after the last coupon date (under the 30/360 convention, each semiannual period is exactly 180 days).

Accrued interest = ($25.00) × (60 ÷ 180) = $8.33

Phase 3: The Settlement Amount.

The practical point: Your trade confirmation will show these as separate line items—principal amount of $1,000.00, accrued interest of $8.33, and total settlement amount of $1,008.33. MSRB Rule G-15 mandates this breakdown appear on the front of every customer confirmation (MSRB Rule G-15).

What happens next: When the next coupon pays, you receive the full $25.00. But you only held the bond for 120 of the 180 days in that coupon period. Your net interest earned is $25.00 − $8.33 = $16.67, which is exactly (120 ÷ 180) × $25.00. The accrued interest payment at settlement ensures both buyer and seller receive interest proportional to their holding period.

Now adjust the scenario. If you had settled 120 days into the coupon period instead:

Accrued interest = ($25.00) × (120 ÷ 180) = $16.67

Your dirty price would be $1,000.00 + $16.67 = $1,016.67. The later you buy within a coupon period, the more accrued interest you owe—but the more of the next coupon effectively belongs to you.

Day-Count Conventions Determine the "Days" in Your Formula

The accrued interest formula is simple, but which numbers you plug in depends on the bond type. The day-count convention determines how you count "days since last coupon" and "days in coupon period."

Convention → Bond type → How it counts:

• 30/360 → U.S. corporate bonds, agency bonds, most municipal bonds → Every month has 30 days, every year has 360 days. A semiannual period is always exactly 180 days.
• Actual/Actual → U.S. Treasury bonds and notes → Uses the actual number of calendar days. A semiannual period ranges from 181 to 184 days depending on which months fall in the period.

The durable lesson: the difference between these two methods typically amounts to 0 to 3 days of interest per period, which translates to roughly $0.00 to $0.50 per $1,000 face value on a 5% coupon bond. That's small on a single bond—but it compounds across large positions, and using the wrong convention will produce a number that doesn't match your confirmation.

The test: before calculating accrued interest on any bond, identify the bond type first and match it to the correct day-count convention. Treasury? Actual/Actual. Corporate or municipal? 30/360. Getting this wrong is the single most common source of accrued interest discrepancies. (For a deeper look at these formulas and their edge cases, see Day-Count Conventions and Settlement Cycles.)

Settlement Timing Changes Your Accrued Interest

As of May 28, 2024, the standard U.S. settlement cycle is T+1—one business day after you execute the trade (SEC Rule 15c6-1 amendment). Previously, it was T+2. U.S. Treasuries already settled T+1 by market convention before the rule change.

Why this matters for clean vs. dirty price: you always calculate accrued interest to the settlement date, not the trade date. Under T+1, that's one business day of additional accrual beyond the day you click "buy."

This distinction catches people. If you trade on a Monday and settle on Tuesday, the settlement date (Tuesday) determines the accrued interest calculation. The move from T+2 to T+1 reduced the window during which accrued interest accumulates between trade and settlement by one full business day—a small but real difference on large positions.

Where Confusion Happens (And How to Avoid It)

Mistake 1: Comparing bond prices without adjusting for accrued interest. You see Bond A quoted at 99.50 and Bond B at 100.00, and assume Bond A is cheaper. But if Bond A is 150 days into its coupon period and Bond B just paid its coupon, the dirty prices might tell a different story. The point is: for apples-to-apples cost comparisons, always use dirty prices (or at minimum, factor in accrued interest).

Mistake 2: Using trade date instead of settlement date for accrued interest. Under T+1, this is a one-day error. On a 6% coupon bond with $1,000 face value, one day of accrued interest is approximately $0.17. Minor on a single bond—but systematic across a portfolio, and your confirmation won't match your calculation.

Mistake 3: Applying the wrong day-count convention. Using 30/360 on a Treasury bond (which uses Actual/Actual) or vice versa. The resulting difference is typically small ($0.00–$0.50 per $1,000 per period on a 5% bond), but it creates unexplained discrepancies that erode confidence in your calculations.

Mistake 4: Forgetting that bonds trade flat in specific situations. When a bond is in default, it trades flat—no accrued interest. The clean price equals the dirty price. Income bonds also trade flat. On coupon payment dates, accrued interest is zero, so clean and dirty prices are equal as well. If you see zero accrued interest on a confirmation and weren't expecting it, check whether the settlement date falls on a coupon date (normal) or whether the bond is in default (very different situation).

Mistake 5: Ignoring the confirmation breakdown. Your trade confirmation separately discloses the principal amount, accrued interest, and total settlement amount. MSRB rules require accrued interest to appear on the front of the confirmation document (MSRB Rule G-15). Always verify the accrued interest figure independently—errors in broker systems do occur, and catching them early is far simpler than correcting them after settlement.

(For more on how bond quotes are displayed and what each number means, see reading bond quotes and price conventions.)

When Clean Price Equals Dirty Price (Flat Trading)

There are exactly three situations where the clean price and dirty price are identical:

1. Settlement falls on a coupon payment date — accrued interest resets to $0.00
2. The bond is in default — trades flat by market convention
3. Income bonds — trade flat regardless of coupon status

In all other situations, dirty price > clean price, and the gap widens every day from coupon date to the next coupon date.

Your Confirmation Review Checklist

Essential (high ROI) — prevents 80% of settlement surprises:

• Verify the accrued interest amount — calculate it independently using the correct day-count convention and settlement date, then compare to the confirmation
• Confirm the settlement date — T+1 for most U.S. securities; ensure accrued interest matches this date, not the trade date
• Check the day-count convention — 30/360 for corporates and municipals, Actual/Actual for Treasuries
• Compare clean price to your expected quote — ensure the principal amount reflects the clean price you agreed to

High-impact (workflow items):

• Review the total settlement amount — confirm it equals clean price + accrued interest (no unexplained fees or discrepancies)
• Note the coupon payment dates — know when the next coupon pays so you can reconcile your net interest received
• Flag flat-trading situations — if accrued interest is $0 and settlement isn't on a coupon date, investigate immediately

Optional (useful for active bond traders):

• Track accrued interest across your portfolio — total accrued gives you a sense of unreceived income at any point
• Compare dirty prices when evaluating similar bonds — clean prices alone can mislead you about relative cost

Your Next Step

Pick one bond in your portfolio (or one you're considering). Look up the last coupon payment date, the coupon rate, and the day-count convention. Calculate the accrued interest as of today using the formula: (semiannual coupon) × (days since last coupon ÷ days in coupon period). Then compare your result to the accrued interest shown on your most recent statement or confirmation. If the numbers match, you understand the mechanics. If they don't, check whether you used the right day-count convention and the correct settlement date—those two errors account for nearly every discrepancy.