2.3 Coinsurance and the Coinsurance Formula

Why Coinsurance Exists

Most property losses are partial, not total. Without a penalty for underinsurance, an owner would insure a $500,000 building for only $100,000, pay a fraction of the premium, and still recover most partial losses — starving the insurer of the premium that matches the real exposure. The coinsurance clause corrects this by penalizing the insured on every loss when the policy limit falls below a required percentage of the property's value. It rewards insurance to value.

Common coinsurance percentages on commercial property: 80% (most common), 90%, and 100%.

The Coinsurance Formula

Claim Payment = (Amount Carried / Amount Required) x Loss

Two hard caps always apply after the formula runs: the payment can never exceed the actual loss or the policy limit, whichever is less. The deductible is subtracted last, after the coinsurance calculation. Memorize the order: compute the penalty ratio, multiply by the loss, then cap, then subtract the deductible.

Example 1 — Requirement Met (No Penalty)

• Building value $500,000; coinsurance 80%; policy limit $400,000; loss $100,000.
• Amount Required = $500,000 x 80% = $400,000.
• Ratio = $400,000 / $400,000 = 100%.
• Claim = 100% x $100,000 = $100,000 — the full loss is paid (less deductible).

Because the insured carried exactly the required amount, there is no penalty. Carrying more than required does not pay extra — the policy limit still caps recovery.

Example 2 — Underinsured (Penalty Applies)

• Building value $800,000; coinsurance 80%; policy limit carried $500,000; fire loss $200,000.
• Amount Required = $800,000 x 80% = $640,000.
• Ratio = $500,000 / $640,000 = 0.78125 (about 78%).
• Claim = 0.78125 x $200,000 = $156,250.

The insured eats roughly $43,750 of the partial loss as a coinsurance penalty for underinsuring. The penalty applies to partial losses; a total loss simply pays the policy limit (here $500,000), never more.

Total-Loss Behavior and How to Avoid the Penalty

On a total loss, coinsurance does not increase recovery — the insurer pays the lesser of the policy limit or the loss. If an $800,000 building insured for $200,000 burns to the ground, the insurer pays $200,000, not $320,000; the limit caps it.

Three ways to neutralize coinsurance:

• Insure to value — carry at least the required percentage of value.
• Agreed value endorsement — the parties agree on a value and coinsurance is suspended for the term.
• No-coinsurance / blanket forms — some commercial programs waive coinsurance for a higher rate.

The single best exam answer to "how do I eliminate the coinsurance penalty?" is agreed value.

Example 3 — With a Deductible (Full Sequence)

Put the whole process together so the order is automatic.

• Building value $600,000; coinsurance 90%; policy limit $450,000; loss $120,000; deductible $2,500.
• Amount Required = $600,000 x 90% = $540,000.
• Ratio = $450,000 / $540,000 = 0.8333.
• Penalty result = 0.8333 x $120,000 = $100,000.
• This is below the $450,000 limit, so no limit cap applies.
• Subtract the deductible last: $100,000 - $2,500 = $97,500 paid.

Note the deductible comes off after the coinsurance math, never before. Reversing that order is a deliberate distractor on the exam.

Coinsurance Pitfalls and Exam Traps

Keep these distinctions straight:

• Coinsurance compares policy limit to required value, not to the loss. Many wrong answers divide the limit by the loss.
• Carrying more than the required amount does not increase recovery beyond the policy limit; over-insuring wastes premium.
• The penalty is judged using the property's value at the time of loss, which can rise with inflation — an inflation guard endorsement helps keep the policy at the required percentage.
• Homeowners forms use a built-in 80% replacement-cost provision: insure the dwelling to at least 80% of replacement cost to collect full replacement cost on partial losses; fall below and partial losses settle on the lesser of ACV or a proportional amount.

Whenever a stem says "agreed value," the coinsurance penalty is off the table by design.

Quick Reference: Reading a Coinsurance Question

When a question gives you a property value, a coinsurance percentage, the amount carried, a loss, and possibly a deductible, work the four steps in order every time:

1. Required amount = property value x coinsurance percent.
2. Coinsurance ratio = amount carried / amount required (cap at 1.0; never pay a bonus for over-insuring).
3. Pre-deductible payment = ratio x loss.
4. Final payment = pre-deductible payment minus deductible, never exceeding the policy limit.

The most missed step is the cap at 1.0: if the insured carried more than required, the ratio is treated as 1.00, the penalty disappears, and the loss is paid in full up to the limit (less deductible). Examiners also slip in a total loss — at a total loss the coinsurance clause is often satisfied automatically up to the limit, so the insured collects the policy limit rather than a penalized amount.

Insurance to Value and the 80% Rule in Practice

The practical takeaway behind every coinsurance question is insure to value. Carriers reward owners who carry adequate limits with full partial-loss recovery; they penalize the rest. On homeowners forms the built-in 80% replacement-cost rule mirrors commercial coinsurance: maintain dwelling coverage at 80% or more of full replacement cost and partial losses pay replacement cost; drop below 80% and the carrier pays the larger of ACV or the proportional amount under the same ratio mechanics.

Because replacement cost climbs with construction inflation, a policy that met the requirement at inception can quietly fall short by renewal. An inflation guard endorsement raises the limit automatically to keep the insured at or above the required percentage, which is the standard recommendation when an exam stem describes an underinsured-by-inflation scenario.