As of 13 August 2026, AI can calculate late payment interest on an overdue invoice.

What to actually do

1. Hand it to a person

   The route this page recommends

   A person who owns the outcome does this end to end, worth it when the failure is dear.

2. Use a tool built for this

   Second choice

3. Do it yourself

   The distant third

   A chat interface, chat-fluent skill, and roughly 15 minutes until you can act on the result.

   How to actually do it

   1. Open the overdue invoice, the customer contract or order terms, and your bookkeeping or invoice ledger.
   2. Gather the invoice amount, VAT treatment, invoice date, agreed payment terms, contractual interest wording, due date, payment dates and amounts, and the date up to which interest should be calculated.
   3. Open the relevant GOV.UK guidance and record the applicable statutory rate and the period for which it applies; paste the source wording or link into the prompt rather than asking the model to guess a current rate.
   4. Paste the prompt and the gathered facts into a chatbot, including every part-payment and any uncertainty about the due date or interest clause.
   5. Export or copy the model's table into a spreadsheet and independently recalculate each period using the outstanding balance, number of days and supplied rate.
   6. Compare the result and assumptions with the invoice ledger, contract and GOV.UK guidance, then have an accountant or solicitor check any disputed entitlement before you send the interest demand.

   Prompt

   Calculate late payment interest on this UK business invoice using only the facts and official rate information I provide.
   
   Invoice amount: [amount and currency]
   Invoice date: [date]
   Contractual payment terms: [terms]
   Due date: [date]
   Date payment was received, or calculation end date: [date]
   Payments received before the final payment: [dates and amounts, or none]
   Contractual interest clause: [paste it, or state none known]
   Applicable statutory rate information from GOV.UK or another official source: [paste the rate, the rate period and the source]
   Whether the customer and supplier are businesses: [details]
   
   First, state whether the supplied facts appear sufficient to calculate the amount. Do not decide that statutory interest is legally available if the facts do not establish that; instead, flag the missing fact or legal question. Calculate separate scenarios for contractual interest and statutory interest where both may be relevant. Show each rate period, outstanding balance, number of days, formula, daily amount, rounding method and subtotal in a table, then give the total interest and total amount due. State whether the calculation uses the invoice amount including or excluding VAT, and do not assume this where the documents do not say. Use no rates, dates or legal facts that I have not supplied, and identify every assumption. Tell me which figures I should compare with the invoice ledger and official GOV.UK guidance before sending a demand. This is not professional advice.

   Open it prefilled in ChatGPT or Claude, or copy it into Gemini, which takes no prefill link.