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Invoice Late Fee Calculator

Compute a simple-daily-interest late fee on an overdue invoice. US-default 18% APR with a configurable grace period and optional flat fee.

Invoice

Amount owed before late fees.

Dates

When the invoice was due.

Defaults to today; change to project a future date.

Rate
Rate type

Days after due date before fees accrue.

One-time fee added on top of interest.

Total due

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Invoice amount
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Days overdue
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Flat fee
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Simple interest
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Late fee total
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Total due
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Export
How this is calculated

The fee uses the simple-daily-interest formula every US incumbent cites: split the annual percentage rate over 365 days and multiply by the number of days past the grace period. 

days_overdue       = max(0, payment_date − due_date − grace_period)
simple_interest    = invoice × (annual_rate / 365) × days_overdue
late_fee           = (days_overdue > 0) ? flat_fee + simple_interest : 0
total_due          = invoice + late_fee

The default 18% APR (= 1.5% / month) matches US small-business norms (FreshBooks, Housecall Pro). Eight US states cap late-fee rates; when the computed APR exceeds 18%, we surface a soft "check your state" warning rather than hard-coding 50 statutes.

The calculator uses simple interest, not compounding. Compound mode is documented as a future enhancement.

How we choose and check our formulas