Late payment fees and interest: how they're calculated

Late payment fees are one of those clauses everyone glances at and nobody calculates. The language is short, usually a sentence or two, and the numbers look small. "Interest shall accrue on overdue amounts at 1.5% per month." Fine. Moving on.

Then an invoice sits unpaid for 120 days, someone finally runs the math, and the late fee is 18% of the original amount. Nobody budgeted for that. The AP team didn't realize the clause was compounding. The buyer's legal team is arguing the cap doesn't apply. And suddenly a $50,000 invoice is a $59,000 dispute.

This walkthrough covers how late payment fees actually get calculated in B2B contracts, the three common structures, where the compounding math bites, and what usury caps actually do in the US and EU.

Why late payment fees exist

A late payment fee does two things. First, it compensates the supplier for the time value of money, cash that's supposed to arrive in 30 days but arrives in 90 is cash the supplier had to finance from somewhere else, usually a revolver at 8–10% or invoice factoring at 12–15%. Second, it creates a financial incentive for the buyer to prioritize the invoice over other AP line items.

Without a late-fee clause, a supplier's only remedy for a late payment is to stop performing, cancel the contract, or sue. All three are expensive and none recover the time cost of the delayed cash. The late-fee clause turns a vague threat into an automatic line item.

Late payment fees are not the same as liquidated damages. Liquidated damages are a pre-agreed estimate of loss for a specific breach, typically used for missed delivery dates or SLA failures. Late fees are compensation for slow cash, governed by different legal standards and different enforcement patterns.

The three common fee structures

Most B2B contracts use one of three structures, sometimes in combination.

1. Flat late fee

A fixed dollar amount charged once when an invoice crosses the overdue threshold. Common language: "A late fee of $75 will be applied to any invoice not paid within 30 days of the due date."

Flat fees are most common in:

• Small-vendor contracts (freelancers, boutique service firms)
• Subscription SaaS billing
• Recurring utility-style services

The advantage is simplicity, the buyer knows the exact cost of being late, the supplier knows the exact recovery. The disadvantage is that $75 is trivial on a $100,000 invoice, so flat fees don't really deter large-invoice delinquency.

2. Percentage late fee

A one-time percentage of the overdue balance, typically 1–5% of the unpaid amount. Common language: "Overdue invoices are subject to a late payment fee equal to 1.5% of the outstanding balance."

This structure scales with invoice size, which makes it effective on larger contracts. The downside: it's a one-time hit, so a buyer who's 30 days late pays the same fee as one who's 180 days late. That removes the incentive to pay faster once the fee has already been assessed.

3. Interest on overdue amounts

The most common structure in mid-market and enterprise contracts. Interest accrues on the unpaid balance at a monthly or annual rate until paid. Common language: "Interest shall accrue on all overdue amounts at the rate of 1.5% per month, or the maximum rate permitted by applicable law, whichever is lower."

Interest-based fees scale with both invoice size and delay duration, which makes them the most economically accurate compensation for slow cash. They're also the most likely to produce surprises when nobody runs the math.

How the math actually works

The headline rate, say 1.5% per month, seems mild. Here's what it looks like on a real invoice.

A $50,000 invoice, 90 days overdue, simple interest at 1.5% per month:

• 3 months × 1.5% × $50,000 = $2,250 in interest

At 180 days overdue: $4,500. At one year: $9,000, or 18% of the original invoice.

The 1.5%-per-month figure is chosen for a reason: multiplied by 12 it equals 18%, which sits just below most US state usury caps for commercial transactions. It's the maximum most suppliers can safely charge.

Simple vs. compound interest

Most B2B late-fee clauses use simple interest, interest accrues on the original overdue balance, not on prior interest. That's the default even when the contract doesn't specify, because compound interest on commercial debt is either capped or prohibited in many US jurisdictions.

A minority of contracts specify compound interest, usually monthly. On a $50,000 invoice at 1.5% monthly compounded for 12 months, the total is roughly $9,800, only $800 more than simple interest, because 18% annual is low enough that the compounding effect is modest. At higher rates the difference grows quickly.

Teams that audit late-fee language usually check three things: whether the rate is monthly or annual, whether it's simple or compound, and whether there's a usury cap reference.

The "maximum permitted by law" clause

Look for language like "or the maximum rate permitted by applicable law, whichever is lower." That phrase does two important things.

First, it protects the supplier from an unenforceable clause. If a contract specifies 2% monthly (24% annualized) and applicable law caps commercial interest at 18%, a bare clause might be struck down entirely, leaving the supplier with no late fee at all. The "maximum permitted" language preserves the clause at the highest enforceable rate.

Second, it shifts the burden. The buyer can't argue the rate is illegal, the clause already concedes that the rate is whatever the applicable law allows, no more.

In the US, commercial usury caps vary by state and typically range from 12% to 24% annualized for non-consumer transactions, with several states having no cap at all for commercial loans above certain thresholds. Most commercial late-fee clauses in the 12–18% range are enforceable in all 50 states.

In the EU, the Late Payment Directive (2011/7/EU) sets a statutory baseline: the European Central Bank's reference rate plus at least 8 percentage points, plus a €40 recovery fee per invoice. That floor applies to commercial transactions even when the contract is silent on late fees, and it applies unless the contract contains a "grossly unfair" departure from the standard.

What triggers the fee

A late payment fee clause typically activates on one of three triggers:

• Automatic. The fee accrues the moment an invoice is overdue, with no notice required. Most common in mid-market and enterprise contracts.
• Upon notice. The supplier must send a written late notice, usually giving a cure period (often 10–15 days) before fees start accruing.
• After a grace period. The invoice is due on day 30 but no fee accrues until day 45 or 60. This is common in long-standing vendor relationships where occasional slippage is expected.

The trigger matters when disputes happen. If a buyer disputes a $50,000 invoice on day 20 and the dispute isn't resolved until day 90, does the clock keep running? Automatic-trigger clauses say yes unless explicitly paused for disputed amounts. Notice-based clauses often require a separate late notice for each dispute resolution.

Well-drafted clauses separate undisputed from disputed amounts: late fees accrue on undisputed portions from day 31, but disputed amounts don't accrue fees until the dispute is resolved in the supplier's favor.

Collection costs and attorney's fees

A separate but related clause often appears alongside late fees: collection costs.

"In addition to late payment fees, Customer shall reimburse Supplier for all reasonable costs of collection, including attorney's fees and court costs."

This clause matters when an invoice goes to collections or litigation. Without it, the supplier bears its own legal costs and only recovers the invoice amount plus interest. With it, the supplier can add legal fees to the claim, which can double or triple the effective recovery on a meaningful invoice.

Enforcement varies by jurisdiction. US courts generally uphold reasonable collection-cost clauses in commercial contracts. Some jurisdictions require the clause to be mutual (both parties can recover costs) for it to be enforceable against either.

Late payment fees in subscription billing

SaaS and other recurring-revenue contracts have a slightly different pattern. Instead of per-invoice late fees, many contracts combine:

• A short grace period (typically 10 days past due)
• A suspension clause allowing the vendor to pause service after the grace period
• Reinstatement fees to resume service
• Interest on the overdue amount until paid

The suspension mechanism is usually more effective than interest at getting paid, losing access to production software tends to focus AP's attention faster than a 1.5% monthly accrual. But suspension has to be contractually available; without it, the vendor can send late notices indefinitely without leverage.

The bottom line

Late payment fees are small numbers that compound into real money. The 1.5%-per-month rate that looks trivial on a single invoice becomes 18% annualized on a persistently overdue account, and once collection costs enter the picture, the math gets worse.

The clauses that work in practice are the specific ones: a named rate, a named trigger, a named cap, and clear handling of disputed amounts. Contracts that just say "late fees may apply" aren't late-fee clauses, they're placeholders. When cash actually runs late, the fine print is what decides whether the supplier recovers their financing cost or writes it off.