Auto Repair Garage: Billed Hours vs Actual Hours, What Does a Diagnostic Overrun Cost in 2026?

You bill one hour of diagnostic time, but your mechanic spends two and a half hours finding the fault: what does that gap cost you? In an auto repair garage, comparing billed hours with actual hours shows how much work goes unbilled and whether your diagnostic price covers the workshop’s costs.

The figures below are teaching assumptions, not average rates. They let you redo each calculation from the costs shown. The example and the rules quoted are French: if you work in another country, check the rules that apply there.

Billed hours and actual hours: what is the difference?

Actual hours are the time really spent looking for the fault. Billed hours are the hours shown on the invoice, under the calculation method given to the customer. The gap can come from a fault that is hard to reproduce, repeated checks or a diagnosis more complex than expected.

The overrun is calculated like this:

Actual hours − billed hours = unbilled hours

For example, if the diagnosis takes 2.5 actual hours and 1 hour is billed, the overrun is 1.5 hours. The share of the time spent that goes unbilled is 60%: 1.5 ÷ 2.5.

At €85 excl. VAT per hour, those 1.5 hours represent €127.50 excl. VAT of potential unbilled labour: 1.5 × €85 excl. VAT. You cannot simply add this amount to the invoice: any extra work must be approved by the customer.

Before you interpret this gap, check how labour is calculated: on the time really spent or on a manufacturer’s time schedule. The DGCCRF, the French consumer protection authority, in its guidance on repair orders (accessed on 2 November 2026) says the customer must be able to know which method applies. The French Government’s reply published on 31 March 2026 also points out that prices are free, but that price information must be clear and given in advance (National Assembly, written question no. 11101, accessed on 2 November 2026).

Hypothetical example: what do 1.5 unbilled diagnostic hours cost?

To assess an overrun, you need the cost of the mechanic’s time, the variable costs tied to the work and the workshop’s fixed costs. Here is an entirely hypothetical annual base.

With 1,600 productive hours in the year, the pay cost per productive hour is €25: €40,000 ÷ 1,600 h.

The annual fixed costs used in the example are:

If the garage bills 1,200 hours in the year, these costs come to €30 excl. VAT per billed hour: €36,000 excl. VAT ÷ 1,200 h. For the example, variable consumables are estimated at €2 excl. VAT per actual hour.

Now suppose a diagnosis is billed at 1 hour at €85 excl. VAT, while it keeps the mechanic busy for 2.5 hours.

Revenue is €85 excl. VAT. Once the costs used in this example are deducted, the estimated result is −€12.50: €85 excl. VAT − €97.50.

The extra cost tied to the 1.5 unbilled hours is €40.50: 1.5 h × €25 of pay, or €37.50, plus €3 excl. VAT of consumables. Fixed costs are not presented as a new expense caused by this diagnosis: they are spread across the year’s billed hours.

Quick tip: record the real time spent on diagnoses, even when you bill a fixed price or a manufacturer’s time. Then compare the observed time with the agreed billing method and spot the types of fault that most often cause an overrun.

What if the overrun blocks another repair?

Labour and consumables do not always capture the effect of an overrun on the day. If the diagnosis delays another job that cannot be postponed, you can also estimate the contribution that repair would have made. Only add it to the calculation if the slot was really lost: an appointment that is simply moved and carried out later is not automatically a lost sale.

Hypothetical example: a brake replacement was due to take 2 hours. Labour is billed at 2 × €85 excl. VAT, or €170 excl. VAT. Parts are billed at €180 excl. VAT and cost the garage €110 excl. VAT. Consumables cost €4 excl. VAT.

If the diagnostic overrun loses this appointment, the €126 of estimated contribution adds to the cost of the overrun. This estimate rests entirely on the assumptions in the table and is not a general forecast for brake repairs. The calculation stays cautious: pay and fixed costs are deducted even though they remain due if the repair does not take place.

Can you automatically bill the extra time to the customer?

No: an overrun must not be added to the invoice without regard to what was agreed. Present the price or its calculation method clearly before the work, for example as a fixed fault-finding price or an hourly rate. If the diagnosis has to go beyond what was planned, inform the customer and record their approval before continuing.

The Government reply published in 2026 also states that using a diagnostic scan tool is not a legal requirement: it is one way of finding a fault. If the problem is simple or already identified by other means, the repairer does not have to use it. Read the full reply on the National Assembly website, published on 31 March 2026 and accessed on 2 November 2026.

To track the profitability of labour and parts more closely, see also our article on the hourly rate and parts margin of an auto repair garage. To test different costs and prices from your own assumptions, ProCalc.app offers margin and break-even calculations. The app is a one-time purchase.

How can you limit diagnostic overruns in an auto repair garage?

You cannot always predict how long fault-finding will take. You can, however, track its effects better by separating, for each job:

• the time really spent and the time billed;
• the billing method given to the customer;
• the consumables used;
• the customer’s approval if further checks are needed;
• any appointment delayed or lost.

Over several weeks, these records will help you spot the diagnoses that often run over and review your fixed prices, your procedures or your organisation. To take this analysis further, you can also read our article on the cost of warranty rework for an auto repair garage.

Frequently asked questions

How do you calculate the cost of a diagnostic overrun in an auto repair garage?

Subtract billed hours from actual hours, then multiply the unbilled hours by the hourly pay cost and the variable costs per hour. In this article’s hypothetical example, a 1.5-hour overrun costs €40.50 in pay and consumables, not counting fixed costs allocated per billed hour.

What is the difference between billed hours and actual hours?

Actual hours are the time spent on the diagnosis. Billed hours are the hours invoiced under the calculation method given to the customer. The difference is time spent that the invoice does not cover.

Can a garage bill the extra time spent on a diagnosis?

The garage must clearly tell the customer the price or how it is calculated before the work starts. If the diagnosis has to go beyond what was agreed, inform the customer and get their approval before continuing or billing anything extra.

How do you know whether a diagnostic overrun loses money?

Compare the diagnostic revenue with pay costs, variable costs and the share of fixed costs spread over billed hours. If the overrun really does prevent another repair, estimate separately the contribution that appointment would have made, and do not count it as lost if it is simply postponed and carried out later.

Does a garage have to use a diagnostic scan tool?

Not in France. According to the Government reply published on 31 March 2026, the scan tool is one way of finding a fault, not a legal requirement. If the fault is simple or identified by other means, the repairer does not have to use it.