2.3 Coinsurance and the Coinsurance Formula
Key Takeaways
- Coinsurance requires the insured to carry a minimum percentage (commonly 80%, 90%, or 100%) of the property's full value or face a penalty on partial losses.
- The coinsurance formula is: (Amount Carried / Amount Required) x Loss = Recovery, never exceeding the policy limit or the loss itself.
- Coinsurance penalties apply only to PARTIAL losses — a total loss simply pays the policy limit regardless of underinsurance.
- Insuring to value avoids the penalty; the deductible is subtracted AFTER the coinsurance calculation.
- Agreed Value and the waiver-of-coinsurance option suspend the coinsurance clause entirely, eliminating penalty risk.
Why Coinsurance Exists
Most property losses are partial, not total. If insurers let policyholders buy thin limits (say, insuring a $500,000 building for only $100,000), those underinsured owners would pay far less premium yet still collect on the small, frequent partial losses that make up most claims. Coinsurance is the contractual fix: it requires the insured to carry insurance equal to a stated percentage of the property's full replacement value — most commonly 80%, but also 90% or 100%. Carry less, and the insurer imposes a penalty on partial-loss payments, effectively making the insured a co-insurer for the shortfall.
The percentage is the coinsurance requirement. Multiply it by the property's full value to get the amount the insured should have carried.
The Coinsurance Formula
The tested formula is universal:
Recovery = (Amount of Insurance Carried / Amount of Insurance Required) x Loss
A memory device used everywhere: (Did / Should) x Loss, where Did is what you carried and Should is what coinsurance required (full value x coinsurance %). Two iron rules:
- Recovery can never exceed the policy limit or the amount of the loss.
- The penalty applies only to partial losses; a total loss pays the limit regardless of underinsurance.
Worked Example 1 - Underinsured (penalty applies)
Building value $500,000, 80% coinsurance, so the insured should carry $400,000. The insured actually carried $300,000. A $100,000 partial loss occurs.
- Did / Should = $300,000 / $400,000 = 0.75
- Recovery = 0.75 x $100,000 = $75,000
- The insured eats the remaining $25,000 as the coinsurance penalty.
Worked Example 2 - Insured to value (no penalty)
Same building, but the insured carried the full $400,000.
- Did / Should = $400,000 / $400,000 = 1.00
- Recovery = 1.00 x $100,000 = $100,000 — paid in full (then less deductible).
Deductible Ordering and Limit Caps
Sequence matters and is a classic trap. Apply the coinsurance formula first, then subtract the deductible, and finally cap at the policy limit.
Worked Example 3 - With a deductible
Building value $1,000,000, 90% coinsurance (should carry $900,000), insured carried $720,000, partial loss $200,000, deductible $5,000.
| Step | Calculation | Result |
|---|---|---|
| 1. Coinsurance factor | $720,000 / $900,000 | 0.80 |
| 2. Apply to loss | 0.80 x $200,000 | $160,000 |
| 3. Subtract deductible | $160,000 - $5,000 | $155,000 |
| 4. Cap at policy limit | min($155,000, $720,000) | $155,000 |
The insurer pays $155,000. Note the deductible came off after the coinsurance reduction, not before.
Total-loss override
If the same building were a total loss, coinsurance is not applied — the insurer simply pays the policy limit ($720,000) less the deductible. The penalty mechanism only bites on partial losses, where underinsurance otherwise lets the owner escape paying for the frequent small claims.
A building worth $500,000 has an 80% coinsurance clause. The owner insures it for $300,000 and suffers a $100,000 partial loss. Ignoring any deductible, how much does the insurer pay?
Suspending Coinsurance: Agreed Value
Commercial insureds who fear miscalculating value can attach the Agreed Value option (CP 04 60 / activated in the Commercial Property Declarations). The insured submits a signed Statement of Values; the insurer agrees that the limit equals the property's value, and the coinsurance clause is suspended for the policy term. No coinsurance condition means no penalty even if the values later prove low.
A related concept is the waiver of coinsurance, sometimes triggered automatically for small losses (e.g., losses under $5,000 may bypass the coinsurance test). Homeowners forms generally do not show a visible coinsurance clause — instead they enforce insurance-to-value through the replacement-cost condition requiring at least 80% of replacement cost at the time of loss; carry less and dwelling losses settle on an ACV basis. Functionally similar, different label.
When does a coinsurance penalty NOT apply to a property loss?
Coinsurance and Inflation: The Silent Trap
Coinsurance compliance is measured at the time of loss, not when the policy was bought. Construction costs rise, so an owner who insured to 80% three years ago may have slipped below the threshold through inflation alone, exposing them to a penalty they never chose. ISO addresses this with the Inflation Guard option, which automatically increases the building limit by a stated annual percentage, and homeowners forms use inflation-protection / index endorsements that bump Coverage A at renewal. These tools keep insurance-to-value current without the insured recalculating each year.
A second subtlety: the value used in the Should figure is the property's value on the valuation basis the policy uses. A replacement-cost policy tests the 80% against full replacement cost, while an ACV policy tests against ACV. Mixing the bases is a common wrong turn — never apply an 80% coinsurance requirement to market value or to land-inclusive figures.
Comparing Coinsurance Outcomes
The penalty scales with how badly the insured undershoots the requirement. Hold the property at $1,000,000, an 80% requirement ($800,000 should be carried), and a $100,000 partial loss, and vary only the amount carried.
| Amount carried | Did/Should factor | Recovery on $100,000 loss | Penalty absorbed |
|---|---|---|---|
| $800,000 (full) | 1.00 | $100,000 | $0 |
| $600,000 | 0.75 | $75,000 | $25,000 |
| $400,000 | 0.50 | $50,000 | $50,000 |
| $200,000 | 0.25 | $25,000 | $75,000 |
Notice the factor can never exceed 1.00 — over-insuring above the requirement does not pay more than the loss or the limit, so buying excess coverage wastes premium. The lesson the exam reinforces: the exact insure-to-value point (the requirement) is the efficient choice, and every dollar of shortfall is borne proportionally by the insured on every partial loss for the whole term.