Reinstatement Premium Calculation: Pro Rata Amount and Time
On this page
- Reinstatement Premium Calculation Errors: How Reinsurers Automate Pro Rata Amount and Time Correctly
- Key Takeaways
- What Is a Reinstatement Premium?
- How Is a Reinstatement Premium Calculated?
- What Do Pro Rata as to Amount and Pro Rata as to Time Mean?
- What Does a Worked Example Look Like?
- Why Do Reinstatement Premium Calculations Go Wrong?
- What Happens When the Treaty Premium Adjusts?
- When Should the Reinstatement Premium Be Booked and Settled?
- How Do Reinsurers Automate Reinstatement Premium Calculation Correctly?
- How Are Reinstatement Premiums Treated Under IFRS 17?
- Where Does Automation Help?
- What Should Heads of Reinsurance Accounting Do Next?
- Key Terms
- Sources
- Frequently Asked Questions
Reinstatement Premium Calculation Errors: How Reinsurers Automate Pro Rata Amount and Time Correctly
Last reviewed: November 2026
After a catastrophe, the claim gets every reviewer's attention. The reinstatement premium deducted from it often gets a spreadsheet, a tired analyst and no second check.
Short answer: Reinstatement premium calculation multiplies the treaty premium by the share of the limit reinstated and, where the clause says so, by the share of the period remaining. Reinsurers get it right by holding clause terms as structured data and recalculating automatically as losses and premiums develop.
Key Takeaways
- Read the clause before the formula. "Pro rata as to amount" and "100% as to time" change the result materially.
- Use the right premium base. A reinstatement calculated on the deposit premium usually needs adjusting later.
- Track the cap. The number of reinstatements limits both recoveries and reinstatement premium.
- Recalculate as losses move. A reinstatement premium is only as final as the loss estimate behind it.
What Is a Reinstatement Premium?
A reinstatement premium is an additional premium charged to restore an excess of loss limit after a loss. It is most common on catastrophe and clash covers.
The Reinsurance Association of America glossary describes it as premium for reinstating coverage reduced by a loss payment. The treaty's reinstatement clause sets the number, rate and basis.
How Is a Reinstatement Premium Calculated?
A reinstatement premium is calculated from four inputs: the premium, the rate, the amount reinstated and, sometimes, the time remaining. The clause decides which of these apply.
Pro rata as to amount, 100% as to time:
Reinstatement premium = treaty premium × reinstatement rate × (loss to layer ÷ layer limit)
Pro rata as to amount and time:
Reinstatement premium = treaty premium × reinstatement rate × (loss to layer ÷ layer limit) × (days remaining ÷ days in period)
A Munich Re training deck on non-proportional reinsurance notes that most treaty conditions it covers are "100% additional premium to time, pro-rata to amount". Market practice varies, so the wording always governs.
What Do Pro Rata as to Amount and Pro Rata as to Time Mean?
Pro rata as to amount charges for the share of the limit used. Pro rata as to time charges only for the share of the period still to run.
| Clause basis | What it charges | Effect of a late-year loss |
|---|---|---|
| Pro rata as to amount, 100% as to time | Full rate for the share of limit reinstated | No reduction for timing |
| Pro rata as to amount and time | Share of limit, scaled by days remaining | Much lower premium |
| Free reinstatement | Nothing, as the cost is priced into the base premium | None |
| Reinstatement at 50% or 150% | The stated percentage of the premium | Depends on the time basis |
What Does a Worked Example Look Like?
A worked example shows how the same loss produces very different amounts. All figures below are illustrative.
Treaty terms (illustrative):
- Cat excess of loss layer: 10,000,000 excess of 10,000,000
- Period: 1 January to 31 December, 365 days
- Annual premium: 1,000,000, with one reinstatement at 100%
- Aggregate limit: 20,000,000 (original limit plus one reinstatement)
Loss 1 on 1 September: ground-up loss of 16,000,000, so the loss to the layer is 6,000,000. There are 122 days from 1 September to 31 December.
- Amount only: 1,000,000 × 100% × (6 ÷ 10) = 600,000
- Amount and time: 600,000 × (122 ÷ 365) = 200,548
Loss 2 on 1 November: loss to the layer is 8,000,000. Only 4,000,000 of reinstatement remains, so only that amount is reinstated.
- Amount only: 1,000,000 × 100% × (4 ÷ 10) = 400,000
- Amount and time: 400,000 × (61 ÷ 365) = 66,849
On the amount-only basis, total reinstatement premium reaches the 1,000,000 cap. The remaining limit is 6,000,000, with no further reinstatement.
These figures are at 100% of the layer. A reinsurer with a 15% line applies its share to both the loss and the premium.
Why Do Reinstatement Premium Calculations Go Wrong?
Reinstatement premium calculations go wrong because each input has its own trap. Most errors come from reading the clause differently from how it was priced.
| Error | What happens | Control |
|---|---|---|
| Wrong premium base | Deposit used where the clause means adjusted premium | Store the defined base in the treaty record |
| Time factor added or omitted | Result wrong by a large margin | Store amount and time bases separately |
| Wrong date | Date of advice or payment used instead of date of loss | Capture date of loss with the claim |
| Cap ignored | Premium charged beyond the reinstatements available | Track used and remaining reinstatement |
| Loss estimate not updated | Premium based on an outdated incurred figure | Recalculate when the loss to layer changes |
| Rate per reinstatement ignored | Second reinstatement charged at the first rate | Store a rate for each reinstatement |
Errors run both ways. Some overcharge the cedent and damage the relationship, while others leave premium unbilled.
What Happens When the Treaty Premium Adjusts?
When the treaty premium adjusts, the reinstatement premium usually has to be recalculated. A calculation based on the deposit premium is then provisional.
The Munich Re deck shows this. Reinstatement premium is recalculated on the adjusted premium, and the difference is billed as additional reinstatement premium.
Illustrative: if the 1,000,000 deposit premium in the example adjusts to 1,100,000, loss 1 on the amount-only basis becomes 660,000. That creates a further 60,000 to bill.
Many teams forget this step. It sits months after the loss, in a different process run by a different person.
When Should the Reinstatement Premium Be Booked and Settled?
The reinstatement premium should be booked when the loss to the layer is recognised and settled as the clause directs. Timing rules differ between treaties.
Some wordings link the premium to losses paid, while others work from the latest incurred estimate. The clause, not habit, should drive which loss figure feeds the formula.
Either way, the amount moves as the loss develops. Each movement needs a matching reinstatement adjustment, or the premium and the claim drift apart.
How Do Reinsurers Automate Reinstatement Premium Calculation Correctly?
Reinsurers automate the calculation by turning the clause into structured terms and rules, then running them on every loss movement. The steps below form one controlled process.
- Capture the clause as data. Record number of reinstatements, rate for each, amount basis, time basis and premium base.
- Link losses to the layer. Hold the date of loss and the latest loss to layer for each event.
- Calculate at 100% and at share. Run the formula on the full layer, then apply each reinsurer's line.
- Track the reinstatement balance. Record limit used, reinstated and remaining after each loss.
- Recalculate on every change. Re-run when the loss estimate moves or the treaty premium adjusts.
- Review before settlement. A second person checks material amounts before they are netted against the claim.
The same Munich Re deck notes that reinsurers commonly offset reinstatement premium against the claim payment. That makes the net claim figure only as reliable as the reinstatement calculation.
How Are Reinstatement Premiums Treated Under IFRS 17?
Under IFRS 17, the treatment depends on whether the reinstatement is mandatory or voluntary. Finance and treaty teams should agree the classification for each treaty.
An IFRS 17 Transition Resource Group staff paper from September 2018 analysed both. For the reinsurer, it treated mandatory reinstatement premiums as equivalent to paying a different amount of claims.
Voluntary reinstatements, where the cedent may choose not to reinstate, were analysed as additional premium within the original contract boundary. Your auditors will expect the classification to be documented.
Where Does Automation Help?
Automation helps by applying the same treaty terms to every loss, every time, with a record of each recalculation. People still interpret the wording and approve the result.
Reinsurance treaty management software holds treaty terms, retention and limit configuration and treaty calculation rules in one place. Premium calculation, calculation history and an audit trail sit alongside amendments and versioning.
That structured treaty data supports consistent reinstatement calculations. Treaty and finance teams validate the configuration and review the results.
Reinstatements are one of several items that go wrong in billing. Our guide to billing the treaty correctly covers the wider picture.
If teams hold different versions of the clause, no formula will help. See why reinsurers need one source of truth for treaty terms.
What Should Heads of Reinsurance Accounting Do Next?
Heads of reinsurance accounting should check that every reinstatement clause is captured as data and recalculated on change. Start with the largest cat programmes.
- Extract reinstatement terms for all excess of loss treaties into one register.
- Confirm the premium base and time basis for each with the underwriter.
- Recalculate reinstatement premium for every open loss after each premium adjustment.
- Add a second review for reinstatement amounts above an agreed threshold.
Key Terms
- Reinstatement premium: Additional premium charged to restore an excess of loss limit after a loss.
- Pro rata as to amount: Charging in proportion to the share of the limit reinstated.
- Pro rata as to time: Charging in proportion to the period remaining after the date of loss.
- Loss to layer: The part of a loss that falls within the layer, after the retention.
- Aggregate limit: The total the layer can pay in the period, including reinstatements.
- Deposit premium: A provisional premium paid in advance and adjusted later to the actual premium.
- Free reinstatement: A reinstatement with no additional premium, priced into the base premium.
Sources
- Munich Re: Features of Non-Proportional Reinsurance (N. Lalloo)
- IFRS Foundation: TRG for IFRS 17, Agenda Paper 3: Commissions and reinstatement premiums in reinsurance contracts issued (September 2018)
- Reinsurance Association of America: Glossary of Reinsurance Terms
- Insurnest: Reinsurance Treaty Management
Frequently Asked Questions
How is a reinstatement premium calculated?
A reinstatement premium is calculated by multiplying the treaty premium by the reinstatement rate and by the loss to the layer divided by the layer limit. If the clause is also pro rata as to time, the result is multiplied by the days remaining after the loss divided by the days in the period.
What does pro rata as to amount mean in reinstatement premium?
Pro rata as to amount means the reinstatement premium is charged in proportion to the part of the limit used and reinstated. A loss using 60% of the layer limit is charged 60% of the reinstatement premium for a full limit.
What does pro rata as to time mean in reinstatement premium?
Pro rata as to time means the reinstatement premium is reduced in proportion to the period left between the date of loss and treaty expiry. A clause that says 100% as to time charges the full amount whatever the date of the loss.
Which premium is used for the reinstatement premium calculation?
The reinstatement premium calculation uses the treaty premium defined in the clause, usually the annual or adjusted premium for the layer. Where the deposit premium is later adjusted, the reinstatement premium is usually recalculated on the final premium, creating an additional or return amount.
What happens when all reinstatements are used?
When all reinstatements are used, the layer is not restored after further losses and no further reinstatement premium is due. The cedent can still recover up to the remaining limit, but the aggregate limit for the period is then fixed.
Why do reinstatement premium calculations go wrong?
Reinstatement premium calculations go wrong when teams apply the wrong premium base, add or omit the time factor, use the wrong date of loss, ignore the reinstatement cap, or fail to recalculate as the loss estimate and treaty premium change.
How are reinstatement premiums treated under IFRS 17?
Under IFRS 17, an IFRS Transition Resource Group staff paper from September 2018 analysed mandatory reinstatement premiums as economically equivalent to paying a different amount of claims. Voluntary reinstatement premiums were analysed as additional premium within the boundary of the original contract.
Can reinstatement premium calculation be automated?
Yes, reinstatement premium calculation can be automated once the clause terms, premium base and loss data are held as structured treaty data with calculation rules. Treaty and finance teams should still validate the configuration and review results before settlement.
Hitul Mistry
CEO, Insurnest
An InsurTech leader with more than a decade of experience across insurance and technology, focused on solving business problems with the help of technology. Has worked with brokers, insurance carriers, and reinsurance firms across the India, UAE, and US markets.
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