2.3 Coinsurance and the Coinsurance Formula
Key Takeaways
- Coinsurance requires the insured to carry a stated percentage (commonly 80%, 90%, or 100%) of the property's full value or face a penalty on partial losses.
- The coinsurance formula is: (Limit Carried ÷ Limit Required) × Loss − Deductible = Amount Paid, never exceeding the policy limit.
- The penalty applies only to partial losses; a total loss pays the full limit regardless of coinsurance compliance.
- Underinsuring to save premium backfires: a building insured to 50% when 80% is required recovers only a fraction of every partial loss.
- An Agreed Value option suspends coinsurance entirely; a margin clause and inflation guard help keep limits adequate.
Why Coinsurance Exists
Most property losses are partial, not total. If insurers let everyone buy a small limit and still collect fully on partial losses, premiums would be inadequate because they are based on the full value at risk. Coinsurance is the rating tool that fixes this: in exchange for a rate discount, the insured promises to carry a limit equal to at least a stated percentage of the property's full value — typically 80%, 90%, or 100%. Carry less, and the insured becomes a 'co-insurer' and shares partial losses proportionally.
The coinsurance percentage appears on the commercial property Declarations and is the default penalty mechanism in the ISO Building and Personal Property Coverage Form.
The Formula
The coinsurance penalty is one of the few formulas you should be able to write from memory:
(Limit Carried ÷ Limit Required) × Loss − Deductible = Amount Paid
where Limit Required = Property Value × Coinsurance %. Two guardrails always apply:
- The result can never exceed the policy limit actually purchased.
- If
Limit Carried ÷ Limit Requiredis 1.0 or greater, there is no penalty — you simply pay the loss minus the deductible.
A useful shorthand is 'did ÷ should': what you did carry over what you should have carried, times the loss.
Read the fraction carefully. The denominator is the required limit (value times coinsurance %), not the property's full value. A common error is dividing the limit carried by the full property value, which overstates the penalty whenever the coinsurance percentage is below 100%.
Worked Scenarios
Assume a building worth $500,000, an 80% coinsurance clause, a $1,000 deductible, and a $100,000 partial loss. Required limit = $500,000 × 80% = $400,000.
Scenario A — Underinsured (penalty applies)
The owner carried only $300,000.
| Step | Calculation | Result |
|---|---|---|
| Coinsurance fraction | $300,000 ÷ $400,000 | 0.75 |
| Apply to loss | 0.75 × $100,000 | $75,000 |
| Subtract deductible | $75,000 − $1,000 | $74,000 |
The insured eats the other $26,000 as the coinsurance penalty plus deductible.
Scenario B — Compliant (no penalty)
The owner carried $400,000 (exactly the requirement). Fraction = $400,000 ÷ $400,000 = 1.0, so the formula is skipped: pay $100,000 − $1,000 = $99,000.
Scenario C — Total Loss
The building is destroyed (a $500,000 loss) while carrying only $400,000. Coinsurance penalties never apply to a total loss beyond the limit itself — the insured collects the full $400,000 policy limit (less deductible). Coinsurance only bites on partial losses.
Scenario D — Over-Insured Beyond the Requirement
Suppose the owner carried $450,000 (above the $400,000 requirement) and suffers the same $100,000 partial loss. The fraction is $450,000 ÷ $400,000 = 1.125, which is capped at 1.0 — there is no 'bonus.' Coinsurance can never pay more than the loss; the insured simply paid extra premium for limit they did not need below a total loss. The penalty mechanism only ever reduces, never increases, recovery on a partial loss.
A building is valued at $1,000,000 with an 80% coinsurance clause. The owner carries $600,000. A $200,000 partial loss occurs with no deductible. How much does the insurer pay?
Under an 80% coinsurance clause, when does the coinsurance penalty NOT reduce the recovery?
Escaping the Penalty: Agreed Value, Margin Clause, Inflation Guard
Underinsurance usually creeps in through inflation and rising construction costs — a building insured to value in 2021 may have slid below 80% by 2026. Several tools manage this risk.
| Tool | What it does |
|---|---|
| Agreed Value option | The insurer accepts a stated value and suspends coinsurance for the term — no penalty on partial losses |
| Inflation guard | Automatically increases the limit by a set percentage during the policy term to keep pace with cost |
| Margin clause | Caps recovery at a percentage of the reported value (used with reporting forms) |
| Peak season endorsement | Raises inventory limits during high-stock periods so coinsurance still matches value |
The practical lesson the exam wants: buying a smaller limit to save premium is a false economy under coinsurance, because the insured silently becomes a co-insurer on every partial loss — and partial losses are the overwhelming majority of claims.
Coinsurance vs. Deductible — Don't Confuse Them
Both shift cost to the insured, but they are different tools. A deductible is a flat retained amount the insured chooses; a coinsurance penalty is a proportional reduction triggered only by carrying an inadequate limit. The order also matters in a settlement: apply the coinsurance factor first, then subtract the deductible. A property question that gives you a value, a coinsurance percentage, a carried limit, a partial loss, and a deductible is asking you to chain all of these in the right sequence — required limit, fraction, times loss, minus deductible, capped at the limit.
Full Chain Worked Example
A building is valued at $1,000,000, written with 90% coinsurance, the owner carries $720,000, a $5,000 deductible applies, and a $300,000 partial loss occurs.
| Step | Calculation | Result |
|---|---|---|
| Required limit | $1,000,000 × 90% | $900,000 |
| Coinsurance fraction | $720,000 ÷ $900,000 | 0.80 |
| Apply to loss | 0.80 × $300,000 | $240,000 |
| Subtract deductible | $240,000 − $5,000 | $235,000 |
The insured recovers $235,000 and absorbs the $65,000 difference plus the deductible. Run the steps in any other order and you will land on a wrong distractor — which is precisely why several answer choices in these questions reflect the misordered arithmetic.