What coinsurance is, and why it is not a deductible
The coinsurance condition on a property policy is a bargain: the insurer charges a lower rate per $1,000 of limit in exchange for your undertaking to insure at least a stated percentage of the property's value. If you break the undertaking, the penalty is proportional. Every covered loss is multiplied by the ratio of the limit you carried to the limit you should have carried.
That last point is the part people get wrong. Underinsurance does not merely mean your recovery is capped at your limit — that is true of every policy. Coinsurance means a partial loss well inside your limit is still cut. Insure a $1,000,000 building for $400,000 under an 80% condition and a $100,000 fire pays $50,000, even though the $400,000 limit is nowhere near exhausted. The same clause appears in UK and Commonwealth wordings under the name average, and in health insurance the word coinsurance means something entirely different — a percentage you share of each bill. Only the property meaning is calculated here.
The rate discount is genuine, which is why the condition survives. Insurers price the expected loss per dollar of limit, and most property losses are partial, so a policy at 80% of value collects proportionally more premium per dollar of expected loss than one at 40%. Coinsurance stops buyers from arbitraging that by insuring only the layer they expect to lose.
The related structures are worth knowing. The property coinsurance penalty calculator works the same condition from the homeowner side, the commercial property blanket limit calculator shows how blanket coverage tests the condition against total values instead of location by location, and the business interruption coverage calculator applies the identical arithmetic to earnings rather than to buildings.
The formula, and the order the three adjustments are applied in
The condition reads: multiply the loss by (limit carried ÷ (value × coinsurance percentage)), subtract the deductible, and pay no more than the limit. The order is fixed and it matters.
The ratio comes first. Applying the deductible before the ratio would reduce the penalty, and it is not what the policy says. On a $12,000 loss with a 50% ratio and a $5,000 deductible, the correct sequence gives 0.5 × 12,000 = $6,000, less $5,000, so the policy pays $1,000. Doing it the other way round — 12,000 − 5,000 = 7,000, halved — would pay $3,500. That is a factor of three and a half on the same claim.
The ratio is capped at 1. Buying more than the required limit does not create a bonus; it simply removes the penalty. That cap is why the recommended action is always to reach the requirement, never to exceed it for coinsurance purposes.
The limit caps the result. This produces a genuinely counter-intuitive consequence. On a loss large enough that the reduced amount still exceeds your limit, the coinsurance reduction costs you nothing, because the limit was going to cap you anyway. In the textbook case — $100,000 value, 80% condition, $60,000 limit — a $40,000 partial loss pays $30,000 against $40,000, a $10,000 penalty; a $100,000 total loss pays $60,000 with or without the condition. Coinsurance penalties are felt on partial losses and vanish on total ones. Since most property losses are partial, that is exactly where insurers wanted the incentive to bite.
One more definitional trap. The value in the denominator is the value at the time of loss, not the value at inception. A building insured to exactly 80% of its value three years ago is likely below the requirement today, without anyone changing the policy, purely because construction costs moved.
Worked example: an $800,000 property with a $600,000 limit
A commercial building has a replacement cost of $800,000 at the date of loss. The policy carries an 80% coinsurance condition, a $600,000 limit and a $5,000 deductible. A fire causes $200,000 of damage.
- Required limit. $800,000 × 80% = $640,000.
- Coinsurance ratio. $600,000 ÷ $640,000 = 0.9375. The limit is short of the requirement, so the ratio is below 1.
- Apply the ratio. $200,000 × 0.9375 = $187,500.
- Subtract the deductible. $187,500 − $5,000 = $182,500.
- Cap at the limit. $182,500 is well below $600,000, so the payment is $182,500.
- Measure the penalty. With the full required limit the payment would have been $200,000 − $5,000 = $195,000. The underinsurance penalty is $195,000 − $182,500 = $12,500.
The insurance-to-value ratio here is $600,000 ÷ $800,000 = 75%, against a requirement of 80%. The penalty is proportional to the shortfall in the ratio: 1 − 0.9375 = 0.0625, and 0.0625 × 00,000 = 2,500. Buying $40,000 more limit — the additional limit output — removes the penalty on this and on every other loss for the rest of the policy period.
How to read the result
The insurance-to-value ratio is the number to watch between renewals, and the comparison that matters is against your coinsurance percentage, not against 100%. At 80% coinsurance, an ITV of 82% is compliant and an ITV of 78% is not; the difference between those two is a 2.5% cut on every claim.
The underinsurance penalty output is scenario-specific. It answers what this particular loss costs you, and it will be zero on a loss large enough to be capped by the limit. Do not read a zero penalty as evidence that the limit is adequate — run a partial loss of 10% to 30% of value, which is the shape of most real claims, and look at the penalty there.
The additional limit needed is the actionable output. It is the cheapest fix available and it is usually inexpensive, because property rates are quoted per $1,000 of limit and the increment is small relative to the base policy. Compare it against the penalty on a realistic partial loss and the decision normally makes itself.
If the additional limit is a large number, the problem is not the policy but the statement of values. A large shortfall means the value in the denominator has moved — typically because construction costs rose, or because a renovation was never reported. Fix the value, not the limit, or you will be short again next year.
What a $200,000 loss pays at each level of underinsurance
| Limit carried | % of requirement | Coinsurance ratio | Claim payment | Penalty |
|---|---|---|---|---|
| $320,000 | 50% | 0.5000 | $95,000 | $100,000 |
| $384,000 | 60% | 0.6000 | $115,000 | $80,000 |
| $448,000 | 70% | 0.7000 | $135,000 | $60,000 |
| $512,000 | 80% | 0.8000 | $155,000 | $40,000 |
| $576,000 | 90% | 0.9000 | $175,000 | $20,000 |
| $640,000 | 100% | 1.0000 | $195,000 | $0 |
| $704,000 | 110% | 1.0000 | $195,000 | $0 |
Each payment is min(loss × ratio − deductible, limit), the expression the calculator evaluates. The last two rows are identical because the ratio is capped at 1 — buying above the requirement removes the penalty but never increases the payment.
Three things that quietly break compliance
Construction cost inflation. The requirement is a percentage of value at the time of loss. A limit set at exactly 80% of value at inception is below the requirement by the time costs rise, with no change to the policy and no notice from anyone. Unreported improvements. A renovation raises the value in the denominator immediately and the limit not at all. Valuation basis mismatch. If your policy settles on replacement cost, the requirement is a percentage of replacement cost, and a statement of values built from depreciated book value or property-tax assessment will be materially low. Any of the three can turn a compliant policy into a non-compliant one without a single decision being made.
Assumptions and things this does not model
- A flat dollar deductible. Percentage deductibles for wind, hail and earthquake are calculated against the limit or the value rather than the loss, and they interact with coinsurance differently.
- One item of covered property. Where a policy schedules building and business personal property separately, each carries its own limit and each is tested separately. Run the calculator once per item.
- No agreed value option. Many commercial forms let you suspend the coinsurance condition by filing a statement of values and accepting an agreed value endorsement. Where that is in force, no penalty applies at all.
- No inflation guard. An inflation guard endorsement raises the limit automatically during the term, which changes the numerator between inception and the date of loss.
- Excludes ordinance or law. The extra cost of rebuilding to current code is covered only if that extension is purchased and it is not part of the value in this calculation.
- Homeowners forms differ. Most homeowners policies use a replacement-cost provision requiring insurance to at least 80% of replacement cost, and settle at actual cash value below that rather than applying a proportional ratio. The mechanism is related but the arithmetic is not identical.
How to stop the penalty applying at all
There are three ways out, and they differ in cost and in effort. The first is simply to carry the required limit and keep it current, which is what this calculator prices. The second is an agreed value endorsement, available on most commercial forms: you file a signed statement of values, the underwriter agrees them, and the coinsurance condition is suspended for the term. That is the cleanest solution and it usually costs little, but it requires accurate values in the first place, so it does not rescue a bad appraisal — it just removes the proportional penalty for one that turns out slightly low.
The third is blanket coverage across multiple locations, where the condition is tested against total values rather than location by location, so an overstatement at one site offsets an understatement at another. That is a genuine advantage, and it is the reason blanket coverage is worth paying for even when a margin clause limits the recovery at any single site.
Whichever route you take, the work is the same underneath: keep the value right. Update annually with a published construction cost index, re-appraise after any renovation, and make sure the basis you value on matches the basis the policy settles on. Everything on this page is a consequence of one number in a denominator, and it is a number nobody looks at until there is a claim.
For the wider programme, the same value discipline drives the extended replacement cost calculator and, on the earnings side, the twelve-month business income figure that the coinsurance condition is tested against in a business interruption policy.