Most judgment calculators assume simple interest. Six states compound.

Interest on a money judgment does not stop the day you win. It runs from the day the judgment is entered until the day it's paid, at a rate the state fixed by statute, and in six states it compounds every year, on interest you already earned. Get the method wrong and the number you hand a court, or a debtor, is wrong.

Docket Interest is a free calculator for that number, for all 50 states and DC. You pick a state, type the judgment amount and the date it was entered, and watch the accrued interest and total due update as you type, with the elapsed days, the daily per-diem, and the governing statute shown alongside. Then one click turns the same figures into a formatted demand letter you can print and mail. The commercial tools that get this right are built for law firms and priced that way. A landlord or small-claims plaintiff collecting a few thousand dollars on their own usually has a spreadsheet and a guess instead.

Simple vs. compound is not an edge case

The mistake a naive calculator makes is assuming simple interest everywhere, because most states do accrue it that way, on the original principal only. But Colorado, Kentucky, Michigan, Oklahoma, South Carolina, and Texas compound annually, and over a multi-year judgment that gap is real money.

So the method is not hardcoded. Every state entry in the data table carries an explicit method field, "simple" or "compound", plus its statutory rate and the governing statute citation, and the calculation engine in src/interest.js dispatches on that field rather than running one formula and hoping. A compound state actually rolls the balance forward year over year; a simple state accrues flat against the principal. The state's rule decides, not the calculator's default.

The rates themselves get the same honesty. Some states fix a flat percentage by statute for years at a time. Others peg the rate to a Treasury yield or the prime rate and reset it annually, semi-annually, or quarterly. The data model records which kind each state is, and the date the rate was last verified, so a formula-based number can be flagged as variable instead of presented with the same false confidence as a rate that hasn't moved since 2019. You know which figures to confirm before you rely on one for a filing.

The whole thing is a data module

There is no backend here. The interest rules for every state ship as a data module inside the JavaScript bundle, and every calculation runs in your browser, so nothing you type ever leaves the page. That keeps it free to run forever and makes it a single static build that drops onto any host or subpath with no configuration. The demand-letter PDF is rendered client-side too, with jsPDF loaded lazily so the calculator stays light until you actually ask for a letter.

Here is the number that makes the compound point concrete. A $10,000 Kentucky judgment entered March 15, 2023, as of today, has run 1,213 days. Kentucky compounds annually under Ky. Rev. Stat. Ann. § 360.040, and the accrued interest comes to $2,141.18, for a total due of $12,141.18. Run the same principal and dates as flat simple interest and you land lower. That difference is exactly the amount a wrong method quietly adds to or subtracts from a demand.

One thing I want to be straight about: this is a calculation aid, not legal advice. Every rate ships with its statute citation precisely so you, or your attorney, can check it against current law, because rates change and some states' rules turn on judgment type, contract vs. tort vs. consumer debt, in ways a general calculator can't fully resolve. The tool is sourced, not authoritative, and it says so.

Try it

Open Docket Interest, pick your state, and type a principal and a judgment date. The total climbs as you type. Switch the state to Kentucky or Texas and watch the same numbers accrue on a compound curve instead of a flat one, then hit "Generate demand letter" to see the whole computation laid out on a letter you could drop in the mail. The data table and the engine are on GitHub, statute citations included.