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Invoice Late-Fee Calculator

Calculate flat late fees, percentage charges, and simple or compounded interest on overdue invoices with optional grace periods and fee caps.

Use this calculator to estimate late-payment charges based on the fee terms you enter. Select a flat fee, percentage fee, monthly interest rate or annual interest rate. Late-fee rules vary by contract and jurisdiction. This tool performs the math only and does not determine whether a fee is legally enforceable. Use the same currency throughout; the dollar sign is generic. Calculations run locally; invoice values are not saved or sent.

Estimate a late charge and updated invoice balance.

Enter any payments already received against the invoice. Clear or leave at zero if no payments have been received.

Days excluded from late-fee calculations before charges start. Clear or leave at zero if no grace period applies.

Optionally cap the total modeled late charge. Enter $0 to cap the charge at zero, or clear to leave it uncapped.

Example: one-time percentage charge

For a $1,000 invoice with $200 already paid, the outstanding principal is $800. A one-time 2% fee on that $800 balance adds $16, bringing the estimated amount due to $816 before any optional fee cap.

About this calculator

How late-charge methods differ

A flat charge is a fixed amount, a percentage fee is applied once to the outstanding principal, and interest methods vary with time and compounding.

  • For monthly interest, months are calculated as effective days overdue divided by 30. Simple interest multiplies the monthly rate by this unrounded fraction; monthly compounding applies the rate to each fractional period.
  • For annual interest, simple interest uses effective days overdue divided by 365. Daily compounding applies the entered annual rate as a daily rate.
  • A grace period reduces the chargeable days. Any maximum late charge applies to the full calculated late charge.
  • Payments reduce the original invoice amount before a fee or interest charge is calculated. The updated amount due is the remaining principal plus the modeled charge.
  • Approximate months overdue describes the full overdue period. Interest uses the days remaining after the grace period. Effective rate comparisons model 30 chargeable days for a month and 365 for a year, before any cap; they do not determine legally permitted rates.

Contract and jurisdiction limits

Late-fee calculations depend on the contract terms used. A flat fee, percentage charge and interest-based charge can produce materially different results.

  • This calculator uses the assumptions you enter and does not determine whether a particular fee or interest rate is permitted under your agreement or applicable law.
  • Late-fee rules vary by contract and jurisdiction. This tool performs the math only and does not determine whether a fee is legally enforceable.

Frequently asked questions

Does this calculator determine whether a late fee is allowed?

No. Fee rules can depend on your contract and jurisdiction. The calculator performs only the math for the amounts, rates and terms you enter; it does not provide legal advice or determine enforceability.

How are monthly and annual interest charges calculated?

Monthly interest uses effective days divided by 30; annual interest uses effective days divided by 365. Select simple interest or the available compounding method to model how the charge changes over time.

What if payments exceed the invoice amount?

Outstanding principal is treated as zero. The calculator will not show a negative principal or updated amount due.