Every progress certification can be calculated in two ways: by the work executed in that period alone, or by the running total since the start of the project. The second is cumulative certification (in Spanish, certificación a origen), and it is the most widely used method in Spain.
It looks like a minor calculation detail. In practice, confusing the cumulative amount with the amount to invoice is one of the most common errors in works administration, and it can end in double invoicing or certifications that do not reconcile with what has actually been collected.
A contractor who invoices the full cumulative total of the cumulative certification instead of the difference from the previous period is invoicing again for work the developer already paid for in previous months.
This guide explains what cumulative certification is, how it is calculated against period-only certification, and how to invoice it without errors. If you first need to review the general certification process, see the complete guide: progress certifications: what they are, types, and how to issue them.
What is cumulative certification?
Cumulative certification is the certification that reflects the total accumulated amount of work executed from the start of the contract up to the certification date, and not only what was executed in the last period.
Each time a new cumulative certification is issued, all the work executed up to that point is measured and valued again, not just what is new since the previous certification. This means each cumulative certification completely replaces the previous one as the snapshot of the project's accumulated status.
Cumulative certification vs. period-only certification
- Cumulative certification: recalculates the total amount executed since the start of the project in each period. If there was a measurement error in the previous certification, it is automatically corrected when the full running total is recalculated.
- Period-only certification: certifies only what was executed in the current period, without revaluing what came before. A measurement error in a past period is carried over until someone detects and manually corrects it.
Cumulative certification is the most widely used standard in Spanish construction precisely because it self-corrects deviations from previous measurements in each new certification, without needing to amend certifications already issued.
How to calculate the amount to invoice
The cumulative amount is not the amount to invoice. What is invoiced in each period is the difference between the running total of the current certification and the running total of the previous certification.
Step 1: Calculate the accumulated amount of the current certification. All the work units executed from the start of the project to date are measured and valued, at the prices of the contracted budget.
Step 2: Locate the accumulated amount of the previous certification. This is the cumulative amount that was certified in the immediately preceding period.
Step 3: Calculate the difference between the two totals. Current cumulative amount minus previous cumulative amount is the amount corresponding to the period to be invoiced.
Step 4: Apply retentions to the period amount. Performance retentions are calculated on the difference invoiced in that period, not on the full running total.
Step 5: Issue the invoice for the net difference. The period amount is invoiced, not the cumulative total, applying the corresponding VAT treatment — including the reverse charge where applicable, as explained in the reverse charge on progress certifications guide.
Worked example
| Item | Amount |
|---|---|
| Cumulative certification for month 3 (running total) | €500,000 |
| Cumulative certification for month 2 (previous running total) | €350,000 |
| Amount to invoice in month 3 (difference) | €150,000 |
| Performance retention (10% on the period) | −€15,000 |
| Net amount to collect for month 3 | €135,000 |
Common errors when invoicing a cumulative certification
- Invoicing the full running total in each period, instead of the difference from the previous certification, which duplicates the collection for work already invoiced.
- Not updating the running total after an approved change order, leaving the cumulative certification out of sync with the actual current budget.
- Applying the performance retention to the cumulative amount instead of to the period difference, inflating the actual retention.
- Confusing cumulative certification with period-only certification when comparing certifications from different projects or different contractors within the same project.
How Trowel helps
| Problem | How Trowel solves it |
|---|---|
| Errors calculating the difference between the current and previous running totals | Automatically calculates the amount to invoice for each period from the history of cumulative certifications, with no manual calculations |
| Retentions applied to the wrong amount | The cost control module applies the performance retention to the period difference, not to the running total |
| Mismatch between the certified running total and approved change orders | Links each change order to the corresponding line item, automatically updating the cumulative total in the next certification |
You can see the technical detail of each module in Trowel Academy.
Frequently asked questions
Do you invoice the cumulative amount or only the period difference?
You invoice the difference between the cumulative amount of the current certification and that of the previous certification, never the full running total.
Why is cumulative certification used instead of certifying only the period?
Because it recalculates everything executed in each certification, which automatically corrects previous measurement errors without having to amend certifications already issued.
Are performance retentions calculated on the running total or on the period?
On the invoiced period amount, that is, on the difference between the current and previous running totals, not on the cumulative total.
What happens if a change order is approved mid-project?
The change order amount is added to the budget and, from then on, is reflected in the cumulative running total of the next certification.
Conclusion
Cumulative certification is the most reliable method for controlling the real financial progress of a project, because each certification recalculates the entire project and corrects previous deviations. The point to master is that the cumulative running total is not the amount to invoice: what is collected in each period is always the difference from the previous certification.
If you want to automate the calculation of cumulative certifications and avoid invoicing errors, request a free Trowel demo.
You may also be interested in
- Progress certifications: what they are, types, and how to issue them →
- Subcontractor certifications on site: how to measure and control them →
- Reverse charge VAT on construction progress certifications →
- How to import a budget with BC3 files in TrowelApp →
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