2.3 Coinsurance and the Coinsurance Formula

Why Coinsurance Exists

Most property losses are partial, not total. If insurers charged the same rate per dollar regardless of how much limit an insured bought, everyone would under-insure — buying a small limit and betting against a total loss. The coinsurance clause corrects this by requiring the insured to carry a limit equal to a stated percentage of the property's value (usually 80%, 90%, or 100%) at the time of loss. Carry enough and the insurer pays partial losses in full (less deductible). Carry too little and the insured becomes a co-insurer and shares the loss.

The Coinsurance Formula

The penalty math the exam expects you to apply is:

Payment = (Limit Carried ÷ Limit Required) × Loss − Deductible

where Limit Required = Property Value × Coinsurance %. The resulting payment can never exceed the policy limit. The phrase to memorize is "did over should, times the loss." The amount the insured did carry over the amount they should have carried, multiplied by the loss.

Worked Example — Underinsured

A building is worth $500,000. The policy has an 80% coinsurance clause, a $250,000 limit, and a $1,000 deductible. A fire causes a $100,000 loss.

• Limit Required = $500,000 × 80% = $400,000
• Coinsurance ratio = $250,000 ÷ $400,000 = 0.625
• Indemnity = 0.625 × $100,000 = $62,500
• Less deductible = $62,500 − $1,000 = $61,500 paid

The insured eats the remaining $38,500 (plus deductible) as the coinsurance penalty for carrying only $250,000 when $400,000 was required.

When the Penalty Does Not Apply

Key traps the exam loves:

• Adequately insured: if the insured carries at least the required limit, the ratio is 1.0 (capped at 1.0 — over-insuring does not boost payment), so partial losses pay in full less deductible.
• Total loss: coinsurance is not applied to a total loss; the insurer simply pays the policy limit (subject to valuation).
• Agreed Value option: attaching the Agreed Value coverage option (form CP 04 10 in ISO commercial property) suspends the coinsurance clause for the policy term.
• Value at time of loss: the required limit uses the value at the time of loss, not the value when the policy was written — inflation can quietly create a penalty.

Coinsurance in Homeowners vs. Commercial

The 80% rule appears in two places. In homeowners, the replacement-cost loss-settlement condition requires insuring the dwelling to at least 80% of full replacement cost at the time of loss to collect replacement cost on partial losses; carry less and the insured recovers the larger of ACV or a proportion of replacement cost. In commercial property, the explicit coinsurance clause on the declarations (commonly 80/90/100%) drives the penalty formula directly.

The distinction matters because the homeowners penalty applies only to the replacement-cost benefit on partial losses, whereas the commercial coinsurance formula reduces the dollar payment itself.

Insurance-to-Value and Inflation Guard

Because the required limit is measured at the time of loss, ordinary inflation can silently push an adequately insured building into a penalty position. Two tools manage this:

• Inflation Guard endorsements automatically increase the limit periodically (e.g., quarterly) to track rising construction costs.
• Agreed Value (commercial) or the Replacement Cost / no-coinsurance options suspend the coinsurance test in exchange for documenting value up front via a statement of values.

A worked check of adequacy: a $1,000,000 building with 90% coinsurance requires a $900,000 limit. If the insured carries $810,000, the ratio is 810,000 divided by 900,000 = 0.90, so a $200,000 partial loss pays 0.90 times $200,000 = $180,000 before deductible. Raising the limit to $900,000 would have paid the full $200,000.

Multiple-Building Coinsurance and Blanket Insurance

When one limit covers several buildings under a blanket basis, coinsurance is tested against the combined value of all covered property at all locations on the blanket, using a single coinsurance percentage and a signed statement of values. This is more forgiving than specific coverage because a higher-than-expected value at one location can be offset by a lower value at another, reducing the chance of a penalty.

Under specific coverage, by contrast, each building's limit is tested separately, so under-insuring one building triggers a penalty on that building even if others are adequately insured.

Step-by-Step Penalty Worksheet

Apply the formula in a fixed order to avoid arithmetic traps:

1. Find Limit Required = property value at time of loss times the coinsurance percentage.
2. Form the ratio = Limit Carried divided by Limit Required, capped at 1.0.
3. Multiply the ratio by the loss to get tentative indemnity.
4. Subtract the deductible.
5. Cap the result at the policy limit.

Example: value $600,000, 80% coinsurance, $360,000 limit, $90,000 loss, $1,000 deductible. Required = $480,000; ratio = 360,000 divided by 480,000 = 0.75; indemnity = 0.75 times $90,000 = $67,500; minus $1,000 deductible = $66,500 paid. The insured absorbs the $22,500 coinsurance penalty plus the deductible for carrying only $360,000 when $480,000 was required.

Why Insurers Use Coinsurance Rates

Coinsurance exists because rates are built on the assumption that most insureds carry a fair proportion of value. If everyone under-insured, premium collected per dollar of exposure would be too low to fund the many partial losses that occur. The coinsurance clause restores rate equity by giving a lower rate to insureds who carry to value and penalizing those who do not. On the exam, when a question asks why a building owner who carried only 50% of value received a reduced claim, the answer ties back to this rate-equity rationale, not to a denial of coverage.