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 an insured bought, everyone would insure to a fraction of value, collect on small losses, and the rate base would collapse. The coinsurance clause forces the insured to carry coverage equal to a stated percentage of the property's full value — usually 80%, 90%, or 100% — in exchange for an adequate rate.

If the insured complies, partial losses are paid in full (up to the limit, after deductible). If the insured carries less than the required amount, the coinsurance penalty applies and the insured becomes a co-insurer, sharing the loss proportionally.

The Coinsurance Formula

The formula the exam expects you to apply:

Recovery = (Did / Should) × Loss − Deductible

Where:

• Did = the limit of insurance actually carried
• Should = the amount required (coinsurance % × full property value at time of loss)
• Loss = amount of the covered loss
• Recovery is capped at the policy limit (you never collect more than the limit)

The penalty applies only when Did < Should. If Did ≥ Should, the (Did/Should) factor is treated as 1 (you do not get a bonus for over-insuring).

Worked Example — Penalty Applies

A building is worth $500,000. The policy has an 80% coinsurance clause, a $400,000 limit, and a $1,000 deductible. A fire causes a $100,000 loss.

1. Should = 80% × $500,000 = $400,000
2. Did = $400,000 (the limit carried)
3. Did ÷ Should = $400,000 ÷ $400,000 = 1.0 — no penalty
4. Recovery = 1.0 × $100,000 − $1,000 = $99,000

Now suppose the insured carried only $300,000:

1. Should = $400,000 (unchanged)
2. Did ÷ Should = $300,000 ÷ $400,000 = 0.75
3. Recovery = 0.75 × $100,000 − $1,000 = $74,000

The insured absorbs the $25,000 coinsurance penalty plus the deductible because of under-insurance.

Coinsurance Penalty Snapshot

Using the $500,000 building, 80% clause (Should = $400,000), $50,000 loss, $1,000 deductible:

Key rules:

• Recovery never exceeds the policy limit.
• Value is measured at the time of loss, not at policy inception — inflation can pull a compliant insured below the threshold.
• An Agreed Value endorsement or an Inflation Guard endorsement suspends/mitigates coinsurance.

Common Traps

• Applying the deductible before the coinsurance factor. The correct order is multiply by (Did/Should) first, then subtract the deductible.
• Using the limit instead of full value for "Should." "Should" is always coinsurance % × property value, not the limit.
• Forgetting the limit cap. Even if the formula yields more than the limit, payment stops at the limit.
• Homeowners policies embed an 80% replacement-cost requirement (the HO version of coinsurance) for full RC settlement; below 80%, contents/structure partial losses settle at the greater of ACV or the proportionate RC amount.

Reading the Coinsurance Clause and Choosing a Percentage

The coinsurance clause states a required percentage (commonly 80%, 90%, or 100%) the insured must carry relative to the property's full value to avoid a penalty. A higher coinsurance percentage earns a lower rate because the insurer collects adequate premium across the pool, so insureds trade a stricter insurance-to-value requirement for cheaper coverage. The clause applies the penalty only to partial losses; a total loss simply pays the policy limit, so coinsurance never reduces a properly limited total-loss payment below the limit.

Step-by-Step Application With a Penalty

Work coinsurance in a fixed order. Compute "Should" = coinsurance % x full value at the time of loss; identify "Did" = the limit actually carried; form the ratio Did/Should (cap it at 1.0, never above); multiply the loss by that ratio; then subtract the deductible; and never pay more than the limit. For a $1,000,000 warehouse with a 90% clause, "Should" is $900,000. If the insured carried only $720,000, the ratio is 0.80, so a $100,000 loss pays $80,000 before the deductible, the $20,000 shortfall is the coinsurance penalty borne by the insured.

Avoiding the Penalty: Agreed Value and Inflation Guard

Insureds escape coinsurance exposure through options that suspend the clause. Agreed value lets the insurer and insured agree on a value, attach a statement of values, and waive the coinsurance condition for the term, so a partial loss is paid in full up to the limit. Inflation guard automatically raises the limit periodically to keep pace with rising values, helping the insured stay above the required percentage. The exam tests recognizing that an insured repeatedly hit by penalties should request agreed value rather than simply raising the limit guesswork.

Blanket Coverage and Margin Clauses

When one limit covers multiple buildings or locations, the policy uses blanket coinsurance: the required amount is the coinsurance percentage applied to the combined value of all covered property, and the single limit floats across locations. Blanket coverage reduces the chance of a penalty at any one location because the full limit is available wherever the loss occurs.

To curb the broad exposure a blanket limit creates, insurers attach a margin clause (maximum amount payable at any single location, often a percentage of that location's reported value), which caps recovery per site even though the limit is blanket. Candidates should remember that blanket coverage requires a signed statement of values, and that misreporting values can reintroduce a coinsurance-style penalty at audit.