Retainers are the backbone of a stable agency: predictable revenue, planned capacity, and clients who stick around. But they are also where margin quietly leaks, because a retainer makes it easy to keep working and easy to forget to invoice. The recurring truth holds here too: the biggest leak is billable time that never reaches an invoice, and a retainer can hide that leak for months.
The short answer
Retainer billing is a recurring arrangement where a client pays a fixed fee each period for an agreed allowance of hours or scope. You bill the same amount on a set cycle, track logged hours against the included allowance, and watch the gap between what was paid for (the drawdown) and what was delivered (logged hours) so you neither over-service nor leave value unbilled.
How a retainer is structured
| Element | What it means |
|---|---|
| Fee per period | The fixed amount the client pays each cycle |
| Included hours | The allowance of work the fee covers (the drawdown) |
| Period type | How often the retainer resets and bills |
| Logged hours | Time your team actually records against it |
| Alert threshold | The usage percentage that triggers a heads-up |
For the formal definition, see the retainer billing glossary entry.
Retainer period types
Not every retainer runs monthly. Hour Cap supports a range of period types so the billing cycle matches the agreement:
- Weekly: resets and bills every week, good for high-tempo support work.
- Fortnightly: a two-week cycle.
- Monthly: the most common cycle for agency retainers.
- Quarterly: a three-month cycle for strategic or advisory work.
- Custom (N days): any fixed number of days when the relationship does not fit a calendar cycle.
Each period carries its own included hours, so usage is measured within the current cycle and resets cleanly when the next one begins.
Drawdown vs logged hours: the number that protects your margin
This is the heart of retainer management. The drawdown is what the client has paid for in the period. The logged hours are what you actually delivered. The relationship between them tells you everything:
Retainer health
Hours remaining = Included hours, Logged hours
Positive and large near period end: you may be under-servicing.
Near zero on schedule: you are on track.
Negative: you are over-servicing and giving away margin.
Hour Cap shows hours used and hours remaining for the current period, so the picture is live, not a month-end surprise. For how these hours relate to invoicing, see billable hours.
Over-servicing: the silent margin killer
Over-servicing is the classic retainer leak. The work keeps flowing, nobody is watching the clock, and by period end you have delivered 60 hours on a 40-hour retainer. That extra 20 hours is unbilled billable time: the exact leak this whole cluster is about. The defense is an alert threshold. Set it to, say, 80 percent and you are warned as usage approaches the allowance, with time to scope additional work, raise a separate invoice, or have the conversation before the margin is gone.
Under-servicing: the quieter risk
The opposite problem is just as real. If you consistently log far fewer hours than the retainer includes, the client starts asking what they are paying for. Visibility into hours remaining lets you proactively fill the gap with valuable work, protecting the renewal rather than waiting for the awkward review.
Invoicing a retainer through Xero: the Xero-native angle
Here is where retainer billing in Hour Cap is genuinely different. You do not export a report and rebuild the invoice by hand in your accounting system. You invoice a retainer period directly to Xero as a draft invoice.
On the Team plan or above, with Xero connected, you bill the period straight from the retainer. The invoice lands in Xero as a draft so you review it before sending, with descriptions intact and the line items grouped the way you choose. Nothing is authorised or emailed automatically. See Xero retainer billing and the general flow in sending tracked time to Xero as draft invoices.
Invoicing past periods so nothing slips
This is the feature that directly attacks the leak. With retainers, it is dangerously easy to let a period pass without raising the invoice, especially when work is busy. Hour Cap lets you invoice past, uninvoiced periods (on the Team plan and above), so a cycle that was missed during a hectic month can still be billed. No period quietly falls through the gap unbilled. That single capability often pays for itself, because a forgotten retainer period is a whole cycle of revenue gone.
A worked example
An agency runs a monthly retainer: $6,000 for 40 included hours, a billable rate of $150, and an alert threshold at 80 percent.
- Month one: the team logs 38 hours. On track. The $6,000 is invoiced to Xero as a draft, reviewed, and sent.
- Month two: a big push. At 32 hours, the 80 percent alert fires. The account lead sees only 8 hours remaining with two weeks to go, flags it, and scopes the overflow as a separate piece of work. Without the alert, the team would have logged 55 hours: 15 unbilled hours, $2,250 of margin gone.
- Month three: chaos in the studio means the period is never invoiced on time. Two weeks later, someone catches it and invoices the past period to Xero anyway. The $6,000 is recovered instead of lost.
Across one quarter, the alert saved $2,250 of over-servicing and the past-period invoice saved $6,000 of forgotten revenue. That is the leak, closed.
Retainer billing vs hourly billing
| Retainer | Hourly | |
|---|---|---|
| Revenue | Predictable, recurring | Variable, per project |
| Main risk | Over-servicing, forgotten periods | Unbilled and written-down hours |
| Key metric | Hours remaining vs included | Utilization and realization |
| What protects margin | Alert thresholds, past-period invoicing | Prompt invoicing to Xero |
What to do this week
- For each retainer, confirm the period type, included hours, and fee match the agreement.
- Set an alert threshold so over-servicing warns you early.
- Check for any past periods that were never invoiced, and bill them to Xero as a draft.
- Make reviewing hours remaining a weekly habit, not a month-end scramble.
Retainers give you predictable revenue, but only if every period is billed and over-servicing is caught early. Watch hours remaining, set your alerts, and invoice each period (including the ones you missed) straight through Xero. For the wider margin picture, see agency profitability: 2026 benchmarks and how to improve it.