Getting started

On a hotel page with your dates picked, Underbook checks the other booking sites Google Hotels lists for the same stay and shows you, on that page, whether one of them lists it for less and whether that lower price names the same room.

It is free, it never books anything, and you always book on the site you choose. A result shows the going rate (the price most sites agree on), any site listing the stay more than 5% below it, whether that listing names a matching room, and whether it shows free cancellation. If you want, Underbook also emails you when the price for that stay drops.

On the first hotel page after you install it, versions before 0.5.30 show a card first: tap "Check this hotel" once, and after that it checks every hotel page by itself. No extension? The web price checker gives the same comparison for any hotel and dates you type. How Underbook works explains where the prices come from and what each word on a result means.

Underbook works on hotel pages: it has rules for Booking.com, Expedia and its sister sites (such as Hotels.com, Hotwire, Orbitz and Travelocity), Agoda, Trip.com, Priceline, Marriott and Hilton, and it recognizes many other hotels' own websites from the page itself.

From version 0.5.30 it also reads the hotel pages of KAYAK, Hyatt, Choice, Wyndham, Accor, Radisson and Best Western, and from version 0.5.32 those of IHG. On any other hotel page, click the Underbook icon in your toolbar: that checks the page you are on.

It does not compare private rentals such as Airbnb listings, and it stays off Google Hotels and Google Maps, which are Google's own hotel pages. The prices themselves come only from the booking sites Google Hotels lists for your stay (where the prices come from).

Open your browser's Extensions menu (the puzzle-piece icon next to the address bar) and pin Underbook there: the pin icon in Chrome, the eye icon in Microsoft Edge, "Pin to Toolbar" in Firefox.

• Chrome: Extensions (the puzzle piece), then the pin next to Underbook.
• Microsoft Edge: Extensions (the puzzle piece), then the eye icon next to Underbook.
• Firefox: the newest version (from 0.5.33) puts its button on the toolbar when you install it. If it is not there, click the Extensions button (the puzzle piece), then the gear or the three dots next to Underbook, and choose "Pin to Toolbar".

Pinned, the icon is one click away, and a click on it checks the page you are on: useful on a hotel site where the banner does not appear by itself. In the newest version (from 0.5.33) the icon also shows what Underbook found: "ON" on a booking site it reads, "…" while it checks, "↓" when another site lists the stay lower, and "✓" when it has checked.

Not the extension: it runs in Chrome, Microsoft Edge and Firefox on a computer, so on a phone use the free web price checker, which compares the same booking sites for any hotel and dates you type, with nothing to install.

The checker can also email you if the price for that stay drops, or watch a stay you already booked (how watching a booking works).