2.3 Coinsurance and the Coinsurance Formula
Key Takeaways
- Coinsurance requires the insured to carry a limit equal to a stated percentage (commonly 80%) of property value; carrying less makes the insured a co-insurer who shares every partial loss.
- The formula is (Amount Carried / Amount Required) x Loss = Claim, where Amount Required = Value x Coinsurance %.
- The penalty applies to PARTIAL losses, not just total losses, and the payment can never exceed the lesser of the actual loss or the policy limit.
- The deductible is subtracted AFTER the coinsurance calculation is performed.
- An Agreed Value clause suspends coinsurance; inflation-guard endorsements and annual value reviews help keep limits adequate.
Why Coinsurance Exists
Most property losses are partial, not total. Without a penalty for underinsurance, an owner would insure a $1,000,000 building for $200,000, pay a small premium, and still recover most partial losses, starving the insurer of premium for the true exposure. The coinsurance clause corrects this by penalizing underinsurance on every loss, pushing owners to insure to value.
Common coinsurance percentages: 80% (most common), 90%, and 100%.
The Coinsurance Formula
Claim Payment = (Amount Carried / Amount Required) x Loss
| Term | Meaning |
|---|---|
| Amount Carried | The policy limit actually purchased |
| Amount Required | Property Value x Coinsurance % |
| Loss | The actual amount of damage |
Two caps always apply: the payment can never exceed the actual loss or the policy limit, whichever is less. The deductible is then subtracted from the result.
Example 1 — Requirement Met
- Building value: $500,000; coinsurance 80%; policy limit $400,000; loss $100,000.
- Amount Required = $500,000 x 80% = $400,000.
- Ratio = $400,000 / $400,000 = 100%.
- Claim = 100% x $100,000 = $100,000 — the full loss is paid (less any deductible).
Example 2 — Coinsurance Penalty
- Same building, but the owner carries only $300,000.
- Amount Required = $400,000; Amount Carried = $300,000.
- Ratio = $300,000 / $400,000 = 75%.
- Claim = 75% x $100,000 = $75,000.
The insured absorbs a $25,000 penalty on a $100,000 loss because they were a co-insurer for 25% of the exposure. If a $1,000 deductible applied, the check would be $74,000.
A building is worth $800,000 and carries an 80% coinsurance clause. The owner insures it for $480,000 and suffers a $200,000 partial loss (no deductible). How much will the insurer pay?
Example 3 — Loss Exceeds the Limit (the Classic Trap)
- Building value $1,000,000; coinsurance 80%; policy limit $700,000; loss $900,000.
- Amount Required = $1,000,000 x 80% = $800,000.
- Ratio = $700,000 / $800,000 = 87.5%.
- Formula result = 87.5% x $900,000 = $787,500.
- But the payment cannot exceed the $700,000 policy limit, so the insurer pays $700,000.
This is the single most common coinsurance trap: students apply the formula and forget the policy-limit cap. Always test the formula result against the limit and the actual loss, and pay the lesser.
Avoiding the Coinsurance Penalty
| Method | How it helps | Caveat |
|---|---|---|
| Carry adequate limits | Insure to at least the required % of value | Values drift; review annually |
| Agreed Value clause | Suspends coinsurance entirely | Needs appraisal / statement of values |
| Inflation guard endorsement | Auto-increases limits ~4-8%/yr | Does not guarantee compliance |
Insurance to Value (ITV) = Amount of Insurance / Property Value. Reaching the coinsurance percentage (for example 80%) avoids the penalty: $400,000 of coverage on a $500,000 building is an ITV of 80%, exactly meeting an 80% requirement.
Blanket Coverage and Special Coinsurance Forms
When one limit covers multiple buildings or locations, blanket insurance with a Statement of Values is common. ISO requires the agreed value (CP 12 30) or a 90%+ blanket coinsurance percentage; the limit floats across covered property, so an underinsured building can borrow capacity from an over-reported one. The trade-off: errors on the statement of values can trigger a margin clause (CP 12 32) capping recovery at a stated percentage of the reported value.
Two related concepts the exam pairs with coinsurance:
- Functional vs. actual value at requirement — the required amount is based on the policy's valuation basis (RCV or ACV). An 80% requirement on a $500,000 RCV building is $400,000; on the same building valued at $350,000 ACV it is $280,000.
- Coinsurance on contents — the clause applies separately to building (Coverage A) and business personal property (Coverage B) limits; meeting it on one does not cure a shortfall on the other.
Finally, remember the deductible sequence: run the ratio, multiply by the loss, cap at the limit, then subtract the deductible. Reversing those steps is the most-missed multi-step calculation on the property exam.
Coinsurance on Contents and the Margin Clause
Coinsurance is not limited to buildings. Business personal property and homeowners contents can carry their own coinsurance requirement, and because inventory values fluctuate, commercial insureds often use reporting forms that adjust the limit to periodically reported values, with a penalty (the full-reporting or honesty clause) for understating values at the last report before a loss.
A margin clause caps recovery at a stated percentage (for example, 125%) of the values shown on the statement of values, working with agreed value to suspend the coinsurance penalty as long as the insured files an accurate statement. The exam contrasts these with the peak-season endorsement, which raises the limit during high-inventory periods so a retailer is not penalized for being underinsured at Christmas. The unifying theme is that every device exists to keep the amount of insurance honestly matched to the values at risk.
Why the Penalty Falls on the Insured
The logic the exam wants candidates to articulate is that the coinsurance clause shifts a participation requirement onto the insured. By agreeing to carry at least the stated percentage of value, the insured earns a lower rate per $100 of coverage. If the insured then carries less than required, the policy treats the insured as a coinsurer for the shortfall, so the insured shares the partial loss proportionally. The penalty therefore never appears on a total loss up to the limit, because the limit itself caps recovery — coinsurance only bites on partial losses where the ratio of carried-to-required is below 1.0.
Agreed Value Suspends the Clause
Insureds who do not want to track values can request the agreed value option. The insurer reviews a signed statement of values, and as long as the insured carries the agreed amount, the coinsurance condition is suspended for the term — no penalty regardless of later value changes. The trade-off is a slightly higher premium and the duty to refile the statement at renewal. Blanket coverage combined with a margin clause achieves a similar effect across multiple locations, paying losses up to the blanket limit while capping any one building at the margin percentage of its reported value.
Which statement about the coinsurance penalty is TRUE?