Quick answer: in Portugal, fuel prices almost always change on Monday and follow international diesel and petrol quotes with a one-week lag. If quotes rose during the week, diesel and petrol will most likely go up the following Monday.
The forecast published on Friday is an estimate, not an official price: each brand sets its own, and taxes can change too.
Below we explain how the pump price is built, why the change happens on Monday, where to find the forecasts and how to check a station's real price before you fill up.
Why do fuel prices change on Monday?
The fuel market in Portugal is free: there is no price set by the State. In practice, oil companies update prices once a week, at the start of the week, based on the average of the previous week's international quotes. That is also how ERSE, the regulator, calculates its reference price: a week's price comes from the previous week's quotes.
Our DGEG data shows this rhythm clearly. In September 2026, the biggest moves in the national average all happened from Sunday to Monday:
- 31 August: simple diesel down 3.7 cents per litre, simple 95 petrol up 2.7 cents;
- 7 September: diesel up 8.7 cents and petrol up 7.3 cents;
- 14 September: diesel up 4.2 cents, petrol down 2.5 cents;
- 21 September: diesel up 5.6 cents and petrol up 4.1 cents.
Between Monday and Sunday the average barely moves: these are mostly adjustments by individual stations.
Will diesel go up or down next week? How the forecast works
The forecasts you read in the papers on Friday compare the average of international quotes for the week that is ending with the week before. If refined diesel became more expensive on north-west European markets, the forecast is a rise; if it became cheaper, a fall.
Diesel and petrol do not always move together.
What matters is not only crude oil (Brent) but the price of the refined product, which depends on refinery supply, demand for diesel for transport and heating, and the euro-dollar exchange rate. That is why, in the same week, diesel can go up while petrol goes down, as happened on 14 September.
There are two reasons a forecast can miss:
- Taxes change. In 2026 the Government has revised the extraordinary ISP discount almost every week, through an ordinance published on Friday. Ordinance no. 437-B/2026/1 of 25 September, for example, cut that discount from 28 September. When the discount shrinks, part of the expected fall disappears.
- Each brand decides. The forecast is an average. A hypermarket station, an independent one and a motorway station may change their prices by different amounts, or not at all.
What makes up the price of a litre of fuel
The retail price adds up several parts:
- the cost of the refined product on international markets;
- transport, storage and mandatory reserves;
- blending of biofuels;
- distribution and station margins;
- ISP (the tax on petroleum products) and the CO2 emissions surcharge, the so-called carbon tax;
- VAT at 23% on the mainland, charged on everything else, taxes included.
To give an idea of the weight of taxes: in the ERSE weekly report for the week of 21 September 2026, the efficient price of simple diesel was €1.450/l before taxes and €2.308/l with taxes. For simple 95 petrol, €1.134/l and €2.111/l. In other words, about 86 cents per litre of diesel and 98 cents per litre of petrol were taxes.
The efficient price is a reference calculated by the regulator, not a mandatory price. It is useful for comparison: when the national average sits well above it, margins are higher.
Where to see today's fuel prices
The forecast tells you where the average is heading. To know how much you will pay, what counts is the price at the station where you fill up. Stations report their prices to DGEG, which publishes them on the DGEG fuel prices portal.
At AUTO.MOTO.pt we use the same data on our fuel prices in Portugal page: the day's national average, the cheapest stations by district and municipality, price history and alerts per station. You can also go straight to today's simple diesel price or today's 95 petrol price.
How to save with the weekly change
- Rise forecast: if your tank is already half empty, fill up by Sunday.
- Fall forecast: if you can, wait until Monday, but check the station's price, because the fall may be smaller because of ISP.
- Avoid the motorway: in the DGEG data we track, on 28 September 2026 simple diesel at motorway stations cost on average about 6 cents per litre more than at other stations.
- Compare in your area: the gap between the most expensive and the cheapest station in the same municipality is often worth more than the week's change.
- Consider LPG or electricity: the LPG Auto price is less than half that of diesel per litre, although an LPG car burns more litres per 100 km. Charging an electric car at home is a different calculation, which we explain in how much it costs to charge an electric car at home.
Frequently asked questions
What time do fuel prices change?
Usually in the early hours of Monday. Some stations only update during the morning, and independent stations may change on another day.
When do next week's forecasts come out?
Usually on Friday, after the week's markets close. The ISP update is normally known on the same day, through an ordinance in the Diário da República.
Is the forecast guaranteed?
No. It is an estimate of the average change, based on quotes. The final price depends on each brand and each station, and on the taxes in force that week.
Why does diesel go up when oil goes down?
Because the station sells refined diesel, not crude oil. If diesel supply from European refineries is tight, or if the euro loses value against the dollar, diesel can rise even while Brent falls.
Are prices the same on the islands?
No. In the Azores and Madeira the regional governments set maximum prices and taxes are different. This article refers to mainland Portugal.
