2.3 Coinsurance and the Coinsurance Formula

Why Coinsurance Exists

Most property losses are partial, not total. Without a penalty, insureds would carry only enough coverage to handle a likely partial loss and pay far too little premium for the limit they buy. The coinsurance clause corrects this by requiring the insured to carry a stated percentage — most often 80%, 90%, or 100% — of the property's full value. Carry less and the insurer pays only a proportionate share of every partial loss. This is one of the most heavily tested calculations on the national exam.

The Coinsurance Formula

Loss Payment = (Limit Carried ÷ Limit Required) × Loss − Deductible

where

Limit Required = Property Value × Coinsurance %

Three rules constrain the result:

1. The payment can never exceed the policy limit.
2. The payment can never exceed the actual loss.
3. If the insured meets or exceeds the required limit, the ratio is capped at 1.0 (full payment, less deductible) — there is no bonus for over-insuring.

Worked Example — Under-Insured

Building value $500,000; coinsurance 80%; the insured carries only $300,000; partial loss $100,000; deductible $1,000.

• Required = $500,000 × 80% = $400,000
• Ratio = $300,000 ÷ $400,000 = 0.75
• Payment = 0.75 × $100,000 = $75,000 − $1,000 deductible = $74,000

The insured eats the $25,000 coinsurance penalty plus the deductible — a direct result of carrying $300,000 instead of the required $400,000.

Worked Example — Compliant

Same building, same 80% requirement, but the insured carries the full $400,000.

• Required = $400,000; Carried = $400,000
• Ratio = $400,000 ÷ $400,000 = 1.0
• Payment = 1.0 × $100,000 = $100,000 − $1,000 = $99,000

No penalty applies. Carrying more than $400,000 would NOT increase the $100,000-loss payment — the loss and limit caps still control.

Traps to avoid: apply coinsurance to value at the time of loss, not the original cost; coinsurance applies to partial losses (a total loss simply pays the limit); and agreed value endorsements suspend coinsurance entirely.

A Repeatable Solving Method

Work every coinsurance problem in the same four steps so you never misplace a figure under time pressure:

1. Required limit = value at time of loss × coinsurance %.
2. Ratio = limit carried ÷ required limit (cap at 1.0; never exceed full payment).
3. Indicated payment = ratio × amount of loss.
4. Final payment = indicated payment − deductible, then check it against the policy limit and the actual loss caps.

Applying this to a 90% example: a $1,000,000 building with a 90% clause requires $900,000. If the insured carries $720,000 and suffers a $300,000 loss with no deductible, the ratio is $720,000 ÷ $900,000 = 0.80, so the insurer pays 0.80 × $300,000 = $240,000. The $60,000 shortfall is the coinsurance penalty the insured absorbs for under-insuring.

Related Provisions That Modify Coinsurance

Several commercial-property options interact with the clause:

• Agreed Value — suspends coinsurance for the policy term in exchange for an executed statement of values; if the insured forgets to renew the agreed-value option, the coinsurance clause snaps back into force.
• Coinsurance waiver — some forms waive the clause for small losses (often under $5,000 or 5% of the limit) so a tiny claim is not subjected to a penalty calculation.
• Blanket insurance — one limit covers multiple buildings or locations; coinsurance is then measured against the combined value of all covered property, which usually makes compliance easier.
• Margin clause — caps recovery at a stated percentage of the values reported on the statement of values, preventing a blanket limit from over-paying any single location.

The Concept Behind the Math

Understand why the penalty falls on the insured. Premium is charged per $100 or $1,000 of limit, so an insured who buys 60% of the needed limit pays roughly 60% of the proper premium. Coinsurance restores fairness by making that insured a co-insurer for the uninsured portion of every partial loss. Phrasing a question around "the insured becomes a co-insurer for the difference" is simply describing this principle in words rather than numbers.

Coinsurance vs. Insurance-to-Value

Do not confuse the coinsurance clause with the broader concept of insurance to value. Insurance to value is the underwriting goal of insuring property close to its full replacement cost; coinsurance is the contractual enforcement mechanism that penalizes failure to do so on partial losses. A policy can require insurance to value through other tools too — an agreed value option, a replacement cost condition that conditions full RC payment on carrying at least 80% of replacement cost, or an inflation guard endorsement that automatically increases the limit during the term to keep pace with rising construction costs.

On homeowners forms the 80% replacement-cost trigger functions exactly like a coinsurance requirement: fall below 80% of full replacement cost and the dwelling claim drops from replacement cost to the larger of ACV or the coinsurance-formula result. Recognizing that a homeowners "80% rule" question is really a coinsurance question in disguise earns easy points.