is there a website that compares flight prices?

Which one suits you depends on what you are after, so here are the main needs one at a time, with what each choice costs you.

If you want to know when to book

Google Flights is Google's flight search, with a date grid, a price graph, price insights and price tracking. The price insights say whether a fare is lower or higher than usual, and you can track a route or flight and get emailed when the price changes. Google's Flights page states no price or fee.

The catch is that historical price data shows only on some routes, according to Google. You get a view of timing on many trips, but not on every route you might search.

If you do not mind where you go

Skyscanner is a comparison site and app for flights, hotels and car hire across airlines and online travel agencies. Its Everywhere search finds low-cost flights anywhere, the calendar view shows the cheapest dates in a month, and Price Alerts tell you when fares move. Its app is free, and it says it does not charge any booking fees.

What you give up is the booking itself. You book with the airline or agency Skyscanner sends you to, not with Skyscanner, so there is a hand-off at the last step.

If you have a fixed budget

KAYAK is a travel search engine that searches hundreds of travel sites for flights, hotels and cars at once. KAYAK Explore shows flight options on any budget, and it adds price alerts and a Trips itinerary tool. The app is free on the UK App Store, and we saw no paid plan.

The cost is that some features, the flight tracker and offline trips, are app-only, per its listing. If you only use the website, you will not see them.

If you would rather use momondo

momondo is a flight and travel search app and site that searches hundreds of travel sites for flights, hotels and cars. Its listing describes an Anywhere search that looks for a deal on a destination you have not thought of yet, momondo Explore for searching to a budget, price alerts and a Trips itinerary tool. Its app is free on the UK App Store.

The thing to know is that its UK App Store listing names KAYAK as the developer and describes the same features as KAYAK's app, so expect the same tools from either. We did not compare their fares.

If you want the cheapest route at any cost in convenience

Kiwi.com is a flight search and booking site that builds routes from thousands of carriers, including trains and buses. It offers self-transfer itineraries combining different airlines, hidden-city fares and throwaway ticketing when cheaper.

You pay for that in convenience, because those are not ordinary single tickets. You book with Kiwi.com under its own guarantee, and its site lists automatic check-in and alternatives for missed connections. Kiwi.com Club membership comes after one booking, with member-only fares and voucher drops worth up to 500 euros, per its site.

When paying is worth it

None of these charge to search, so there is nothing to upgrade to. Skyscanner says it never charges booking fees, and Kiwi.com says it has no membership fees and no hidden charges. KAYAK and momondo are free apps with no paid plan seen, and Google's page states no fee.

The real trade-off is how you book. Skyscanner compares and sends you to the airline or agency to book, while Kiwi.com books itself and adds its own guarantee. We compared no fares, so we cannot say which site turns up the lowest price on a given trip; it depends on the route.

The fare you see is not always the total you pay. Google says the example fares it shows include required taxes and fees for one adult in Economy, and that optional charges and bag fees may apply. If you are taking a suitcase, the total can move.

Kiwi.com needs a closer read. Its hidden-city fares put a layover in your intended destination, which is a different thing from flying there directly, and its self-transfer trips combine separate airlines' flights, so the journey is stitched together from pieces.

KAYAK's app can measure your hand luggage with the camera to check it fits your flight's size limit, which is worth knowing before you get to the gate.