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Why people leave, and what it is actually costing you

July 2026 · 6 min read

Ask most teams for their turnover rate and you get a pause, then a guess. Ask what it cost last year and you get a longer pause. Yet every leaver is a recruitment bill, a stretch of lost output and a chunk of knowledge walking out of the door - and until those things are counted, retention stays an opinion competing against budgets backed by numbers.

A leaver log fixes that, but only if it does more than store names. The difference between a list and an instrument is four calculations. Here is how each one works, and why it changes the conversation.

Attrition rate, done properly

The formula is simple: leavers in the period divided by average headcount over the same period. The trap is the denominator. People divide by today's headcount, or by starting headcount, and quietly overstate or understate the rate. Use the average - roughly (opening + closing) / 2 - and be consistent about the window, usually a rolling twelve months so seasonal spikes do not distort it.

One rate on its own is a number without a verdict. What makes it useful is comparison: this quarter against last, this team against the organisation, this year against the one before. A single figure tells you nothing; a trend tells you whether the thing you tried in spring worked.

Tenure buckets: where the real story hides

Average tenure at exit is a headline, and like most headlines it hides the plot. Split leavers into bands - under one year, one to two, two to five, five-plus - and a shape appears. A cluster of under-one-year leavers is not a pay problem or a culture problem; it is an onboarding-and-fit problem, and it points straight at how people are hired and their first ninety days. Long-tenure leavers tell a different tale entirely, usually about growth ceilings or life stage.

The same overall rate can mean two completely different things depending on where the leavers sit. Buckets are what let you tell them apart, and they let tenure compute itself from the start and leave dates rather than being worked out by hand for every person.

The exit-reason Pareto

Record why each person left, then rank the reasons most-common-first. This is the Pareto principle applied to people: a small number of reasons usually accounts for most of the leavers, and those are where retention effort earns its return. Chasing the long tail of one-off reasons feels thorough and achieves little.

Two cautions keep the Pareto honest. First, use a controlled list of reasons rather than free text, or nothing ranks. Second, exit-interview reasons are politely edited - "career development" often means something more pointed - so read the ranking as a direction to investigate, not a confession.

Putting a pound figure on it

This is the calculation that unlocks budget. Capture what each leaver cost to replace: advertising and agency fees, the hiring team's time, onboarding and training, and the lost productivity while the seat is empty and the replacement gets up to speed. Total it, and turnover stops being an HR abstraction and becomes a line a finance director recognises.

The number is almost always larger than people expect, and that is the point. A retention initiative that looks expensive against nothing looks cheap against the six-figure cost of the churn it prevents. You are not arguing for a nice-to-have; you are showing a saving.

The one split most trackers miss

Not every leaver is a loss. A planned retirement and the departure of a strong performer you fought to keep are both "attrition", and treating them the same wastes the metric. Flag each leaver as regretted or not, and the regretted-leaver percentage tells you whether you have a genuine retention problem or simply normal, healthy churn.

A high overall rate made up of planned exits is manageable. A modest rate that is mostly regretted leavers is a fire. Only the split tells you which one you are looking at.

Log each leaver once, and every one of these figures should compute itself and land on a single page you can take into a leadership review. That page is the argument. Handle it with care, too: leaver records are personal data, so keep them under your organisation's UK GDPR obligations like any other employee information.