2.3 Coinsurance and the Coinsurance Formula

Why Coinsurance Exists

Most property losses are partial, not total. If insurers charged the same rate regardless of how much coverage an insured bought, everyone would under-insure — buying a small limit, paying a small premium, and still collecting on the small fires that make up the bulk of claims. Coinsurance corrects this by requiring the insured to carry coverage equal to a stated percentage (usually 80%, 90%, or 100%) of the property's full value at the time of loss. Carry enough, and partial losses are paid in full; carry too little, and the insured shares (co-insures) the loss.

The coinsurance percentage is shown on the Declarations page next to the property limit, and on ISO commercial property it is built into form CP 10 30 / CP 00 10.

The Coinsurance Formula

The formula tested on every state exam:

Payment = (Carried ÷ Required) × Loss − Deductible

Where Required = Property Value × Coinsurance %. The payment can never exceed the policy limit carried.

Worked example (the classic one):

• Building value at time of loss: $500,000
• Coinsurance requirement: 80% → Required = $500,000 × 0.80 = $400,000
• Limit actually carried: $300,000
• Loss: $100,000, deductible $1,000

Step 1 — Ratio: $300,000 ÷ $400,000 = 0.75 (75%). Step 2 — Apply: 0.75 × $100,000 = $75,000. Step 3 — Deductible: $75,000 − $1,000 = $74,000 paid.

The insured underinsured, so they absorb the missing 25% plus the deductible. The $25,000 shortfall is the coinsurance penalty.

Reading the Result and Avoiding Traps

Three exam traps recur:

• "Did I carry enough?" — Compare carried to required. If carried ≥ required, the ratio is capped at 1.0 (you never get a bonus for over-insuring) and partial losses are paid in full up to the limit.
• Total loss ignores coinsurance. If the loss equals or exceeds the limit carried, the insurer simply pays the limit — the formula does not reduce a total loss below the policy limit.
• Value is measured at the time of loss, not at policy inception. Inflation that raises building value can silently push the insured below the coinsurance threshold.

An Agreed Value endorsement suspends the coinsurance clause; an inflation guard endorsement automatically raises the limit to help stay compliant.

Blanket Coverage and the Margin Clause

Blanket insurance insures two or more buildings or locations under a single limit, giving the insured flexibility to draw on the full amount wherever the loss occurs. Coinsurance still applies, but it is tested against the combined value of all blanketed property, so the insured must report values accurately on a statement of values. A margin clause caps recovery at the reported value plus a stated percentage (e.g., 110%), preventing an insured from low-balling reported values to dodge premium yet collecting the full blanket limit.

Contrast with specific (scheduled) insurance, where each item carries its own limit and its own coinsurance test — a shortfall on one building does not borrow from another.

A Second Worked Example

Lock in the mechanics on a contents loss: business personal property valued at $250,000, 90% coinsurance, limit carried $180,000, loss $60,000, deductible $2,500.

1. Required = $250,000 × 0.90 = $225,000.
2. Ratio = $180,000 ÷ $225,000 = 0.80.
3. Apply = 0.80 × $60,000 = $48,000.
4. Less deductible = $48,000 − $2,500 = $45,500 paid.

The insured eats the 20% penalty ($12,000) plus the deductible because they carried only 72% of value against a 90% requirement. The fastest exam shortcut: divide the percentage of value actually carried by the required percentage — 72% ÷ 90% = 0.80 — to get the ratio in one step.

Why Coinsurance Exists and Its Boundaries

The coinsurance clause exists because most property losses are partial, not total. If insurers charged the same rate per dollar of coverage regardless of how much value an insured covered, owners would insure to a low figure (say, expected partial losses only) and underpay for the catastrophic-loss protection the pool provides.

Coinsurance fixes this by requiring the insured to carry a stated percentage (commonly 80, 90, or 100 percent) of the property's value; insure to that level and partial losses are paid in full up to the limit, fall short and the penalty formula (carried divided by required, times the loss) reduces every partial-loss payment.

The penalty applies only to partial losses. At a total loss, the policy limit (not the coinsurance formula) caps recovery, so an underinsured insured collects the full limit even though it is less than the property value. The formula also never pays more than the loss, the limit, or the value, whichever is least, after subtracting the deductible. Coinsurance is tested with the value measured at the time of loss, which can rise with inflation and silently push a once-compliant insured into a penalty, a problem the inflation guard endorsement addresses by automatically increasing the limit.

Two provisions remove the penalty risk. The agreed value option suspends coinsurance when the insured documents value by appraisal and carries that amount. A waiver of coinsurance for small losses (often under $5,000) appears in some forms. Knowing that agreed value turns off the penalty is a frequent exam point.

Worked total-loss contrast: using the same building insured for $180,000 at a $250,000 value, a total loss pays the $180,000 limit (the formula is not applied at total loss), whereas the $60,000 partial loss above was penalized to $48,000. The difference between partial-loss penalty and total-loss limit recovery is the distinction examiners most want you to draw.

Key Takeaways

Coinsurance requires carrying a stated percentage (often 80 percent) of value so partial-loss premiums are equitable; carrying less triggers the carried-divided-by-required penalty on partial losses. The penalty never applies at a total loss, where the policy limit caps recovery, and value is measured at the time of loss, so inflation can create a penalty that inflation-guard endorsements prevent. Agreed value suspends coinsurance entirely.