Coinsurance: Formula, Penalty, and Worked Examples

Why Coinsurance Appears on Every P&C Exam

Coinsurance is a property insurance provision that requires the insured to maintain coverage at least equal to a stated percentage of the property's value at the time of loss—commonly 80%, 90%, or 100%. If the insured carries less than the required amount, a coinsurance penalty reduces the claim payment even when the loss is far below the policy limit. Nevada exam items frequently supply dollar figures and expect you to calculate the payable amount before the deductible.

Coinsurance exists because insurers price premiums based on the total exposure. An insured who deliberately or carelessly underinsures shifts part of the risk back to the insurer without paying commensurate premium. The penalty restores balance: you cannot insure a $500,000 building for $100,000 and collect full partial losses.

Core Definitions

• Coinsurance percentage: The minimum percentage of value that must be insured (e.g., 80%).
• Value at time of loss: Usually replacement cost or ACV per policy definition—read the scenario.
• Amount required: Value × Coinsurance percentage.
• Amount carried: The limit of insurance applicable to the damaged property (or blanket limit allocated to that property).
• Covered loss: The amount of covered damage before deductible and before coinsurance adjustment.

The Coinsurance Formula

When the insured fails to meet the coinsurance requirement at the time of loss:

Step 1 — Calculate amount required: Amount Required = Property Value at Time of Loss × Coinsurance %

Step 2 — Compare amount carried to amount required: If Amount Carried ≥ Amount Required → no penalty (pay covered loss up to limit, minus deductible). If Amount Carried < Amount Required → apply penalty ratio.

Step 3 — Apply penalty to covered loss: Loss Payment = (Amount Carried ÷ Amount Required) × Covered Loss Amount

Step 4 — Subtract deductible (if not already netted in the scenario).

The ratio Amount Carried ÷ Amount Required is the coinsurance fraction. An insured at 50% of the requirement receives 50% of the covered loss—before deductible.

Worked Example (Exam-Style Numbers)

A Sparks warehouse suffers a covered fire loss. Policy facts at time of loss:

Step 1 — Amount required: $500,000 × 80% = $400,000 required

Step 2 — Underinsurance check: Carried $250,000 < Required $400,000 → penalty applies

Step 3 — Coinsurance fraction: $250,000 ÷ $400,000 = 0.625 (62.5%)

Step 4 — Apply to covered loss: 0.625 × $100,000 = $62,500

Step 5 — Subtract deductible: $62,500 − $1,000 = $61,500 payable

Note: The insured had a $250,000 limit—more than enough for a $100,000 loss—but still suffered a penalty because they were underinsured relative to total building value. This surprises real clients and appears constantly on exams.

When No Penalty Applies

No coinsurance penalty when:

• Amount carried ≥ amount required at time of loss
• Agreed value endorsement is in force for the period (coinsurance suspended; stated value governs)
• Waiver of coinsurance endorsement applies
• Policy is written at 100% coinsurance and limit equals full value
• Some blanket policies modify allocation rules—read endorsements

Coinsurance vs. Other Concepts

Partial Loss vs. Total Loss

Coinsurance applies to partial and total losses alike when underinsured. A common trap: students assume a $50,000 loss on a $500,000 building with a $300,000 limit is fully paid because the loss is "within the limit." If required insurance was $400,000 (80% of $500,000) and only $300,000 was carried, the penalty still applies: 300/400 = 75% of the covered loss.

Nevada Producer Scenarios

Las Vegas retail strip (CP 00 10): Building value $800,000, 90% coinsurance, limit $500,000, hail loss $60,000. Required = $720,000. Ratio = 500/720 = 0.694. Payment before deductible = 0.694 × $60,000 = $41,667. Producer should have recommended a building appraisal.

Rural Nye County dwelling: Client insures dwelling for mortgage balance ($180,000) but replacement value is $300,000 with 80% coinsurance. Required = $240,000. Underinsurance guarantees partial-loss penalty on every claim until limits are corrected.

Endorsements That Modify Coinsurance

• Agreed value: Insurer and insured agree on value; coinsurance waived during term if insured maintains agreed limit.
• Replacement cost / inflation guard: Helps keep pace with rising values; does not eliminate coinsurance math if still under 80%.
• Blanket insurance: Single limit over multiple buildings; loss allocation rules determine applicable carried amount.

Exam Checklist

1. Identify value at time of loss and coinsurance %.
2. Compute amount required.
3. Compare to amount carried.
4. If under, multiply covered loss by carried ÷ required.
5. Subtract deductible last unless question asks for pre-deductible figure.
6. Confirm answer does not exceed policy limit.

Coinsurance math rewards methodical test-takers. Practice until the penalty ratio is automatic—your Nevada P&C candidates will face it on both property concepts items and commercial property scenarios.