Exposure: counting risk, not policies
In the previous guides you met the P&C world and its policies — the coverages, perils, limits and deductibles that say *what* is insured. Now we ask a humbler-sounding but foundational question: *how much* insurance is a company actually carrying? You cannot just count policies, because one policy might cover a single bicycle and another a fleet of a hundred trucks. Instead an insurer counts exposure — a unit of risk that scales with how much loss could happen.
The yardstick chosen for a line is the exposure base — the thing you multiply a rate by to get a premium. Each line has a natural one. In auto it is the car-year: one insured car for one year. In homeowners it is often the amount of insurance, per 1,000 of dwelling value. In workers' compensation it is payroll, per 100 of wages — because more workers and higher wages mean more and bigger injury claims. A good base is *responsive* (it rises with the risk), *practical* (easy to measure and audit), and *hard to manipulate*. Premium is then, roughly, rate per exposure unit times number of units.
Written premium: the money on day one
When a customer buys a one-year policy and pays 1,200, the insurer records written premium of 1,200 — the full price of the coverage *sold*. Written premium is a sales figure: it tells you how much business the company put on the books in a period, and it is the number that moves when marketing succeeds or a rate increase takes hold. It is recognised at the moment the policy incepts, regardless of how the customer pays. Whether they hand over 1,200 in cash or agree to twelve monthly instalments, the written premium is 1,200 on day one.
Here is the crucial point, and the source of nearly all confusion that follows. On day one the insurer has been *paid* for a whole year of protection but has *delivered* almost none of it. If a fire happens in month two, the company must pay the claim; the promise runs for the full policy period. So that 1,200 is not the insurer's to keep yet — it is, in a real economic sense, an advance payment for a service that will be rendered slowly, over the coming twelve months. Written premium answers *what was sold*. It does not answer *what has been earned*.
Earned premium: coverage actually provided
Earned premium is the slice of the written premium for which the insurer has *already* stood the risk — coverage genuinely delivered so far. The standard assumption is that risk runs evenly across a policy: a year of protection is earned smoothly, day by day, at 1/365 per day. So our 1,200 policy earns about 3.29 each day, or 100 each month. Three months in, the company has earned 300; nine months of promise still lie ahead. This even, time-proportional earning is the written-versus-earned distinction in a nutshell.
Earned premium is the figure that belongs in any honest measure of profit, because it lines up *in time* with the losses the policy is generating. To judge how a book of business performed over a calendar year, you compare the losses incurred *during* that year against the premium *earned during* that year — never against premium merely written. Matching earned premium to incurred losses is exactly how the loss ratio is built, and getting this matching right is the difference between a true picture and a flattering or alarming illusion.
The unearned premium reserve
What about the part of the premium that has *not yet* been earned? At any moment, written premium splits cleanly into two pieces: what has been earned, and what has not. The unearned slice is a liability the insurer carries on its balance sheet — the unearned premium reserve, or UPR. It is the company's honest admission: *we have been paid for protection we still owe.* For our 1,200 policy three months in, 300 has been earned and 900 sits in the unearned premium reserve, waiting its turn to be earned over the remaining nine months.
One 1,200 annual policy, 3 months elapsed: Written premium = 1,200 (recognised on day one) Earned premium = 1,200 * 3/12 = 300 Unearned (UPR) = 1,200 * 9/12 = 900 Identity (always): Earned + Unearned = Written Over time: Earned rises -> UPR melts to 0 by month 12
Be honest about what this reserve is and is not. The UPR is *not* idle cash sitting in a vault, and it is *not* a guess about future claims — that is a different liability entirely. It is a simple, almost mechanical bookkeeping entry: the unexpired, prepaid portion of premium the insurer would, in principle, have to refund if every policy were cancelled tomorrow. It is unwound by arithmetic, not by judgement. (One honest refinement: if a line's risk is *not* spread evenly over the year — hurricane cover earns faster in storm season — the even time-proportional rule is replaced by an earning pattern that follows the true shape of the risk.)
Policies in force, and the accounting backbone
One more headline number completes the picture: policies in force — the count of policies (and the exposure they carry) providing live coverage at a given instant. Written premium is a *flow* measured over a period; in-force business is a *stock* measured at a point in time, like a photograph of the book on 31 December. Together with exposure they answer three different questions an executive will ask in the same breath: how much did we sell, how much are we currently on the hook for, and how much have we genuinely earned?
Step back and the whole machine clicks together. Exposure measures *how much* risk; written premium records the *sale*; earned premium is what time has *converted into income*; the unearned premium reserve holds the *not-yet-delivered* remainder; and in-force counts the *live promises*. This is the accounting backbone of a P&C insurer — the scaffolding on which every later craft is hung. When you reach reserving, you will match incurred losses to *earned* premium; when you reach ratemaking, your pure premium will be losses per *earned exposure*. Nothing downstream is trustworthy unless these distinctions are kept straight first.