About

In litigation and debt recovery, the calculation of interest is governed by strict statutory rules that vary by jurisdiction and time. This tool addresses the complexity of Variable Rate Calculations. Unlike simple fixed-rate calculators, legal interest often tracks economic indicators (e.g., the Prime Rate or Treasury Yields) that fluctuate weekly or quarterly. A debt originating in 2008 will accrue interest at drastically different rates than one in 2023.

This calculator handles both Pre-Judgment (interest from the cause of action to the verdict) and Post-Judgment (interest from verdict to payment). It utilizes a historical database of indices to apply the correct rate for each specific time segment of the accrual period.

Formulas

For simple interest jurisdictions:

I = P × r × d365

For US Federal compounded interest (iterated annually):

A_n = A_n-1 × (1 + r_n)

Where r_n is the specific statutory rate effective at the compounding interval.

Reference Data

Jurisdiction	Standard Rate Basis	Compounding
US Federal (28 USC § 1961)	1-Year Treasury Constant Maturity (Weekly)	Annually
California (State)	10% per annum (Civil Code § 3289)	Simple (Usually)
New York (State)	9% per annum (CPLR § 5004)	Simple
Texas (State)	Prime Rate (Floor 5%, Ceiling 15%)	Simple or Compounded
United Kingdom (Statutory)	Base Rate + 8%	Simple

Frequently Asked Questions

This tool defaults to a 365-day year (Actual/365), which is the standard for US Federal courts and most state jurisdictions. Some specific contracts may specify a 360-day year, but statutory law typically favors the calendar year.

The calculation counts the actual number of days between dates. If the period includes February 29th, that day is included in the numerator d.

Yes. The database contains historical Federal Treasury yields and Prime rates going back over 20 years. The tool will look up the specific rate applicable on the date of entry and (if variable) adjust accordingly.