Money Math
Simple Interest Calculator
Calculate simple interest on any principal amount over any time period.
This is an estimate for settlement discussion purposes only. It is not legal advice and does not account for all possible factors.
How it works
Judgments, prejudgment interest, and many settlement terms accrue simple — non-compounding — interest. This calculator computes it for any principal, any annual rate, and any period entered in days, months, or years.
The formula is the classic one: principal × annual rate × time in years. Time is converted using a 365-day year. Because the interest never compounds, the result grows in a straight line — interest on a judgment does not itself earn interest under most statutes.
Worked example
A $100,000 judgment at 8% simple interest for 18 months: $100,000 × 0.08 × 1.5 = $12,000 of interest, for a total of $112,000. The same calculation works for prejudgment interest measured in days — 245 days at 8% on $100,000 is $100,000 × 0.08 × (245/365) ≈ $5,369.86.
When to use it
Use it to compute prejudgment or post-judgment interest for a demand, to check the other side's interest math, or to price the time element of a structured deal where interest is part of the negotiation.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it accrues in a straight line. Compound interest is calculated on principal plus previously accrued interest, so it accelerates. Statutory judgment interest is usually simple; if your deal compounds, use the Payment Over Time Calculator instead — its amortization math compounds each period.
Does this use a 360-day or 365-day year?
365 days. Some courts and financial institutions use a 360-day convention, which produces slightly higher interest for the same nominal rate. If your jurisdiction uses 360, expect the numbers to differ by about 1.4%.
What interest rate applies to a judgment?
It varies by jurisdiction and case type — some states set a fixed statutory rate, others float with a benchmark, and federal post-judgment interest follows the weekly one-year Treasury yield. Look up the rate for your jurisdiction; this tool accepts any rate, including rates above the slider via typing.