Blog

How Interest Accrues on a Tax Lien Certificate, Day by Day: The Worked Math

The three definitions: base, rate, and time

Start with the base. Interest accrues on the obligation the statute says it accrues on. Colorado's dataset rule states it in so many words: interest accrues only on taxes and subsequent taxes, not on any premium bid. Florida's certificates accrue on the certificate amount at the rate bid. In neither system does interest run on your research costs, your travel, your foreclosure or deed-application expenses (unless a specific statute folds a cost into the redemption amount), or the premium you paid to win. When you see a rate quoted for a state, the first underwriting question is never 'how high is it?' It is 'what does it multiply?'

The rate definition differs by system in ways that change the day-by-day math not at all — and the expected total a great deal. A bid-down rate (Florida: from 18% down in 0.25% increments) is set by your competition. A county-fixed rate (Maryland: 6% by statutory default, higher where local law sets it, capped at 10% for owner-occupied residential property) is set before you arrive. A formula rate (Colorado: federal discount rate plus nine points, set each October 1, fixed for the certificate's life) is set by an index. The accrual arithmetic that follows uses Colorado's 14% — the dataset's rate for certificates in the October 1, 2025 to September 30, 2026 period — and Florida's 18% maximum, because both are dataset figures rather than illustrations invented for this article.

Worked example: a Colorado certificate at 14%, day by day

Take a Colorado certificate on $12,000 of taxes, at the dataset's 14% rate, with no premium for the first pass. Annual interest is $12,000 × 0.14 = $1,680. Spread evenly as simple interest across a 365-day year — the illustration method used throughout this article; the selling treasurer's calculation controls the actual redemption figure — the daily accrual is $1,680 ÷ 365 = $4.60 per day, to the nearest cent ($4.6027 before rounding). From there the timeline is multiplication: 30 days accrues 30 × $4.6027 ≈ $138.08. Ninety days: 90 × $4.6027 ≈ $414.25. One hundred eighty days: 180 × $4.6027 ≈ $828.49. A full year: $1,680 exactly, closing the loop.

Read the shape of that table, not just its numbers. Accrual is linear: the certificate earns the same $4.60 on day 300 as on day 3, because simple interest does not compound inside the certificate. Your annualized return on the redemption branch is therefore hostage to when redemption happens only through the costs around the certificate — fixed costs (buying, servicing, any legal spend) are spread over however many days the owner gives you. A redemption at day 30 pays $138.08 of interest against all of your fixed costs; the same certificate at day 365 pays $1,680 against roughly the same fixed costs. Early redemption is not bad — capital comes back for redeployment — but any model that assumes a full year of interest on every certificate is assuming the one timeline owners are least obliged to provide.

Now add the premium: the dollars that sit at 0%

Repeat the Colorado example with competition. Suppose winning the same $12,000 certificate took a $900 premium, so $12,900 left your account. The accrual table does not change by one cent: interest still runs on $12,000 of taxes at 14%, because under the dataset rule interest accrues only on taxes and subsequent taxes, and the premium is not refunded on redemption either. What changes is your denominator. If the owner redeems at day 180, you receive $12,000 + $828.49 = $12,828.49 against $12,900 spent — less than you paid out, on a certificate carrying a '14%' rate. At day 365 you receive $13,680 against $12,900, a gain of $780 on $12,900 ≈ 6.0% for the year — respectable, and less than half the headline rate.

This is the single most consequential piece of arithmetic in lien buying, and it is why premium decisions belong in the underwriting file rather than the auction room. The break-even day for that certificate is the day accrued interest equals the premium: $900 ÷ $4.6027 ≈ day 196. Redeem before it and the redemption branch loses money outright; redeem after it and the branch profits, thinly at first. Every certificate bought with premium has such a day, computable before the sale from the dataset rate and your maximum premium. If you cannot state a target certificate's break-even day in the redemption branch, you do not yet have a maximum premium — you have a hope with a paddle.

A second dataset rate: Florida's 18% and the 5% floor

Run the same machinery on a Florida certificate struck to the county and bought over the counter at the dataset's 18% maximum — the rate at which unsold Florida certificates are struck. On a $10,000 certificate: annual interest is $10,000 × 0.18 = $1,800; daily, $1,800 ÷ 365 ≈ $4.93; ninety days ≈ $443.84; one hundred eighty days ≈ $887.67. Compare a certificate bid down at auction to 9%: annual interest halves to $900, daily accrual to ≈ $2.47. Same county, same parcel size, half the accrual — the bid-down auction's price is paid out of every subsequent day's interest, which is why low winning rates deserve the same skepticism as high premiums.

Florida adds a floor the Colorado example lacks: a 5% mandatory minimum on redemption applies to most certificates. Five percent of $10,000 is $500. On a certificate bid down to a very low rate and redeemed early, the minimum — not the day count times the bid rate — can set the interest figure, per the dataset's description. Treat the mechanics as a verification item for any Florida purchase: confirm how the minimum applies to your certificate and your redemption scenario with the selling authority, because the difference between 'rate times days' and 'statutory minimum' is exactly the kind of difference that only appears at the redemption counter, in the other party's favor, if you assumed instead of asking.

What the day count cannot tell you

Three cautions close the math. First, accrual in this article is shown as simple interest spread evenly over 365 days for illustration; individual systems may calculate interest by month, by fraction of year, or under their own statutory convention, and the selling authority's redemption figure is the authoritative one. Never quote a redemption payoff to an owner or a court from an article's arithmetic — including this one. Second, subsequent taxes change the base mid-stream: when a holder pays a later year's taxes to protect the position, that payment joins the interest-bearing base from its own date, so a real certificate's accrual is a stack of tranches, each with its own day count, not one clean line.

Third, and most important: interest is only one branch of the outcome tree. The day-by-day table describes redemption. It says nothing about the deed path — its costs, its auctions, its title work — and nothing about the certificates that expire, are voided for missed deadlines, or tie up capital for years at rates that looked better in the brochure. LienToDeed publishes state rates and sale terms alongside its lists so the base, rate and timeline can be checked per parcel before bidding (free for basic alerts, Bidder $49/mo, Pro $99/mo). The arithmetic itself fits on an index card: base times rate times days over 365. The discipline is refusing to bid until you have run it — premium, break-even day and all — on the specific certificate in front of you.