Methodology

Outputs

With full access, the calculator produces on-screen totals and a full calculation schedule.

• Schedule lines correspond to each base-rate period (rate-change to rate-change).
• Each line shows: start date, end date, days, rate basis (base rate ± margin), interest for the period, and running total.
• Export formats: PDF and RTF.
• Fluctuating rates are synced automatically.

What this calculator does

This calculator performs arithmetical interest calculations only. It applies the selected interest basis to the user-provided inputs (dates, principal amounts, and any payment events) using fixed, disclosed rules. It does not exercise judgment, discretion, or interpretation.

The calculator does not determine which interest regime applies in any legal context.

Inputs and event handling

Calculations are performed using:

• Start date and end date (or judgment date where entered),
• Principal amount(s),
• Optional interim payments (amount and date), processed strictly in chronological order,
• Optional further losses (additional amounts and dates), also processed chronologically.

Where interim payments (receipts) are entered:

• Interest accrues on the outstanding principal up to (but excluding) the payment date;
• The user selects how the payment is credited:
  • Credit to Principal: the payment reduces principal directly;
  • Credit to Interest first: the payment reduces accrued interest first, with any remainder applied to principal.
• Interest then accrues on the adjusted balance;
• The breakdown table displays how each receipt was allocated between interest and principal.

Where further losses are entered:

• Interest accrues on the existing principal up to (but excluding) the date of the further loss;
• The further loss is added to the principal;
• Interest then accrues on the increased balance.

Rates and sources

Where a predefined rate is selected, the calculator uses the named benchmark exactly as published by the identified authority or source, together with any explicit margin stated in the rate definition.

Each predefined rate discloses:

• Benchmark name,
• Publishing authority or source,
• Last update date (the date the rate was last updated in our database, not the moment of retrieval),
• Any fixed margin (where applicable),
• Any prescribed calculation mechanics (for example, compounding).

Where a user-specified rate is selected, the calculator applies the rate exactly as entered.

The calculator does not infer margins, substitute benchmarks, or adjust rates based on outcomes.

Available rate options

The following rate bases are available, organised by jurisdiction:

United Kingdom

United States — Federal

United States — State-specific

Canada

Time basis and day count

Interest is calculated on a daily basis using actual calendar days.

• Each calendar day is counted once only;
• The divisor is 365, or 366 for leap years;
• No day-count convention (such as 30/360) is applied unless explicitly stated in the selected rate definition.

Compounding

Interest is calculated on a simple (non-compounding) basis by default, unless the selected interest basis explicitly requires compounding.

Where compounding applies, the compounding rule and interval are stated and applied mechanically.

For the Bank of England base rate + margin basis, when compound interest is selected, an optional 6-monthly compounding rests toggle is available:

• When enabled, accumulated interest is capitalised (added to principal) every 6 months from the calculation start date;
• When disabled, compounding occurs annually as standard;
• The methodology statement and exports reflect the selected compounding frequency.

Outputs and transparency

Outputs include:

• Headline totals (principal and total interest),
• A breakdown sufficient to reproduce the result from the stated inputs,
• A method statement describing the rules applied,
• Rate source and authority information (where a predefined rate is used),
• Explicit assumptions and exclusions.

Permanent links

Users may request a permanent link to a calculation output.

Each permanent link:

• Is guaranteed to be unique by the database;
• Uses an 8-character identifier generated from 48 bits of randomness, giving approximately 281 trillion possible combinations;
• Is not derived from the calculation inputs;
• Provides read-only access to the calculation as generated at the time the link was created.

The identifier contains no personal data, case identifiers, or user information.

The design makes valid links practically impossible to guess, while allowing links to be safely shared or cited as stable references to a specific calculation output.

Assumptions and exclusions

The calculator assumes:

• Inputs are accurate as provided;
• Dates reflect the intended interest period;
• Any selected rate basis is appropriate for the user's purpose.

The calculator does not:

• Provide legal advice;
• Determine entitlement to interest;
• Interpret statutes, contracts, judgments, or procedural rules;
• Apply equitable discretion or judicial adjustment;
• Resolve conflicts of law or classification issues.

United States — Additional Notes

In the United States, the calculator does not determine:

• Whether federal or state law governs interest;
• Whether interest is pre-judgment or post-judgment;
• Whether a statutory, contractual, or discretionary regime applies;
• Any issue arising from diversity jurisdiction or Erie-related characterisation.

Jurisdictional labels in the rate menu are used solely to identify commonly referenced benchmarks or published rates, not to prescribe applicability.

Privacy and data retention

We do not retain details of the calculations made by users. Calculations are only preserved if downloaded by the user or a permanent link is created.

We cannot trace permanent links to a particular user. We never ask for party names or the name of the email address holder. Privacy and confidentiality is therefore assured.