Late Payment Interest Calculator (Invoices)
How many days late is this invoice, and how much interest and late fees apply?
Spec sheet
late_interest_simple · v1.0.0Simple daily interest from the due date
- 01
days late = paid date − due date - 02
interest = amount × annual rate × days ÷ 365 - 03
monthly rate: annual = monthly × 12
- Late fees and interest must be allowed by your contract and local law; some states cap them.
- Simple interest; no compounding.
- Simple interest, actual/365formula
Worked example
computed by the same formula$2,500 due Aug 1, 1.5 %/month
- Invoice amount
- 2500
- Due date
- 2026-08-01
- Paid (or today's) date
- 2026-10-01
- Interest rate
- 1.5 %
- Rate is
- per month
- Flat late fee
- 0
- Total now due
- $2,575.21
- Days late
- 61
- Interest
- $75.21
Questions
How is the late payment interest calculator calculated?
›
Simple daily interest from the due date. days late = paid date − due date · interest = amount × annual rate × days ÷ 365 · monthly rate: annual = monthly × 12
Example: $2,500 due Aug 1, 1.5 %/month?
›
Total now due: $2,575.21; days late: 61.
What does this calculator not cover?
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Late fees and interest must be allowed by your contract and local law; some states cap them. Simple interest; no compounding.
Where do the numbers come from?
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Sources: Simple interest, actual/365. The formula is versioned (late_interest_simple v1.0.0) and covered by automated tests.
Is my data stored?
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No. The calculation runs in your browser and nothing is sent unless you choose to save a shareable link.
Also searched aslate payment interest calculator · invoice late fee · overdue invoice interest · how many days overdue · 1.5 percent per month late fee