2.3 Coinsurance and the Coinsurance Formula

Why Coinsurance Exists

Most property losses are partial, not total. If insurers charged the same rate regardless of how much coverage you bought, everyone would underinsure (buy a $200,000 limit on a $1,000,000 building) and still collect on small claims. Coinsurance corrects this by requiring the insured to carry a limit equal to a stated percentage of the property's value at the time of loss — typically 80%, 90%, or 100%. Carry enough and losses are paid in full (up to the limit); carry too little and the insured is penalized as a co-insurer of the shortfall.

The coinsurance clause appears in CP 00 10 (Building and Personal Property Coverage Form) and in dwelling/homeowners building coverage.

The Coinsurance Formula

The formula every P&C candidate must memorize:

Payment = (Did / Should) × Loss – Deductible

Where:

• Did = limit of insurance actually carried.
• Should = property value at time of loss × coinsurance percentage.
• Loss = amount of the covered loss.

The payment can never exceed the policy limit. Apply the deductible after the coinsurance calculation. Memorize it as "Did over Should, times the loss, minus the deductible."

Worked Example — Underinsured Building

A building is worth $500,000 at the time of loss. The policy has an 80% coinsurance clause, the insured carries a $300,000 limit, the loss is $100,000, and the deductible is $1,000.

The insured eats $26,000 (the $25,000 coinsurance penalty plus the $1,000 deductible) for carrying only 75% of the required limit. Had they carried $400,000+, the $100,000 loss would have been paid in full minus deductible.

Edge Cases and Traps

• Total loss: the penalty disappears — the insurer simply pays the policy limit (you cannot collect more than the limit anyway).
• Compliance: if Did ≥ Should, the ratio is capped at 1.0 — no penalty, never a bonus for over-insuring.
• Agreed Value endorsement suspends coinsurance entirely by substituting a signed Statement of Values.
• Value at time of loss, not at policy inception, drives "Should" — inflation can quietly push an insured below compliance, which is why Inflation Guard matters.
• The deductible is always subtracted last, after the ratio is applied.

Coinsurance on Contents and Multiple Locations

Coinsurance applies separately to each coverage and each location unless a blanket limit with the proper margin clause is used. A building can be in compliance while the business personal property at the same address is penalized, because each is tested against its own "should-carry" amount. On blanket policies, ISO requires a signed Statement of Values and applies a margin clause (for example, 110%) to cap recovery at any single location.

Why the Penalty Is a Premium-Equity Tool

Examiners test the purpose of coinsurance: it equalizes rates between insureds who fully insure and those who gamble on never having a total loss. Because most losses are partial, an underinsured policyholder would otherwise pay far less premium yet recover nearly as much on common small claims. The penalty restores rate equity by reducing partial-loss recovery in proportion to the underinsurance.

Order of Operations

Common Distractors

• Coinsurance never pays a bonus for over-insuring - the ratio caps at 1.0.
• On a total loss, the penalty is irrelevant because recovery is limited to the policy limit anyway.
• Inflation guard and agreed value are the two cures the exam expects you to name when an insured keeps slipping out of compliance.
• The percentage in a coinsurance clause is the required-to-carry figure, not the share the insured pays - a frequent wording trap.

Coinsurance vs. an Inland-Marine Floater

A useful exam contrast: scheduled inland-marine floaters (such as the Personal Articles Floater) carry no coinsurance because each item is insured at an agreed value, whereas commercial property and dwelling building coverage almost always carry a coinsurance condition.

When a question asks how to eliminate a recurring coinsurance penalty on volatile-value property, the best answer is often to schedule the items on an agreed-value floater rather than to keep raising a blanket limit. Remember too that the coinsurance percentage is selected by the insured at policy inception in exchange for a rate credit - a higher coinsurance commitment earns a lower rate, which is the economic reason an insured accepts the penalty risk in the first place.

Agreed Value as the Clean Cure

When an insured repeatedly slips out of coinsurance compliance, the Agreed Value option is the definitive fix: the insurer and insured agree on a value via a signed Statement of Values, the coinsurance clause is suspended for the term, and a covered loss is paid up to the agreed figure without any penalty calculation. The trade-off is a higher premium and the need to refresh the statement each renewal, but it eliminates the partial-loss penalty entirely - the answer the exam expects for a high-value, fluctuating-value account.