2.3 Coinsurance and the Coinsurance Formula
Key Takeaways
- Coinsurance requires carrying a stated percentage (80/90/100%) of property value to receive full payment on partial losses.
- Formula: (Did Carry / Should Carry) × Loss − Deductible = Payment; Should Carry = value × coinsurance %.
- Underinsuring makes the insured a co-insurer who shares the loss; meeting the requirement pays the loss in full (less deductible).
- Adequacy is tested at the time of loss, so inflation can trigger a penalty; inflation guard or agreed value protects against this.
- The penalty applies to partial losses only; Agreed Value suspends coinsurance entirely.
Why Coinsurance Exists
Most property losses are partial, not total. If insurers charged the same proportional rate regardless of how much coverage an insured bought, everyone would underinsure — buying a small limit and gambling that no total loss occurs. The coinsurance clause combats this by requiring the insured to carry insurance equal to a stated percentage (commonly 80%, 90%, or 100%) of the property's value at the time of loss. Carry enough, and partial losses are paid in full (less deductible). Carry too little, and the insured becomes a co-insurer and shares the loss.
The Coinsurance Formula
The formula tested on every P&C exam:
( Did Carry / Should Carry ) × Loss − Deductible = Payment
Where:
- Did Carry = the actual limit of insurance purchased
- Should Carry = property value × coinsurance percentage
- Payment is capped at the policy limit and never exceeds the actual loss
A useful sanity check: if Did Carry ≥ Should Carry, the ratio is 1 (or treated as 1) and the loss is paid in full up to the limit, minus deductible.
Worked Example 1 — Underinsured
A commercial building is worth $500,000. The policy has an 80% coinsurance clause, so the insured should carry $400,000. The insured actually carries only $320,000. A fire causes a $100,000 loss; the deductible is $1,000.
| Step | Calculation | Result |
|---|---|---|
| Should Carry | $500,000 × 80% | $400,000 |
| Coinsurance ratio | $320,000 / $400,000 | 0.80 |
| Loss × ratio | $100,000 × 0.80 | $80,000 |
| Less deductible | $80,000 − $1,000 | $79,000 |
The insured absorbs the $20,000 coinsurance penalty PLUS the deductible. The penalty exists because they carried only 80% of the required amount.
Worked Example 2 — Adequately Insured
Same building, same 80% clause (Should Carry = $400,000). This time the insured carries the full $400,000. The same $100,000 loss occurs with a $1,000 deductible.
- Ratio = $400,000 / $400,000 = 1.00
- Payment = ($100,000 × 1.00) − $1,000 = $99,000
No coinsurance penalty applies because the insured met the requirement. Key trap: coinsurance is measured against the value at the time of loss, not at policy inception — so inflation can push a once-adequate policy into penalty territory. An inflation guard or agreed value endorsement protects against this.
Coinsurance Traps and Related Provisions
- The penalty applies to partial losses only; on a total loss the insured collects the full policy limit (which is usually below the should-carry amount anyway).
- Agreed Value suspends coinsurance entirely.
- A margin clause caps recovery at a stated multiple of reported values (common in blanket commercial policies).
- Coinsurance is a property concept — a similar but distinct idea, percentage participation, appears in health insurance; do not confuse them.
- ISO commercial property defaults to 80% coinsurance unless a higher percentage is selected; selecting 90% or 100% lowers the rate per $100 of coverage because the insured carries more.
A building worth $500,000 carries an 80% coinsurance clause. The insured purchased $320,000 of coverage. A $100,000 loss occurs with a $1,000 deductible. How much will the insurer pay?
Which statement about the coinsurance clause is CORRECT?
A Third Worked Case — Exactly at the Requirement
Coinsurance penalizes underinsurance using (Did / Should) x Loss = Payment, capped at the limit and reduced by the deductible. Practice all three outcomes:
- Underinsured: Building RC $500,000, 80% clause requires $400,000; insured carries $300,000; loss $100,000, deductible $1,000. Payment = ($300,000 / $400,000) x $100,000 = $75,000, minus $1,000 = $74,000.
- Exactly insured: Same building, insured carries the full $400,000; ratio = 1.0, so the policy pays the loss in full (up to the limit) minus the deductible.
- Total loss: Coinsurance never pays more than the limit; if the building is destroyed, the insured collects only the carried limit, which is why underinsurance hurts most on a total loss.
Coinsurance Variations and How to Avoid the Penalty
Several provisions interact with coinsurance:
| Provision | Effect |
|---|---|
| Agreed Value option | Suspends coinsurance if the insured files a statement of values and insures to the agreed figure |
| Blanket coverage | One limit over multiple items/locations, easing per-item shortfalls |
| Margin clause | Caps recovery at a percentage of reported values under blanket coverage |
| Inflation guard | Automatically raises the limit to keep pace with rebuilding costs |
Exam alert: Coinsurance is a property concept tied to partial losses; it does not apply to liability coverage and rarely bites on a total loss. The penalty is the insurer's tool to push insureds to carry an amount close to full value so premiums fairly reflect exposure. The cleanest way to avoid the penalty entirely is to elect the Agreed Value option or insure to 100% replacement cost.
Reading a Coinsurance Question Quickly
Every coinsurance problem reduces to four numbers: the property's replacement cost, the coinsurance percentage (usually 80%, 90%, or 100%), the limit carried, and the loss. The routine never changes: multiply RC by the percentage to get the amount that should have been carried; divide the amount did carry by that figure; multiply the ratio by the loss; cap the result at the limit; subtract the deductible. If the ratio is 1.0 or higher, there is no penalty and the loss is paid in full up to the limit.
The most common exam mistake is dividing by the full replacement cost instead of by the required amount (RC times the coinsurance percentage) — always insert the percentage. A second mistake is forgetting that coinsurance applies to partial property losses; on a total loss the insured simply collects the limit, and on liability coverage coinsurance does not apply at all.