CNR diesel index: automate your fuel surcharge in 2026

Why fuel indexation has become vital for carriers

The price of professional diesel represents between 25 and 35% of the operating costs of a road transport vehicle. With the volatility observed since 2022, carriers who invoice a fixed rate without an adjustment mechanism see their margins melt away in a matter of weeks. Conversely, poorly informed shippers face sudden price hikes without understanding their origin.

It is precisely to smooth this commercial relationship that the legislator imposed, via the Gayssot law and then the law of 5 January 2006, a mandatory mechanism for passing on variations in fuel prices. The CNR diesel index published by the Comité National Routier has become the essential reference for applying this pass-through transparently and indisputably.

Yet, in 2026, a majority of transport SMEs continue to manually recalculate their fuel surcharges in Excel files, at the cost of errors, invoicing delays and tensions with customers. It is time to automate this process.

What exactly is the CNR diesel index?

The Comité National Routier (CNR) is a joint body created in 1949, tasked with observing and publishing road freight transport costs in France. Each month, it publishes several reference indices, including the famous professional diesel index.

The different CNR indices to know

• Professional diesel index: average price of diesel excluding recoverable taxes, used for heavy vehicles (>7.5 tonnes).
• Non-professional diesel index: price at the pump including all taxes, relevant for light commercial vehicles.
• CNR long-distance, regional, parcel indices: composite indices integrating several cost items.

For indexing a fuel surcharge, it is generally the monthly professional diesel index that serves as the basis. It is published around the 10th of each month for the previous month, freely accessible on the CNR website.

The legal framework: the mandatory CNR clause

Article L. 3222-1 of the French Transport Code requires that each invoice mention the fuel item and its variations. In the absence of a negotiated contractual clause, the carrier can apply, as of right, the legal formula based on the CNR index. Refusing this pass-through exposes the shipper to criminal penalties.

The mathematical formula to index your fuel surcharge

The standard monthly recalculation formula is based on a simple principle: measure the gap between the index of the reference month (that of the contract signature or the rate in force) and the index of the invoicing month.

New rate = Initial rate × [1 + (Diesel share × ((Index M – Index M0) / Index M0))]

Let’s break down the variables:

• Initial rate: the price negotiated at contract signature.
• Diesel share: percentage that fuel represents in the total cost (generally 25 to 35%, to be set contractually).
• Index M0: value of the CNR index in the reference month.
• Index M: value of the CNR index in the invoicing month.

A concrete example

Imagine a transport rate of €1,000 negotiated in January 2026, with a diesel share set at 30%. The January CNR index (M0) is 135.20. In April 2026, the index rises to 142.80.

Calculation: 1,000 × [1 + (0.30 × ((142.80 – 135.20) / 135.20))] = 1,000 × [1 + (0.30 × 0.0562)] = 1,000 × 1.01686 = €1,016.86

The fuel surcharge to be invoiced is therefore €16.86 for this service in April. Multiply that by hundreds of monthly invoice lines, and you understand why automation becomes essential.

The 5 mistakes that undermine your manual recalculations

1. Confusing professional and non-professional indices

Using the non-professional index for heavy trucks means overestimating your costs (since it includes non-recoverable TICPE). Result: disputed invoices, refunds to be made.

2. Forgetting to update the M0 index during renegotiations

When you renegotiate a rate, the reference index must be reset. Many accounting departments forget this step and continue to calculate on an obsolete M0.

3. Applying an unrealistic diesel share

Arbitrarily setting the diesel share at 20% when your real cost structure is 32% makes you lose money with every price hike. Have an annual audit of your cost structure carried out.

4. Applying the index with an uncontrolled lag

The index for month M is published around the 10th of month M+1. Invoicing in real time therefore requires using the M-1 index. This lag must be clearly indicated in the contractual clause.

5. Not keeping the history

In case of inspection or dispute, you must be able to justify each recalculation for several years. A lost or overwritten Excel file can cost you dearly.

How to automate monthly recalculation: methodology

Step 1: digitise your contracts

Centralise in a structured database the key information of each customer contract: base rate, negotiated diesel share, M0 index, type of index used, review date. Without this clean base, no automation is possible.

Step 2: automate index retrieval

The CNR publishes its indices in the form of downloadable tables. Several solutions exist:

• Monthly scraping of the CNR site via a Python script or a no-code tool (Make, n8n).
• Automated manual entry via a dedicated interface in your TMS.
• Native connectors offered by some SaaS transport solutions.

Step 3: integrate the formula into your invoicing software

Ideally, your TMS (Transport Management System) or invoicing software should automatically apply the formula to each invoice generation. The surcharge then appears as a separate line, with mention of the index used — a regulatory requirement.

Step 4: audit and control

Automation does not exempt you from control. Set up monthly reporting comparing the surcharges invoiced to your simulations, and investigate any discrepancy greater than 2%.

Everest: the solution that automates your fuel surcharges

At Everest, we have designed a platform specifically built for carriers who want to put an end to manual recalculations. Our Fuel Indexation module natively integrates CNR indices automatically updated each month, applies the legal formula to all your contracts and generates compliant invoices with the mandatory mention of the index.

Concretely, you gain:

• Time: no more hours spent in Excel at the beginning of each month.
• Peace of mind: zero calculation errors, guaranteed legal compliance.
• Margin: no diesel price increase is forgotten in invoicing.
• Customer transparency: your shippers receive a clear breakdown of the recalculation.

Everest integrates with your existing tools (TMS, ERP, accounting) and keeps a complete history of indices and calculations to satisfy any audit or inspection request.

Conclusion: switch to automatic indexation from 2026

The CNR diesel index is not an administrative constraint: it is a tool to protect your margin and ensure commercial transparency. Applied correctly and automatically, it guarantees fair invoicing for you as well as for your customers, and shields you from legal disputes.

In 2026, continuing to recalculate your surcharges manually is no longer sustainable. Between the persistent volatility of energy prices, compliance requirements and pressure on margins, automation has become a decisive competitive advantage. Carriers who have taken the plunge see an average recovery of 1 to 3% of margin over the year — simply because they no longer let any index variation slip through.

It is time to leave Excel behind and adopt tools that match the challenges of your business.