Pennsylvania Late Fee Calculator for Invoices: The 6% Default Rate, What a Written Agreement Unlocks, and the 1%-Per-Month Construction Rule

If you're looking for a Pennsylvania late fee calculator for invoices, you've probably got an overdue invoice in front of you and one question: what can I actually tack on? The short answer has three layers.

If you never agreed on a rate, Pennsylvania gives you 6% a year. If you put a rate in writing before the work started, you can charge a lot more than that. And if you're in construction, a separate statute hands you 1% per month whether the contract mentions it or not.

Here's the math for each, plus the clause to put on your next invoice. Standard disclaimer: I'm not a lawyer, and for anything big enough to litigate, talk to one.

The default: Pennsylvania's 6% legal rate of interest

Pennsylvania's legal rate of interest is 6% per year, set by 41 P.S. § 202. That's what applies when a contract says "with interest" but doesn't name a rate, and it's what a court awards as prejudgment interest on an unpaid contract debt when nothing was agreed.

So if your invoice or contract is silent on late fees, 6% simple interest is your fallback. Not 6% per month. Six percent per year, which works out to about 0.5% a month or 0.0164% a day.

Working the 6% math

The formula is simple interest:

Late interest = Invoice amount × 0.06 × (days late ÷ 365)

Take a $4,000 invoice that's 45 days past due:

$4,000 × 0.06 = $240 per year
$240 ÷ 365 = $0.66 per day
$0.66 × 45 days = $29.59

At 90 days late, it's $59.18. On a $12,500 invoice that's 60 days overdue, you're looking at $123.29.

That's the honest picture of the pa late payment interest rate when you haven't set your own: it's real money, it's legally solid, and it's not nearly enough to change anyone's behavior. A client who's ignoring a $12,500 invoice isn't going to be moved by $2 a day.

If you want to skip the arithmetic, the late fee calculator does simple interest at any rate and any number of days.

One catch: the 6% is mostly a courtroom number

You can add 6% interest to a reminder email even if your contract never mentioned it, but the client has no obligation to pay it until a judge says so. In practice, the statutory rate matters most when you're filing in magisterial district court (Pennsylvania's small claims venue, capped at $12,000) and asking for interest on top of the principal.

For everything before that point, the number that actually works is the one you wrote down in advance.

What a written agreement unlocks

Once a late fee or interest rate is in a contract or on an invoice the client accepted before the work, you're no longer stuck at the pennsylvania legal interest rate 6 percent default. You're enforcing an agreed term.

Pennsylvania's usury law (the Loan Interest and Protection Law) is aimed at loans, mostly consumer loans of $50,000 or less. A late charge on a business invoice for services isn't a loan, and courts generally treat it as liquidated damages, enforceable as long as it's a reasonable estimate of what the delay costs you and not an obvious penalty.

That's why 1.5% per month (18% a year) is the standard you'll see on commercial invoices across Pennsylvania. It's high enough to matter and low enough that nobody seriously argues it's punitive.

The same invoices at 1.5% per month

$4,000 invoice, 45 days late:  $4,000 × 0.015 × 1.5 months = $90
$4,000 invoice, 90 days late:  $4,000 × 0.015 × 3 months   = $180
$12,500 invoice, 60 days late: $12,500 × 0.015 × 2 months  = $375

Compare $180 against $59 on the same 90-day-late $4,000 invoice. Three times the leverage, and you only get it by writing one sentence down before the project starts.

A few rules that make the clause stick:

• It has to be in place before the work. Adding "1.5% monthly interest" to invoice number four when the first three said nothing is asking for a dispute you'll lose.
• Keep it in the reasonable range. Anything above 2% per month starts looking like a penalty, and a judge can strike it entirely rather than trim it down.

If you'd rather adopt a full written policy than a one-line clause, the late payment policy template is built for exactly that.

The construction exception: CASPA's 1% per month

If you do construction work in Pennsylvania, forget the 6% default. The Pennsylvania Contractor and Subcontractor Payment Act (CASPA, 73 P.S. § 501 and following) sets its own rules, and the pennsylvania contractor subcontractor payment act interest rate is 1% per month, or 12% a year.

Here's how it works:

• Payment deadlines are built in. If the contract is silent, an owner must pay a contractor within 20 days of the end of the billing period or delivery of the invoice, whichever is later. A contractor must pay a subcontractor within 14 days of receiving the owner's payment.
• Interest starts on day 8 after the due date. If payment isn't made within seven days of when it's due, interest runs from the eighth day at 1% per month or fraction of a month. That "fraction of a month" language matters: 32 days late counts as two months.
• You don't have to write it in. The statute applies unless the parties agreed otherwise, and since the 2018 amendments, the Act's protections can't be waived by contract.
• Bad-faith withholding adds a penalty. If the owner or contractor withholds payment without a good-faith reason, the court can add another 1% per month on top of the interest.
• The winner gets attorney fees. The substantially prevailing party in a CASPA claim is awarded reasonable attorney fees and expenses.

CASPA math on a real invoice

Say you're a subcontractor with a $12,500 invoice. The contractor was paid by the owner on March 1, so your payment was due March 15. It's now May 14, 60 days later.

Interest started March 23 (the eighth day). March 23 to May 14 is 52 days, which is one full month plus a fraction, so it counts as two months:

$12,500 × 0.01 × 2 months = $250 in interest

If a court finds the money was withheld in bad faith, the penalty doubles it:

$250 interest + $250 penalty = $500

Plus your attorney fees if you win. That's a very different conversation from "6% a year."

One important carve-out: CASPA doesn't apply to residential projects of six or fewer units under construction at the same time. A kitchen remodel on a single-family home falls back to the general rules above, so put your own clause in the contract.

The clause to put on your invoices

Copy this onto your invoice template and into your contract's payment terms. Pick one version.

General services (most freelancers and small businesses):

Payment is due within 30 days of the invoice date. Balances not paid by the due date accrue a late charge of 1.5% per month (18% per year) on the outstanding balance, calculated from the due date until paid in full. Client is responsible for reasonable costs of collection, including attorney fees and court costs.

Construction work covered by CASPA:

Payment is due within 30 days of the invoice date. Unpaid balances accrue interest at 1% per month or fraction of a month beginning on the eighth day after the due date, as provided by the Pennsylvania Contractor and Subcontractor Payment Act, 73 P.S. § 501 et seq. Contractor reserves all rights and remedies under the Act, including recovery of attorney fees and penalties for payments withheld in bad faith.

The second version doesn't create any new rights. It just tells the client you know the statute, which usually does more for your cash flow than the interest itself.

What actually gets the invoice paid

Interest is leverage, not a collection strategy. The clause gives you something to point at on day 31, and the numbers above tell you what to put on the reminder. But the reminder still has to go out, on time, and then again on day 45, and again on day 60.

If you want the exact wording for each of those stages, the late payment email toolkit has the sequence from friendly nudge to final notice, and the paragraph about interest slots into all of them.

So, to close the loop on the Pennsylvania late fee calculator for invoices question: 6% a year is what the state hands you for free, 1.5% a month is what a written clause gets you, and 1% a month plus penalties and fees is what CASPA gives construction trades automatically. Pick the one that fits, put it on the invoice before the work starts, and let automated payment reminder software handle the chasing.