2.3 Coinsurance and the Coinsurance Formula
Key Takeaways
- Coinsurance requires the insured to carry a stated percentage (often 80%, 90%, or 100%) of the property's value or face a penalty on every loss.
- Payment = (Limit Carried / Amount Required) x Loss, where Amount Required = Value x Coinsurance %.
- The settlement is the LESSER of the formula result, the actual loss, or the policy limit.
- A ratio of 100% or more is capped at 100% — you never collect a bonus for over-insuring.
- Agreed value or full insurance to value is the clean way to avoid the coinsurance penalty.
Why Coinsurance Exists
Most property losses are partial, not total. Without a penalty for underinsurance, an owner could insure a $1,000,000 building for only $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 inadequate limits on every covered loss. The clause is found in most commercial property forms and in dwelling-fire policies, and it is one of the highest-yield calculation topics on the property portion of the exam.
The clause rewards owners who carry limits close to full value with lower per-dollar rates, and it makes the insured a co-insurer for any shortfall below the required percentage. Coinsurance percentages are stated in the policy declarations; 80% is the most common, with 90% and 100% options available for lower rates in exchange for carrying more coverage. Coinsurance applies only to partial losses — a total loss simply pays the limit.
Higher coinsurance percentages buy a lower rate per $100 of coverage because the insurer is assured of collecting premium on a value close to the true exposure. The trade-off is that the insured must carry a higher limit to escape the penalty, leaving less room for error if values are underestimated. The clause is an example of the insurance principle of insurance to value, aligning the premium collected with the risk transferred.
The Coinsurance Formula
Memorize this exactly:
Payment = (Amount Carried / Amount Required) x Loss
where Amount Required = Property Value x Coinsurance %. The final settlement is the lesser of the formula result, the actual loss, or the policy limit. Three numbers, one cap — that is the whole mechanic.
A common memory aid is "did/should over should": divide what you did carry by what you should have carried, then multiply by the loss. Note that value in the formula is measured at the time of loss, not when the policy was bound, so an appreciating building can quietly slide into underinsurance. Always read the question for the policy limit because it caps the formula result no matter how large the calculated payment is.
Coinsurance Calculation Steps
| Step | Operation |
|---|---|
| 1 | Amount Required = Property Value x Coinsurance % |
| 2 | Ratio = Amount Carried / Amount Required (cap at 100%) |
| 3 | Formula Payment = Ratio x Loss |
| 4 | Settlement = lesser of formula payment, loss, or policy limit |
Example 1 — No Penalty
- Building value: $600,000; coinsurance: 80%; limit carried: $480,000; loss: $120,000.
- Amount Required = $600,000 x 80% = $480,000.
- Ratio = $480,000 / $480,000 = 100%.
- Payment = 100% x $120,000 = $120,000 — the full partial loss is paid because the owner insured to value.
Example 2 — Coinsurance Penalty
- Same building and loss, but the owner carries only $360,000.
- Amount Required = $480,000; Amount Carried = $360,000.
- Ratio = $360,000 / $480,000 = 75%.
- Payment = 75% x $120,000 = $90,000.
The insured absorbs a $30,000 penalty because, by underinsuring, they acted as a co-insurer for 25% of the exposure.
Example 3 — the Policy-Limit Cap Trap
- Building value $1,000,000; coinsurance 80%; 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 payment cannot exceed the $700,000 limit, so the insurer pays $700,000. Forgetting the cap is the most common error on this calculation.
Notice that in Example 3 the insured both under-insured to value and suffered a near-total loss, so two limits bite: the coinsurance penalty reduces the formula result, and the policy limit then caps the actual check. When a deductible is also present, subtract it after the coinsurance step but recognize that the policy limit still governs the maximum. Practicing several variations until the order of operations is automatic is the surest way to bank these points on test day.
Avoiding the Penalty and Insurance to Value
Insurance to value (ITV) is the ratio of coverage carried to property value. Meeting or exceeding the coinsurance percentage avoids any penalty; a ratio of 100% or more is capped at 100% — over-insuring earns no bonus. Buying agreed value removes coinsurance entirely. Because values drift upward, limits should be reviewed annually.
Worked ITV check: $480,000 of coverage on a $600,000 building gives an ITV of $480,000 / $600,000 = 80%, exactly satisfying an 80% requirement. If construction-cost inflation pushed the building's value to $640,000, the same $480,000 would only be 75% insured to value, re-opening the penalty on the next loss. This is why annual limit reviews and inflation-guard endorsements matter.
An inflation-guard endorsement automatically increases the limit by a stated percentage over the policy term to keep pace with rising replacement costs, helping the insured stay above the coinsurance threshold. A peak-season endorsement does the same for inventory that swings seasonally. Both are tools that protect the insured from sliding into a penalty as values change, and the exam may ask which endorsement addresses fluctuating or rising values.
Exam Cheat-Sheet
- Amount Required = value x coinsurance %.
- Payment is the lesser of the formula result, the actual loss, or the policy limit.
- A ratio of 100% or more is treated as 100% — no bonus.
- Coinsurance applies to partial losses only; agreed value removes it. Watch the policy-limit cap on large losses — it is the most common miss.
A $500,000 building has an 80% coinsurance clause and is insured for $300,000. A covered loss of $50,000 occurs. How much will the insurer pay (before any deductible)?
An insured carries MORE than the required amount under a coinsurance clause. What effect does the excess have on a partial loss settlement?