Friday to Monday is three nights, and a $150 room can be $193 a night

Check in on Friday and check out on Monday and you have stayed three nights, not four. And a room advertised at $150 a night can come to $193 a night once tax and a nightly fee are added. Both mistakes are easy to make, and both are simple sums.

Count nights, not days

A hotel charges for nights. The number of nights is the number of days between the two dates, with the check-out day left out. Friday to Monday touches four calendar days and has three nights in it.

• Friday to Monday: 3 nights.
• The 28th to the 3rd, in a 31-day month: 6 nights.
• The same dates in a 30-day month: 5 nights.

Stays that cross the end of a month are where the count goes wrong. The date difference calculator gives the number of days between any two dates, and that number is the number of nights.

What a $150 room really costs

The advertised rate often leaves out tax, and some hotels add a fixed nightly charge under names such as a resort or facilities fee. Take three nights at $150, with 12% tax on the room and a $25 nightly fee. All three figures are examples.

The total is $579, which is $193 a night. That is 29% above the rate in the advertisement.

A fixed fee weighs more on a cheap room than an expensive one. $25 is 17% of a $150 night and 6% of a $400 night. In some places the fee is taxed as well, which this example leaves out.

Comparing two prices

Two listings can only be compared at the same stage. A price with tax and fees included will look worse than one without, and may be the lower of the two. The figure to compare is the total for the whole stay, on the last screen before you pay.

If the price is in another currency, your card may add a fee of its own. This article works out what paying by card abroad costs.

What this leaves out

• The real tax and fees. They differ by city and by hotel. The booking page shows them before you pay.
• Parking, breakfast and deposits. These are often charged at the desk and are not in the booking total.
• Cancellation terms. A cheaper rate is often one that cannot be refunded.

An estimate from example figures, not financial advice.