2.3 Coinsurance and the Coinsurance Formula

Why Coinsurance Exists

Most property losses are partial, not total. Without a penalty for underinsurance, an owner could insure a $1,000,000 building for only $200,000, pay a small premium, and still fully recover the typical small partial loss — leaving the insurer underpaid for the real exposure. The coinsurance clause corrects this by penalizing underinsurance on every loss, pushing owners to insure to value.

Common coinsurance percentages: 80% (the most common), 90%, and 100%.

The clause requires the insured to carry a limit equal to at least the stated percentage of the property's value at the time of loss. Carry less, and the insured becomes a co-insurer of the difference. Insurers offer a lower rate in exchange for the coinsurance requirement — the higher the percentage the insured agrees to carry, the lower the rate per $100 of coverage, because the spread of insurance-to-value across the book becomes more uniform and predictable.

The Coinsurance Formula

Claim Payment = (Amount Carried / Amount Required) x Loss

Two caps always apply afterward: the payment can never exceed the actual loss or the policy limit, whichever is less. The deductible is subtracted last, after the coinsurance calculation.

If the insured carries at least the required amount, the fraction equals or exceeds 1, no penalty applies, and the insurer pays the loss in full (up to the limit, less deductible).

Worked Coinsurance Example (Penalty Applies)

A building is worth $500,000. The policy has an 80% coinsurance clause, so the Amount Required = $500,000 x 0.80 = $400,000. The owner insured for only $300,000 (Amount Carried). A fire causes a $100,000 loss; the deductible is $1,000.

Step 1  Required = $500,000 x 80% = $400,000
Step 2  Ratio    = $300,000 / $400,000 = 0.75
Step 3  Payment  = 0.75 x $100,000 = $75,000
Step 4  Less deductible = $75,000 - $1,000 = $74,000

The insurer pays $74,000; the insured absorbs the remaining $26,000 as the coinsurance penalty plus deductible. Because the owner carried only 75% of what was required, the insurer pays only 75% of the loss.

Worked Example (No Penalty)

Same $500,000 building and 80% clause (required = $400,000). The owner carried $450,000. A $100,000 loss occurs, deductible $1,000.

Because $450,000 exceeds the $400,000 required, the ratio is greater than 1, so the formula is not applied as a penalty. The insurer pays the full loss up to the limit: $100,000 - $1,000 = $99,000.

When the insured carries more than required, the insurer simply pays the loss (capped at the limit) — there is no bonus for over-insuring. This is why agents target the required amount precisely and reassess values at renewal.

Coinsurance Traps

• The penalty is based on value at the time of loss, not the value when the policy was written — rising replacement costs can silently push the insured below the required amount.
• The coinsurance ratio is never allowed to exceed 1 to create a windfall; carrying extra does not pay more than the loss.
• Subtract the deductible last, after applying the ratio.
• Agreed Value coverage suspends coinsurance; do not apply the formula when an agreed-value endorsement is in place.

Total Loss and the Coinsurance Clause

A subtle exam point: on a total loss, the coinsurance penalty effectively disappears because the policy limit caps the payment anyway. If the $500,000 building above burns to the ground but the owner carried only $300,000, the insurer pays the $300,000 limit (less deductible) — there is no separate penalty to apply because the limit already reflects the underinsurance.

Some states also enforce a valued policy law for total losses to real property: on a total loss by a covered peril, the insurer must pay the full face amount of the policy regardless of ACV, which can override both ACV valuation and coinsurance. Always check whether the state has a valued policy law when a fact pattern describes a total loss to a dwelling or building.

Watch the difference between coinsurance (a property concept that penalizes underinsurance) and co-insurance in health insurance (a percentage the insured shares of each covered claim). The exam mixes the two terms to test whether you know that on a P&C property form the clause rewards insuring to value and applies a proportional penalty only when the insured fails to do so.

The Vacancy and Agreed-Value Interactions

Coinsurance can be suspended or modified by other provisions, which the exam combines into multi-step items. An agreed value (agreed amount) endorsement waives the coinsurance clause for the policy term in exchange for the insured filing a statement of values the insurer accepts — so no penalty applies even if the limit later proves slightly low. Conversely, the vacancy condition can reduce a commercial property recovery by 15% once a building is vacant beyond 60 consecutive days, applied on top of any coinsurance result.

Remember the order of operations on a penalty claim: apply the coinsurance ratio first, then subtract the deductible, and cap at the policy limit — never the reverse.