Coinsurance is the condition that turns a $200,000 claim into a $150,000 check on a policy the owner thought was fine. It applies to partial losses, which is what makes it a surprise.

What the clause requires

A coinsurance percentage — commonly 80%, 90%, or 100% — is the share of the property's total value you agree to carry. Insure for less, and you become a partial self-insurer, so the carrier reduces every claim payment by the same proportion you fell short.

The formula:

(Limit carried ÷ Limit required) × Loss − Deductible = Payment

An 80% coinsurance example

A building has a replacement cost value of $1,000,000. The policy carries 80% coinsurance, so the required limit is $800,000. The owner, trying to hold down premium, insures for $600,000. A fire causes $200,000 in damage. The deductible is $5,000.

  • Required: $1,000,000 × 80% = $800,000
  • Carried: $600,000
  • Ratio: $600,000 ÷ $800,000 = 75%
  • Payment: 75% × $200,000 = $150,000, less the $5,000 deductible = $145,000

The owner is out $55,000 on a $200,000 loss — even though the $600,000 limit was three times the size of the claim. That is the part that stuns people. There was plenty of limit available. The penalty had nothing to do with running out of coverage.

Why the values drift

Almost nobody underinsures on purpose. It happens because:

  • The limit was set years ago and construction costs have climbed since
  • Equipment and inventory were added without updating the schedule
  • The owner insured to market value instead of rebuild cost — the two are different numbers and land is not insurable
  • The value was estimated on an actual cash value basis while the policy settles on replacement cost

That last one catches agents too. If the policy pays replacement cost, the coinsurance calculation uses replacement cost value.

How to get rid of the exposure

  • Agreed value. You submit a statement of values, the carrier accepts it, and the coinsurance clause is suspended for the term. This is the clean fix and the one to ask for on any account where values are uncertain.
  • Blanket coverage. One limit across multiple buildings or locations, which gives room to absorb a value that came in low at one spot.
  • Review values annually. Not at renewal in five minutes — actually price the rebuild and add the equipment purchased this year.
  • Higher deductible instead of a lower limit. If premium has to come down, take it out of the deductible. Cutting the limit buys a penalty on every future claim.

One thing coinsurance never does

It cannot increase your payment above the limit you bought, and it does not apply to a total loss that exceeds the limit — there, you simply collect the limit. It only bites on partial losses, which is the overwhelming majority of property claims.

Related: Actual cash value vs. replacement cost

Educational only — confirm against the actual policy form and the coinsurance percentage shown on the declarations.