Why trust Underbook? Updated October 1, 2026

How Underbook works, and how to check us

Underbook compares the price of a hotel stay across the booking sites Google Hotels lists: on the hotel page you are looking at, with the Chrome extension, or on the web, with the price checker. This page says where the prices come from, what the words on a result mean, how often we measured them to hold, what we never do and how we make money. Each point names where you can check it: our privacy policy, our price study or our corrections log.

In short

The prices are what each booking site lists on Google Hotels for your hotel, dates and party size, looked up when you check.
"All-in" means taxes and fees as the sites report them. When no site reports a before-tax price, we say "listed rate" instead, because fees may be missing.
"Same room name" is strong evidence, not proof. Our blind tests found it right 84% to 92% of the time, so we still tell you to check the room.
When we clicked through to lower prices, most of the pages we could read showed them. Offers also disappear within hours, so check the final total before you book.
We never see your payment details, never book for you and never sell your data. We may earn a commission when you book through our links, at no extra cost to you, and it never changes what we show.

Where the prices come from

When you check a hotel, our price service looks up that hotel, your dates and your party size on Google Hotels and reads the price each booking site lists there. It gets Google Hotels' results as data through SerpApi, a search data service. The extension and the web price checker use the same price service.

Only what Google Hotels lists. A booking site that Google Hotels does not list for that stay is not compared. The prices are the public ones a signed-out visitor sees, so a discount you would only get after signing in to a site, such as Booking.com's Genius price, is not in them.
The extension does not read the price on your page. On a hotel page it reads the hotel's name, the town when the page shows it, your dates and the number of adults, and sends those to the price service. The price of the site you are on comes from Google Hotels' listing for that site, when there is one.
Prices can be up to 30 minutes old. A priced result is kept for 30 minutes so a repeat check is fast (the privacy policy says so), and booking sites change prices often. The site you open shows the final price.
A backup source. If the main lookup fails outright, the price service can use a second data provider, MakCorps. Its prices come without a tax breakdown, so they are shown as listed rates, in US dollars.

What "all-in" covers, and when it does not

For most listings Google Hotels gives two figures: the price per night with taxes and fees as the booking site reports them, and the price before taxes. Underbook calls the first one all-in and compares on it when it can.

When no site at that hotel reports a before-tax price, we cannot tell whether taxes and fees are in the numbers. The result then says "listed rate" and "Resort/property fees may not be included, verify at checkout." In the first snapshot of our study that was the case at 31% of hotels.
Some sites show a price before taxes on their own page and add taxes at checkout. When the lower listing has a before-tax price, the banner shows it too, as the price that site's page will show before taxes, with taxes and fees added at checkout.
All-in is what the sites report. We have not checked prices on a checkout page (see what we checked), so the final total is always the one on the booking site.
Prices are per room per night, for the number of adults on the page or in the form. Every result says which currency its figures are in. In the extension that is the currency of the booking site you are on, not one taken from your location.

What the words on a result mean

Going rate: The price most of the booking sites agree on for that hotel and dates. Exactly: the most common nightly price (before taxes, where the site reports it) among the listings that name a room or offer free cancellation (among all listings if none do), with ties going to the lower price. It is the usual price, not the lowest one.
A lower rate, and "Save ~$X/night": A site whose nightly price (before taxes, when the site reports it) is more than 5% under the going rate. The "Save" figure is the gap between the price for the site you are on (or the going rate, when that site is not in the list) and the lower listing, all-in when the sites report it. It is the difference between two listed prices at the time of the check, not a promise.
All-in, or listed rate: "All-in = taxes & fees included" when the sites report a before-tax price we can compare it with. "Listed rate" when none does, with a reminder to check the total at checkout.
Same room name: Shown only when a room on a listing more than 5% under the going rate has a name that matches, under strict rules, a room sold at the going rate. A dorm bed against a private room, the bed type and number, the room category, its grade, view, bathroom, smoking and accessibility must not conflict, and every detail one name states must also be stated on the other. The result names the matched room. A matching name is strong evidence, not proof (our tests), so check the room before you book.
Check the room: The lower listing names no room, or names one that does not match the room at the going rate. The web checker, and the extension from version 0.5.29, say which: "<site> doesn't name the room. Check it before you book.", "Different room.", "Similar, but not proof." or "Not clear it is the same room." Earlier versions of the extension say "Different room type possible, check before booking." and name the room the lower price is for when the site states it. This is common: in our study, at 2,505 of the 4,181 hotels with a listing 5% or more below the going rate (59.9%), no such listing named a room at all.
"Already lists the lowest price we found" and "You've got the best rate": Said only about a site that is in Google Hotels' list for the stay: the first when it has the lowest price we found, the second when no site is more than 5% under the going rate and the site you are on is within 5% of the lowest price. When the site you are on is not in the list, the banner says "If this page's price is about the same or lower, you've got the best rate." because it never read your page's price.
Free cancellation listed: Google Hotels showed a free-cancellation label for that listing. Deadlines and terms are on the booking site; read them before you pay.
Checked N booking sites: How many different booking sites the lookup returned for that stay.

What we checked about our own accuracy

Does the lower price hold when you click through? (September 30, 2026)

We took 20 hotels from our September 30 price snapshots where a site was listed 5% or more below the going rate. About 2 hours later we opened that site's link from Google Hotels for the same stay, in an automated browser, without signing in, booking or opening a checkout page. A price "held" when the page showed the same hotel, dates and party, at our price, a lower one or one at most 5% higher.

10 of the 20 pages could be read. 7 held, 2 were higher and 1 hotel was sold out. 9 of those 10 pages showed exactly the price Google Hotels listed for that site at that moment. Both higher prices were cases where Google Hotels' own price had already risen since our snapshot. No link landed on the wrong hotel.
The other 10 could not be read by an automated browser: a "Verify you are human" check on Super.com and Evendo, KAYAK's bot page, and an error page on two hotels' own booking pages. So 7 of 20 held in all, or 7 of the 10 we could read.
Checked again the same day in a normal Chrome browser, from fresh lookups: 5 of 5 Super.com offers, 8 of 8 KAYAK, momondo and HotelsCombined offers and 5 of 5 Agoda offers held. The KAYAK bot page did not appear.
What did not last: the offers themselves. At 6 of 10 hotels where Agoda had been cheapest, its offer was gone from Google Hotels or sold out within 2 to 3 hours. This is why the extension and the checker look prices up when you ask, and why a result can change between two checks.
Other things the pages showed. KAYAK's lower price was KAYAK's own rate, so it held only when booking with KAYAK itself. Two of three Vio.com prices said "Sign in to book". On Traveluro and trivago DEALS the "Total" was the before-tax price times the nights, with no tax shown on the hotel page.

Limits: 1 to 5 offers per site is a handful, not a rate for each site, and no checkout page was opened, so taxes and fees added at checkout were not checked. Source: our value audit of September 30, 2026, which records every hotel, price and page.

How often is "same room name" right? (October 1, 2026)

We wrote the method and the pass mark down before the first search, then priced 300 hotels with every room each site listed. Of the 194 with a listing 5% or more below the going rate, 53 (27%, likely between 22% and 34%) had one whose room name matched a room at the going rate under our strict rules.

A separate reader, an AI model that saw only the two room names and none of our verdicts, judged 42 of 50 sampled matches (84%) to be the same room. Our bar for calling a name match proof was 95%, so the test failed on that point.
The 8 it did not judge the same room were accessible rooms matched with standard rooms or with a different kind of accessible room, and room words our rules did not know: Western against Japanese style, "Comfort" against "Grand", a "Design" room against a duplex, "Cozy" or "Deluxe" stated on one side only.
We tightened the rules the same day (accessibility, grade words, style and layout must now agree). On 100 fresh pairs that the new rules call the same room, a reader who had not seen them, again an AI model given only the names, agreed on 92 (92%). Better, still short of 95%. That is why a result still tells you to check the room, even when the names match.

The 300-hotel test is also reported on the study page. Our write-ups of both tests list every pair the reader disagreed with.

The price study

Between September 22 and October 1, 2026 we took 15 price snapshots: 6,254 hotel prices in 348 cities. In every snapshot, 61% to 81% of hotels had some site listing the stay 5% or more below the going rate. The per-hotel data is free to download on the study page (CC BY 4.0), and when we found errors in it, we said so there and in the corrections log.

What we never do

See your payment details. We never see, store or send them. You always book directly on the site you choose.
Book for you. Underbook never books anything. If you ask us to watch a stay you already booked, we don't see the booking and we don't contact the hotel or the booking site: if you rebook, you book and cancel yourself. We never need your booking number, name, phone, address or card.
Sell or share your data. We don't, and we don't build an advertising profile of you.
Track your browsing. On a page that is not a hotel page, the extension sends nothing about the page on its own. It shows a one-time tip on booking sites' search results, sends an anonymous once-a-day usage ping with no address, title or page content, and checks a page only if you click its toolbar button. It uses no cookies or tracking pixels.
Charge you. Underbook is free, and we never take payment from you.
Let a commission choose what you see. See the next section.

Each of these is stated in our privacy policy or on the price checker.

How we make money

In the words of our privacy policy:

When you open a booking site through Underbook and complete a booking, we may earn an affiliate commission from that site, at no extra cost to you (you pay the same price you'd pay going there directly). This never changes which rates we show you or the order we show them: we always surface the genuinely lowest rate we find, and we'll link you to a cheaper option even when it earns us nothing. We never take payment from you, and we don't sell your data.

How the link works. A click on a booking-site link passes through our service, which adds one to a daily click total, turns the link into a partner link where the booking site has one, and sends you on. We do not record who clicked.
Where the order comes from. The price service sorts the listings by price. The partner link is added only at the moment you click, after the result is already on your screen.

What we send and keep

A summary. The privacy policy has every detail and is the one that counts.

What a check sends: the hotel name, its town when the page shows it, your dates, the number of adults, the booking site's market and currency, and the extension's version. Never your identity, your browsing history or payment information.
What the price service keeps: a priced result for 30 minutes, with nothing about who asked. Lookups that found no rates, and hotel names we had to pick from close matches, go in short lists (the most recent 50, at most 30 days) that record nothing about who made them.
Usage statistics: anonymous events counted as daily totals, so we can see where people stop getting value. They never contain a web address, a hotel name, your dates or your email, and no IP address is stored.
Your email, only if you ask: for a price-drop alert or a watch on a stay you booked, we keep your email and that stay, and use them only to email you about it, with at most one drop alert and a one-click unsubscribe. A watch on a booked stay is deleted the day after check-out.
On your device only: the last 25 stays you checked, so a return visit can say what the price did. They never leave your device.
The web checker: counts checks as daily totals. To stop automated misuse, each visitor can run up to 40 checks a day, counted under a one-way hash of the IP address that changes every day and is deleted after about 26 hours; the address itself is never stored.

Who runs Underbook

Underbook makes three things that work together: the browser extension (on the Chrome Web Store and on Microsoft Edge Add-ons), the web price checker on underbook.co, and the price service both of them use, at underbook-backend.underbook.workers.dev.

One address reaches the people who run it: hi@underbook.co. Questions, requests to delete your data and corrections all go there.

When we get something wrong

We have. The corrections log lists every correction to a figure or claim we published, newest first: what we said, what was wrong, what we changed and when. The first and largest: the first version of our price study called 45 listings "confirmed same-room savings" when our check had only seen that the cheaper site named a room. We retracted it the next day, on the study page itself.

If you find something wrong, on this site or in a result, email hi@underbook.co.

Questions

Where do Underbook's prices come from?

From Google Hotels. For your hotel, dates and party size, our price service reads the price each booking site lists there. A site Google Hotels does not list for that stay is not compared, and neither is a price you would only see after signing in to a site.

Is the lower price still there when I click through?

Usually, if it is still offered. In our click-through test of September 30, 2026, 7 of the 10 pages we could read showed our price, a lower one or one at most 5% higher, and a same-day check in a normal browser held for every Super.com, KAYAK-group and Agoda offer that was still listed. But offers go: at 6 of 10 hotels where Agoda had been cheapest, its offer was gone or sold out within 2 to 3 hours. Check the final total on the site before you book.

Does "same room name" mean it is the same room?

It is strong evidence, not proof. In blind tests on October 1, 2026, an AI reader that saw only the two room names agreed with 84% of our matches, and with 92% after we tightened the rules, short of the 95% we set as the bar for proof. Check the room before you book.

How does Underbook make money?

When you open a booking site through Underbook and complete a booking, we may earn an affiliate commission from that site, at no extra cost to you. It never changes which rates we show or the order we show them in. Underbook never takes payment from you and does not sell your data.

Check a hotel, and read the result for yourself.

Every site Google Hotels lists for your dates, the going rate, and whether the lower listing names the same room.

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