Retainer invoice: how to bill a monthly retainer, with an example

The short answer

A retainer invoice bills a fixed monthly fee for an agreed amount of work, sent at the start of the month the fee covers, before the work is done. It has the period it covers in the description, the hours or the scope included, a due date in the first week or two of that month, and a separate line for any hours used beyond the allowance in the previous month.

Same day every month, same format every month. The client's accounts team should be able to pay it without reading it.

What a retainer actually is

People use the word for three different arrangements, and the invoice looks different for each:

Most of my retainers have been the first kind: a studio that needs 3D product renders every month and does not want to brief someone new each time. The hours version is the easiest to invoice and the easiest to argue about, so most of this guide is about that one.

Write down which type you have agreed before the first invoice goes out. A retainer that one side thinks is "20 hours" and the other thinks is "whatever we need this month" will end badly.

Bill in advance

Invoice on the first working day of the month for that month. Not at the end, after the work.

Billing in advance is what makes a retainer different from hourly billing with a regular client. The client is paying to reserve your time, and you are planning your month around it. If the client pays late, you find out before you have done the work, not after.

Payment terms of 7 to 14 days are normal for a retainer invoice. If the client's payment run is monthly, ask which day it runs and send the invoice a few days before it.

Unused hours

Decide this before you start, and put it in the agreement. The common options:

• Use it or lose it. Unused hours do not roll over. This is the standard, because the client paid to reserve your time and you kept it free. It feels harsh to clients who had a quiet month, so say it plainly at the start.
• Limited rollover. Unused hours carry into the next month only, up to a cap such as 25% of the allowance, then expire.
• Full rollover. Hours bank up indefinitely. I would avoid it. After a slow quarter you can owe a client a month of work you were paid for long ago.

I use limited rollover for clients I like working with and use it or lose it for everyone else. Either way, show the balance on each invoice so it never becomes a surprise.

Overage

When the client uses more than the allowance, bill the extra hours in arrears, on the next month's retainer invoice, at an agreed hourly rate. Some freelancers charge overage at a higher rate than the retainer works out to, because the retainer price includes a discount for the guaranteed volume. That is fair, but agree it up front.

If you are not sure what hourly rate the retainer should be built on, the hourly rate calculator works out what you need per hour to match a salary, and the day rate calculator does the same from a target income.

For overage, keep a simple log of what you did and how long it took. You will need it the first time a client asks why the invoice is higher this month. If you bill other clients by the hour too, the guide on invoicing hourly work covers how much detail to show.

A worked example

The agreement: 20 hours a month for 2,000.00 a month, which works out to 100.00 an hour. Overage at 110.00 an hour, billed the following month. Unused hours roll over for one month only, up to 5 hours.

In September the client used 24 hours, so 4 hours of overage. The October invoice:

Then a short status block under the table:

September: 24 of 20 hours used, 4 hours overage billed above.
Rollover into October: none.
October allowance: 20 hours.

Due 14 October 2026. Please use invoice 2026-047 as the payment
reference.

Had the client used only 16 hours in September, there would be no overage line, and the status block would say "Rollover into October: 4 hours (expires 31 October)". The retainer line stays the same either way. Do not reduce the fee for an under-used month; that turns the retainer back into hourly billing.

What the description line should say

The line item is the part that gets read in six months when someone audits the client's supplier payments. It should stand on its own:

• The word retainer.
• The period covered, as a month and year, not "this month".
• What the fee covers: hours or scope.
• The agreement it comes from, with a date.

Everything else a normal invoice needs is still required, from your details to the tax lines. The checklist is in what to put on an invoice. If you charge VAT, the retainer is usually taxed like any other advance payment: the tax point is generally when you issue the invoice or receive the payment, whichever is first. HMRC's VAT Notice 700 and your own tax office have the details. This is general information, not tax advice.

Wording for the agreement

The invoice only works if the agreement behind it is clear. Something like this, adapted to your work:

Retainer: 2,000.00 per calendar month for up to 20 hours of work,
invoiced on the first working day of each month and due within
14 days.

Unused hours roll over to the following month only, up to 5 hours,
and then expire. Hours beyond the monthly allowance are billed at
110.00 per hour on the next month's invoice.

Either party may end the retainer with 30 days' written notice.
The fee for the notice period remains payable.

The notice period matters more than it looks. Without one, a client can stop paying on the first of the month, and you have already turned down other work for that month. The contract checklists list the other clauses a freelance agreement should have.

Keep it running

• Send it on the same day every month. A late retainer invoice tells the client the retainer is optional.
• Use one invoice number sequence for everything, retainers included.
• Review the allowance every three to six months. If the client is in overage every month, propose a bigger retainer. If they are always under, they will cancel eventually, so raise it first.
• If a retainer invoice is overdue, pause new work after a reminder or two, as the agreement says. The guide on following up an unpaid invoice has the sequence.

Questions