2.3 Coinsurance and the Coinsurance Formula

2.3 Coinsurance and the Coinsurance Formula

Coinsurance is a property-insurance condition that requires the insured to carry a limit equal to a stated percentage (commonly 80%, 90%, or 100%) of the property's value at the time of loss. It exists because most losses are partial — without coinsurance, insureds would buy small limits, underpay premium, and still expect full payment on small claims. Coinsurance enforces insurance to value.

If the insured carries less than the required amount, the policy imposes a penalty by paying only a proportion of the loss. The mechanic is the coinsurance formula:

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

"Amount Required" = coinsurance % × property value at the time of loss. The classic mnemonic is "Did Over Should" — Did carry, Over the amount you Should have carried, times the loss.

Worked coinsurance example (underinsured)

A building is worth $500,000. The policy has an 80% coinsurance clause, so the required limit is 0.80 × $500,000 = $400,000. The insured actually carries only $300,000. A partial loss of $100,000 occurs, with a $1,000 deductible.

The insured eats the $25,000 coinsurance penalty plus the deductible because they carried only 75% of the required limit.

Two ceilings on the coinsurance result

The coinsurance calculation never increases recovery above two ceilings:

1. The actual loss amount (less deductible), and
2. The policy limit.

When the insured carries equal to or more than the required amount, the ratio is 1.00 (capped at 1.0) and there is no penalty — the loss is paid in full up to the limit, less deductible. The penalty only bites when the insured is underinsured. Many exam items test whether you correctly cap the ratio at 1.0 rather than over-paying.

Why insurance to value matters

Coinsurance aligns premium with exposure. Two buildings of equal value should pay similar premium for similar coverage; an underinsured owner who paid less premium should not collect the same as a fully insured neighbor on a partial loss. The penalty is the equalizer.

Key traps:

• Apply the coinsurance percentage to value at the time of loss, not the original purchase price or the policy limit.
• Subtract the deductible after applying the coinsurance ratio.
• The agreed value option (Section 2.2) suspends coinsurance entirely; if a question states agreed value applies, do not run the formula.

Coinsurance with a Sublimit and Multiple Buildings

When a policy schedules several buildings on separate limits, apply the coinsurance test separately to each building unless a blanket limit with an agreed value applies. A blanket limit covering multiple locations generally requires a signed statement of values and often an agreed-value endorsement, which then suspends the per-building penalty.

Worked Adequately-Insured Example

A building worth $600,000 carries 90% coinsurance, so the required limit is $540,000. The insured carries $550,000 and suffers a $120,000 loss with a $2,500 deductible. Because carried ($550,000) exceeds required ($540,000), the ratio is capped at 1.00 - there is no penalty. The insurer pays the full $120,000 minus the $2,500 deductible = $117,500, never exceeding the $550,000 limit.

Why Examiners Love Coinsurance

Coinsurance items test three skills at once: computing the required amount from value at the time of loss, capping the ratio at 1.0, and subtracting the deductible after the ratio. Miss any one and the answer is wrong, which is why this single concept may appear in three or four scored questions.

Exam summary: Did/Should x Loss - Deductible. Apply the percentage to value at time of loss; cap the ratio at 1.0; subtract the deductible last; agreed value turns the penalty off entirely.

Coinsurance Penalty Intuition

The penalty is not arbitrary; it is the price of paying too little premium. An owner who insures to only 60% of the required amount paid roughly 60% of the proper premium, so the formula limits recovery to that same fraction of every partial loss. Full payment on a partial loss returns only when the insured carries the full required limit.

Common Coinsurance Distractors

• Applying the percentage to the original cost or the policy limit instead of value at the time of loss.
• Subtracting the deductible before multiplying by the ratio.
• Forgetting to cap the ratio at 1.00 when the insured is over-insured, which would wrongly inflate the payment.
• Running the formula even though an agreed-value endorsement is in force, which suspends coinsurance entirely.

Work the steps in order every time - required amount, ratio (capped at 1.0), times loss, minus deductible, capped at the limit - and these distractors fall away. Because coinsurance underpins both commercial property and many dwelling claims, the few minutes spent drilling it return some of the highest-yield points on the national portion.