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 limit, owners could buy a small limit, pay a small premium, and still collect fully on common partial losses. Coinsurance corrects this by requiring the insured to carry a limit equal to a stated percentage of the property's value — typically 80%, 90%, or 100%.

Meet the requirement and partial losses are paid in full (up to the limit, after the deductible). Fall short and the insured becomes a co-insurer, sharing the loss in proportion to the underinsurance.

The Coinsurance Formula

The tested formula is:

(Did Carry ÷ Should Carry) × Loss = Recovery

• Did Carry = the limit of insurance actually purchased.
• Should Carry = property value at the time of loss × coinsurance %.
• The result is then capped at the policy limit and reduced by the deductible.

A frequent error is using the value when the policy was issued. Always use the value at the time of loss, because property appreciates and replacement costs rise over the policy year.

Worked Example

A building is worth $500,000 at the time of loss with an 80% coinsurance clause, so the insured should carry $400,000. Suppose the insured actually carries only $300,000 and suffers a $100,000 loss with a $1,000 deductible.

1. Did ÷ Should = $300,000 ÷ $400,000 = 0.75.
2. 0.75 × $100,000 loss = $75,000.
3. Subtract the $1,000 deductible = $74,000 paid.

The insured absorbs $26,000 — the $25,000 coinsurance penalty plus the deductible — because the building was underinsured.

Effect of Coverage Level on a $100,000 Loss ($500K Building, 80% Coinsurance)

Suspending Coinsurance

The ISO Agreed Value option (CP 12 30) suspends the coinsurance clause for the policy period. The insured submits a signed Statement of Values, and as long as the limit equals the agreed value, partial losses are paid in full with no penalty.

Key exam reminders:

• Coinsurance can never make the insurer pay more than the policy limit or more than the actual loss.
• On a total loss, coinsurance does not reduce payment below the limit — the limit is simply paid.
• Higher coinsurance percentages earn a lower rate because the insurer collects adequate premium on the full value.

Coinsurance on Contents and in Other Lines

Coinsurance is not limited to buildings. The ISO Building and Personal Property Coverage Form (CP 00 10) can apply a coinsurance percentage to business personal property, valued at the time of loss. The same Did-÷-Should math applies, using the contents value rather than the building value.

Coinsurance-type concepts also appear in health and inland marine lines, but the property version is the one tested numerically. A subtle distinction: a deductible is subtracted after the coinsurance factor is applied, never before. Reversing that order is a classic wrong answer because it understates the penalty the insured actually bears.

Avoiding the Penalty — Insurance to Value

The practical lesson of coinsurance is insurance to value (ITV): carry a limit at or above the required percentage of full value. Producers help clients with replacement-cost estimators, and carriers add inflation guard so the limit tracks rising costs through the year.

A building written at exactly the required percentage receives no penalty on partial losses, which is why most homeowners forms build in an 80% replacement-cost trigger. If the insured drops below 80% of replacement value, the dwelling settlement converts from replacement cost to the larger of ACV or the proportionate amount — a direct application of the coinsurance idea inside a personal-lines policy.

The Coinsurance Formula Step-by-Step

The penalty formula is (Did / Should) x Loss, where Did is the limit actually carried, Should is the required limit (coinsurance percentage times full value at the time of loss), and the result is then reduced by the deductible and capped at the policy limit. The ratio is never allowed to exceed 1.0, so carrying more than the required amount earns no bonus but avoids any penalty.

The insured recovers $80,000 (minus the deductible) and absorbs the $20,000 shortfall as the coinsurance penalty. Two common traps: (1) the required limit is measured at the time of loss, so inflation can push a once-adequate limit below the threshold; and (2) total losses are typically paid at the policy limit without applying the ratio, so the penalty mainly bites partial losses. This is why insurance-to-value monitoring and inflation guard are the producer's primary defenses against an unexpected penalty.

When Coinsurance Does and Does Not Apply

Coinsurance is a commercial property and dwelling concept; it does not apply to liability coverage or to most personal property scheduled on a floater. On personal homeowners forms it operates indirectly through the 80% replacement-cost trigger: meet 80% of replacement value and the dwelling settles at full replacement cost on partial losses; fall below and settlement drops to the larger of ACV or the proportionate amount. The Agreed Value option (common on commercial property) suspends coinsurance entirely in exchange for the insured carrying a value the insurer accepts in advance, removing the penalty risk.

Recognizing which coverages carry a coinsurance clause - and that Agreed Value turns it off - prevents the common error of applying the penalty formula where it does not belong.

Comparing Penalty Outcomes

The table captures the practical takeaway: the penalty exists only when the insured carries less than the required percentage and only on partial losses. Producers who keep limits current through reappraisal and inflation guard, or who use Agreed Value where available, protect clients from a penalty that can turn an adequate-seeming policy into a painful out-of-pocket shortfall at claim time.