Your stay's price will probably change after you book. Here's how to catch it.

Why the price moves

Hotels have priced by demand for decades. Rentals caught up: hosts now hand their calendars to pricing tools that recalculate every night against the season, the day of the week, local events, how fast the calendar is filling and how far away the date is. PriceLabs alone says it prices more than 600,000 properties. A common setting in those tools is a last-minute discount, a percentage off when a date is still empty a week or two out.

The result is that a price is a reading, not a fact. When we checked one Seville hotel on Booking.com on October 19, its total for the same dates moved three times in 25 minutes: $828, then $843, then $828 again. Every number Stay Otter shows carries the time it was read for exactly this reason.

Do prices really drop after you book?

Sometimes, and often enough to be worth a look. NerdWallet's analysis of about 2,500 hotel rates across seven major brands found that booking closer to the stay was usually cheaper than booking four months ahead. On one of our own watches, a one-bedroom near Rainey Street in Austin dropped from $1,314 to $1,097 for the same five nights in November: $217, or 16.5%, read on October 6.

Prices also go up. On one of my own bookings, a long stay in Medellín, the rate read well above what I paid. So the move is not to wait for a better price before booking. It is to book something you like with free cancellation, and then keep an eye on it.

Free cancellation is the whole trick

A lower price only helps you if you can get out of the higher one. That is what a free-cancellation rate buys: until its deadline, you can rebook the cheaper rate and cancel the original at no cost. Travellers already use it heavily. In D-EDGE's study of 680 European hotels, close to 40% of booked revenue was cancelled before arrival by 2018, and about half of Booking.com bookings were.

A non-refundable rate is usually a little cheaper on the day and fixed forever after. If your plans are settled and the deadline would be days away anyway, take it. If you're booking weeks ahead, the refundable rate is the one that lets a later drop reach you.

How to check it yourself

1. When you book, note the free-cancellation deadline and put two reminders in your calendar: a week before it and the day before.
2. A few times before the deadline, open the property's own page on the site you booked, with the same dates and the same number of guests, signed out if you can (member prices can hide the public rate).
3. Compare the same room type and the same cancellation terms. A cheaper room that is non-refundable, or sleeps fewer people, is a different product.
4. If it is lower by enough to matter to you, book the new rate first and keep the confirmation. Then cancel the original, before the deadline. Rebook first, cancel second, never the other way round.
5. For a hotel chain booked direct (Marriott, Hilton, Hyatt, IHG), check the chain's own price-match terms too: most only apply within a day of booking, and the rules changed this year.

For rentals, compare the total at checkout, not the nightly price: cleaning and service fees and taxes can change the picture, and they are part of what you would rebook.

What Stay Otter does for you

I built Stay Otter to be the friend who checks. It never books or cancels anything; it tells you when it's worth acting, and in what order.

• Before you book: verified picks with the price and the time it was read.

  The otter reads Booking.com, Vrbo and Expedia live for your dates and shows the all-in total, the street (quiet or lively) and what's within a ten-minute walk, ranked by your criteria, never by commission. On Pro, it re-runs your search every day and emails you only when a verified pick drops 5% or $50.

• After you book: a free watch until your deadline.

  Paste or forward your confirmation on the Booked page. The otter keeps five facts (property, dates, what you paid, the deadline, where you booked), reads the rate every morning, and writes only when a rate you can still cancel into is lower. You also get reminders a week, three days and a day before the deadline. Hotels booked direct with a chain are read across the sellers on Google's rate list. It's free with an account.

• Found a place yourself? Check it.

  Check a listing reads any Booking.com, Vrbo or Expedia page, or a hotel by name, into the same receipts: the street, what's nearby, and with dates, the lowest total and the lowest you can still cancel.

Searches and booking watches are free. Pro ($5.99 a month or $49 a year, free for 30 days with no card) adds the daily price-drop alerts on searches, the chain playbooks and a second seat. See pricing.

The short version

• Prices move after you book, in both directions.
• Only a free-cancellation rate lets you take a drop.
• Rebook first, cancel second, before the deadline.
• Or let the otter watch it and tell you when.