2.3 Coinsurance and the Coinsurance Formula

Why Coinsurance Exists

Most property losses are partial, not total. If carriers charged the same rate per $100 regardless of limit, insureds would deliberately under-insure (buy a $200,000 limit on a $1,000,000 building) and still recover most small losses. The coinsurance clause corrects this by requiring the insured to carry a stated percentage of the property's value — commonly 80%, 90%, or 100% — in exchange for an adequate rate. Fail to carry enough, and the insured becomes a co-insurer and shares the loss.

The Coinsurance Formula

The formula is the most calculation-heavy item on the property exam:

Did Carry / Should Carry  ×  Loss  −  Deductible  =  Payment

where Should Carry = Coinsurance % × Property Value at the time of loss. Three guardrails:

• If the ratio is 1.0 or higher, the penalty disappears — the insured carried enough.
• The payment is capped at the policy limit (the "Did Carry" amount).
• The payment is capped at the actual loss.

The lesser of (formula result, policy limit, actual loss) — then subtract the deductible — is paid.

Worked Example: 80% Coinsurance, Under-Insured

• Building value at time of loss: $500,000
• Coinsurance requirement: 80% → Should Carry = 0.80 × $500,000 = $400,000
• Policy limit actually carried (Did Carry): $300,000
• Loss: $100,000; Deductible: $1,000

($300,000 / $400,000) × $100,000 = 0.75 × $100,000 = $75,000
$75,000 − $1,000 deductible = $74,000 paid

The insured eats the remaining $26,000 as the coinsurance penalty. Had the insured carried $400,000 (the full 80%), the ratio would be 1.0 and the policy would pay $100,000 − $1,000 = $99,000.

Coinsurance Comparison Table

Traps to memorize: (1) coinsurance penalties apply to partial losses — a total loss to the limit pays the limit regardless; (2) value is measured at the time of loss, not when the policy was written, so inflation can silently push an insured below the requirement; (3) the Agreed Value option suspends coinsurance entirely.

Coinsurance Step-By-Step and the 100% Trap

Apply the formula in a fixed order to avoid errors. (1) Compute Should Carry = coinsurance % x property value at the time of loss, not at inception. (2) Form the ratio Did Carry / Should Carry, capping it at 1.0. (3) Multiply the ratio by the loss. (4) Subtract the deductible. (5) Cap the answer at the policy limit. The most common wrong answer applies the deductible before the coinsurance penalty — the order is penalty first, then deductible.

A frequent trap raises the coinsurance requirement to 90% or 100%. Higher coinsurance lowers the rate per $100 but punishes under-insurance harder. On a total loss, coinsurance never reduces payment below the policy limit, because the limit is the cap and the ratio question is moot — coinsurance penalties bite only on partial losses.

Blanket Insurance and the Margin Clause

When one limit covers several buildings or locations, blanket coverage applies the coinsurance test against the combined value of all covered property, giving the insured flexibility to move value between locations without penalty. To prevent abuse, insurers attach a margin clause capping recovery at a stated percentage (e.g., 110%) of any single location's reported value. Blanket coverage typically requires a signed statement of values filed at inception; understating those values reintroduces a coinsurance penalty at audit.

Coinsurance vs. Agreed Value and the Insured's Incentive

Coinsurance is fundamentally a rate-equity device: it rewards insureds who carry adequate limits with a lower rate and penalizes those who gamble on partial losses. An insured who dislikes the penalty risk can buy an agreed value option, which suspends coinsurance in exchange for documenting and insuring the full value. The exam contrasts the two: coinsurance keeps the clause active and penalizes shortfalls; agreed value removes the clause but requires accurate up-front valuation.

Inflation guard endorsements help keep "Did Carry" rising with replacement costs so the insured does not silently drift below the required percentage as building costs climb during the policy term.

Common Coinsurance Question Stems

Exam writers recycle a few patterns. The first gives you property value, coinsurance percentage, amount carried, loss, and deductible, and asks for the payment — apply the five steps. The second flips it: it gives the payment and asks how much the insured should have carried, testing whether you can read the ratio backward. The third asks why a fully insured total loss has no penalty — answer: coinsurance applies only to partial losses and the limit caps any recovery. The fourth contrasts coinsurance with agreed value to test which device a worried insured should buy.

Keep "value at time of loss" front of mind, because using the inception value is the classic distractor.

Inflation Guard and Why Limits Drift

Building replacement costs rise during a policy term, so an insured who met 80% at inception can silently fall below it by renewal. An inflation guard endorsement automatically increases the limit by a stated percentage through the term to keep "Did Carry" tracking value. Without it, a long-tenured policy is the classic setup for a coinsurance penalty on a partial loss — a pattern the exam uses to test whether you understand that the requirement is measured at the time of loss, not when the policy was written.