2.3 Coinsurance and the Coinsurance Formula

Coinsurance: Sharing the Risk of Underinsurance

Coinsurance is a property-policy clause that requires the insured to carry insurance equal to a stated percentage of the property's value (commonly 80%, 90%, or 100%). Its purpose is insurance to value — because most losses are partial, an insured could otherwise carry a small limit and rarely be penalized, undercutting the rate structure. The coinsurance clause penalizes underinsurance at the time of loss.

The national portion virtually guarantees at least one coinsurance calculation. Memorize the formula and the order of operations.

The Coinsurance Formula

(Amount of insurance carried ÷ Amount of insurance required) × Loss = Amount paid

where Amount required = Property value × Coinsurance %.

The result is then capped at the policy limit and reduced by the deductible. If the insured carries at least the required amount, the ratio is 1 (or more, but never above 1 for payment purposes) and the loss is paid in full up to the limit.

Steps:

1. Required = Value × Coinsurance %.
2. Ratio = Carried ÷ Required (cap at 1.00).
3. Payment = Ratio × Loss.
4. Subtract deductible; cap at policy limit.

Worked Example — Underinsured

Building value $500,000, 80% coinsurance, insured carries $300,000, loss $100,000, $1,000 deductible.

1. Required = $500,000 × 80% = $400,000.
2. Ratio = $300,000 ÷ $400,000 = 0.75.
3. Payment = 0.75 × $100,000 = $75,000.
4. Less $1,000 deductible = $74,000.

The insured absorbs the $25,000 coinsurance penalty plus the deductible because they carried only 60% of value instead of the required 80%.

Worked Example — Adequately Insured

Same $500,000 building, 80% coinsurance, but insured carries $400,000 (exactly the required amount), loss $100,000, $1,000 deductible.

1. Required = $400,000.
2. Ratio = $400,000 ÷ $400,000 = 1.00.
3. Payment = 1.00 × $100,000 = $100,000.
4. Less deductible = $99,000.

No penalty. Trap: the coinsurance ratio applies even when the loss is far below the limit — it is tied to the value of the property, not the size of the loss. A total loss is also paid only up to the policy limit, and coinsurance never increases recovery above the limit.

Quick-Reference Penalty Table

Carrying more than required does not pay more than the loss — the ratio is capped at 1.00. Agreed value endorsements suspend coinsurance entirely; blanket coverage and a properly filed statement of values also help avoid penalties.

Avoiding the Penalty: Agreed Value, Blanket, and Inflation Guard

The coinsurance penalty exists to enforce insurance to value, but several tools suspend or soften it:

• Agreed value endorsement — insurer and insured agree on a value (via a signed statement of values); the coinsurance clause is suspended for the policy term, so no penalty applies even if the building is technically under-valued.
• Blanket insurance — one limit covers multiple buildings/contents; a properly filed statement of values prevents per-item penalties and lets a loss at one location draw on the full blanket limit.
• Inflation guard — automatically increases the limit periodically to keep pace with rising replacement costs, reducing the chance the insured drifts below the required percentage.
• Peak season / reporting forms — for fluctuating inventory values, the insured reports values periodically and the limit adjusts, avoiding both under- and over-insurance.

Trap: agreed value does not mean the insurer pays more than the loss — it only removes the coinsurance penalty. The insured still recovers actual loss up to the limit.

Why Insurers Use Coinsurance and the Insurance-to-Value Logic

Coinsurance exists because most property losses are partial, not total. Without a coinsurance clause, an owner of a $1,000,000 building could insure only $200,000, pay a small premium, and still fully recover the typical $50,000 partial loss — collecting the same protection as someone who insured to full value but paying a fraction of the premium. That under-funds the loss pool and is unfair to fully insured policyholders.

The coinsurance clause restores fairness by rewarding insurance to value: carry the required percentage (commonly 80%, 90%, or 100% of value) and partial losses are paid in full up to the limit; carry less, and the penalty ratio reduces every partial-loss payment. The math is checked only at the time of loss, using the property's value then, which is why inflation guard matters — a building that appreciates can silently drop below the required percentage even though the insured never changed the limit.

Exam tip: the penalty applies to partial losses; a total loss is paid only up to the policy limit regardless of the ratio, and carrying more than required never pays above the actual loss.

Final Coinsurance Drill and Recap

Drill the four-step process until it is automatic. Required equals value times the coinsurance percentage. The ratio is insurance carried divided by that required amount, capped at 1.00. Multiply the ratio by the loss, subtract the deductible, and cap at the policy limit.

Remember that the penalty applies only to partial losses tied to property value, not loss size; that carrying more than required never pays above the actual loss; and that agreed value suspends the clause entirely. A building that appreciates can silently slip below the required percentage, which is why inflation guard and periodic reappraisal protect the insured from an unexpected coinsurance penalty at claim time.