Late Payment Interest Calculator
Enter your invoice details to estimate interest and total amount due on an overdue payment.
How late payment interest works
Late payment interest is an additional amount charged when an invoice is paid after its due date. Businesses use it to compensate for delayed cash flow and to encourage on-time payments. This calculator helps you estimate that extra cost quickly, whether you use simple daily interest or daily compounding.
Basic formula (simple daily interest)
For most invoices, the quick formula is:
Interest = Principal × (Annual Rate ÷ Day Basis) × Late Days
Where:
- Principal = original invoice amount
- Annual Rate = agreed yearly rate (for example, 8%)
- Day Basis = 365 or 360, depending on contract/policy
- Late Days = days overdue after any grace period
How to use this calculator correctly
- Enter the invoice amount and annual rate from your contract or policy.
- Choose either manual late days or provide due date and payment date.
- Add a grace period if your terms allow several days before interest starts.
- Select Simple Daily (most common) or Daily Compounding (more aggressive).
- Include a fixed late fee if applicable.
If both dates are entered, the calculator uses the date difference and fills the late-days calculation automatically.
Example calculation
Suppose an invoice is $5,000, annual rate is 10%, payment is 30 days late, and no grace period applies.
- Daily rate = 10% ÷ 365 = 0.00027397
- Interest = 5,000 × 0.00027397 × 30 = $41.10 (approx.)
- If no fixed fee is charged, total due = 5,000 + 41.10 = $5,041.10
Simple vs compound late interest
Simple daily interest
Interest is calculated only on the original invoice amount. This is straightforward, easy to audit, and widely used in invoice terms.
Daily compounding
Interest is calculated on principal plus accumulated interest each day. Over long delays, this can produce a noticeably higher amount due.
Always follow your contract terms and local regulations when choosing the method.
Common mistakes to avoid
- Using an annual rate that is not documented in your invoice terms.
- Forgetting the grace period and overcharging too early.
- Using 360-day basis when your agreement assumes 365 days (or vice versa).
- Adding fees that are not legally allowed in your jurisdiction.
- Applying compound interest when only simple interest was agreed.
Best practices for businesses and freelancers
- Include payment terms, late interest rate, and fees clearly on every invoice.
- State when interest starts (immediately after due date, after grace period, etc.).
- Send reminders before and after due dates.
- Keep a clear record of due dates, payment dates, and calculations.
- Offer easy payment options to reduce delays.
Important legal note
Late payment rules vary by country, state, and industry. Some places cap interest rates or restrict administrative fees. This calculator provides an estimate for planning and communication; it is not legal or tax advice.
Tip: If you are collecting a substantial overdue amount, confirm your terms with a qualified legal or accounting professional.