Farecast, the start-up that predicts whether airfares are about to go up or down, has launched nationwide.

We wrote about this useful service in late June, when the company launched a test version covering flights originating from Seattle or Boston. As of Monday, it covers 1,800 markets, including 55 origination cites — including San Francisco, Oakland and San Jose — for our readers in Silicon Valley.

Farecast, backed with $7 million from Greylock and others, shows whether the lowest fares for the trip are rising or falling over the next seven days. It takes into account past experience, and lets you know the degree of certainty it has in its prediction. Take, for example, a search for San Francisco to Kauai. You will get a page that looks like the one below (click to enlarge, for partial screenshot). It advises you to buy, because it predicts with 80 percent certainty that prices will increase over the next seven days; It also shows prices are at a two-month low.

Airline tickets are one of the big-ticket expenses we face every year, and which we seem to have the least knowledge about. If we delay our ticket-buying plans by a week, we’ve found ourselves paying $300 more– or sometimes less — than had we booked the week before. Farecast helps crack this opaque process, using historical patterns as its prime guide.

Farecast has unveiled an additional feature: A way for you to subscribe to regular alerts (via an RSS feed) that tracks the going fare for your chosen route, plus predictions.

Advertising is contextually driven. If you are flying to Chicago, Farecast will show ads about Chicago hotels or rental car companies.

Farecast’s VP of Marketing Mike Fridgen said hundreds of thousands of people have already accessed the site. Some 50,000 people clicked its “add my city” button since the company launched two months ago.

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