2.3 Coinsurance and the Coinsurance Formula

Why Coinsurance Exists

Most property losses are partial, not total. Without a penalty for underinsurance, an owner would insure a $500,000 building for $100,000, pay a small premium, and still recover most partial losses — underfunding the insurer for the real exposure. The coinsurance clause corrects this by requiring the insured to carry insurance equal to a stated percentage of value (commonly 80%, also 90% or 100%). Fall short and the insured becomes a co-insurer, sharing every partial loss in proportion to the shortfall.

Coinsurance is part of the broader concept of insurance-to-value (ITV): ITV = Amount of Insurance ÷ Property Value. Reaching the coinsurance percentage avoids the penalty.

The Coinsurance Formula

Claim Payment = (Amount Carried / Amount Required) x Loss

Three caps and rules govern the result:

1. The payment is never more than the actual loss.
2. The payment is never more than the policy limit.
3. The deductible is subtracted after the coinsurance calculation.
4. If Amount Carried ≥ Amount Required, the factor is capped at 1.0 — no penalty (and no bonus for over-insuring).

Worked Example — The Penalty Applies

Building value $500,000, 80% coinsurance, so Amount Required = $400,000. The owner carries only $300,000. A $100,000 partial fire loss occurs; $1,000 deductible.

The owner recovers $74,000 and absorbs the rest — the penalty for carrying 75% of what was required. Note the penalty hits a partial loss; it is not just a total-loss issue.

When carried meets the requirement

If the same owner carried $400,000+, the factor is 1.0: payment = $100,000 − $1,000 = $99,000 (capped at the loss and limit).

Avoiding the Penalty and Exam Traps

Practical defenses against a coinsurance penalty:

• Agreed Value endorsement — suspends coinsurance entirely.
• Inflation-guard endorsement — automatically raises the limit to track rising replacement cost.
• Annual value reviews — keep the limit at or above the required percentage.
• Peak-season / value-reporting forms — for fluctuating inventory.

Traps:

• The factor never exceeds 1.0 — over-insuring earns no extra recovery.
• Apply coinsurance to the loss, not to the building value, then cap at the limit, then subtract the deductible — in that order.
• Coinsurance is based on value at the time of loss, not the value when the policy was written — inflation can silently create a shortfall.
• A common distractor multiplies the factor by the building value rather than the loss — wrong.

Coinsurance vs. Insurance-to-Value Endorsements

The coinsurance percentage is a minimum, not a target. Carrying exactly 80% on an 80% clause means a total loss still leaves the insured 20% short, because the limit caps recovery. Most advisors recommend insuring to 100% of replacement cost and relying on inflation guard and a guaranteed/extended replacement cost endorsement to absorb rebuild-cost spikes after a catastrophe.

Key related endorsements and how they interact with coinsurance:

Commercial nuance: the ISO BPP can be written with an Agreed Value option (the insured files a statement of values), which suspends coinsurance for the policy term. If the statement is not renewed, coinsurance snaps back — a tested gotcha.

Blanket Coinsurance and the Margin Clause

When one limit covers several buildings or locations on a blanket basis, coinsurance is measured against the total values reported on a statement of values. To prevent an insured from underreporting values to dodge the penalty, insurers attach a margin clause that caps recovery at a stated percentage (e.g., 110%) of the value shown for the damaged location. The interplay of blanket limits, the statement of values, and the margin clause is a step up in difficulty the exam sometimes reaches for.

Trap: Filing a low statement of values to cut premium can trigger both a coinsurance penalty and a margin-clause cap at claim time - the insured loses twice.

Reading a Coinsurance Question in the Right Order

Multi-step numeric questions resolve cleanly only if you process them in a fixed order. Lock in this sequence and the distractors fall away:

1. Compute required = value at loss x coinsurance %.
2. Compute the factor = carried / required, capped at 1.0.
3. Multiply the factor by the loss (not the value).
4. Cap the result at the policy limit and the actual loss.
5. Subtract the deductible last.

Trap: Coinsurance is measured against value at the time of loss, so inflation can silently push an insured below the required percentage even though the limit never changed - the reason inflation-guard and agreed-value endorsements exist.