How to Calculate Billable Hours and Realization Rate
Most firms measure their fee earners with three numbers: billable hours, utilisation, and realisation. They sound similar, they are often confused, and each one answers a different question. This guide gives you the formulas, a worked example for a single timekeeper, and the reason the hardest part of the calculation is not the arithmetic — it is capturing accurate hours in the first place.
If you would rather see where your own unbilled time is sitting than run the maths by hand, Timentry surfaces captured-but-unbilled hours, realisation, and utilisation in real time. But the formulas below are worth understanding whichever tool you use.
What counts as a billable hour
A billable hour is time spent on work you can invoice to a client under your engagement terms. It excludes internal admin, business development, training, and non-chargeable pro bono unless your firm books those separately.
The unit matters. Most firms bill in increments — commonly six-minute units (tenths of an hour) in legal work, or fifteen-minute units elsewhere. A three-minute phone call rounded to a tenth of an hour is 0.1 billable hours, not 0.05. Your increment policy changes the total, so it belongs in any honest calculation.
Billable hours for a period = the sum of every billable time entry, rounded to your firm's increment.
The trap here is not the sum — it is the entries that never get recorded. A two-minute email answered between meetings, a corridor question from a partner, a quick document review before lunch: each is billable, each is easy to forget by the time the timesheet is filled in at 6 p.m. This is where most billable time leaks away. We cover the mechanics of that leak in Why Law Firms Lose Billable Hours.
Utilisation rate: how much of your capacity is billable
Utilisation measures how much of a timekeeper's available time was spent on billable work. It is a productivity and capacity number.
Utilisation rate = (billable hours ÷ available hours) × 100
"Available hours" is a policy choice. Some firms use a fixed target (for example, 1,800 hours a year, or a 40-hour week); others use actual hours worked. Be consistent, and state which you use — utilisation numbers are only comparable when the denominator is defined the same way.
Worked example. An associate records 32 billable hours in a 40-hour week.
- Utilisation = (32 ÷ 40) × 100 = 80%
That 80% tells you the associate spent four of every five working hours on chargeable work. It says nothing yet about whether the firm was actually paid for those hours. That is realisation's job.
Realisation rate: how much of your billable work becomes revenue
Realisation measures how much of the value you recorded actually turns into collected fees. It is the number that connects timekeeping to the bank account, and it is usually the most revealing of the three.
There are two common versions, and mixing them up is the most frequent mistake:
Billing realisation = (hours billed ÷ hours worked) × 100 — how much of the recorded time survived write-downs before the invoice went out.
Collection realisation = (amount collected ÷ standard value of hours worked) × 100 — how much of the full value you ultimately banked, after write-downs and unpaid or discounted invoices.
Worked example. The same associate records 32 billable hours at a standard rate of £250/hour — a standard value of £8,000.
- A supervising partner writes down 4 hours as non-recoverable before billing. Hours billed = 28.
- Billing realisation = (28 ÷ 32) × 100 = 87.5%
- The client later negotiates the invoice down, and the firm collects £6,000.
- Collection realisation = (6,000 ÷ 8,000) × 100 = 75%
The gap between 87.5% and 75% is the money lost after the work was billed. The gap between 100% and 87.5% is the money lost to write-downs. Both gaps are worth tracking; they have different causes and different fixes.
Putting the three together
| Metric | Formula | Question it answers | |---|---|---| | Billable hours | Σ billable entries (rounded to increment) | How much chargeable work did we record? | | Utilisation | (billable ÷ available) × 100 | How much of our capacity was chargeable? | | Realisation (billing) | (hours billed ÷ hours worked) × 100 | How much recorded time survived to the invoice? | | Realisation (collection) | (amount collected ÷ standard value) × 100 | How much value did we actually bank? |
A firm can have strong utilisation and weak realisation — busy people whose time is written down or discounted. It can also have the reverse. You need all three to see the whole picture.
The number the formula can't fix: uncaptured time
Every formula above starts with recorded billable hours. If that first number is understated because time was reconstructed from memory at the end of the day, every metric downstream is understated too — and the loss is invisible, because you cannot write down time you never recorded.
This is the part a calculator cannot solve. Timentry addresses it at the source with passive time capture: a lightweight desktop agent, plus cloud connectors for Microsoft 365, Google Workspace, Zoom, and telephony providers, records billable activity as it happens — emails, meetings, calls, and documents — with no timer to start. An AI activity-clustering layer drafts narrative time entries from that captured activity for you to review, so the small entries that usually evaporate are already on the sheet.
The result is that your billable-hours figure reflects the work you actually did, not the work you remembered at 6 p.m. — which makes the utilisation and realisation numbers you calculate from it trustworthy rather than optimistic.
This article explains general professional-services metrics for educational purposes; it is not accounting or financial advice. Rate, increment, and write-down conventions vary by firm and jurisdiction — use your own firm's policies.
See your own numbers
To move from formulas to your firm's real figures — captured-but-unbilled hours, utilisation, and realisation across timekeepers and matters in real time — see how Timentry works or talk to the team. For the broader problem of billable time going unrecorded, read Why Law Firms Lose Billable Hours.