My car (like many) uses some sort of pressure/weight sensor to detect if a passenger is sitting in the front seat. If there is not (or they don't meet a certain weight threshold, like a child) then the passenger side airbag is turned off. Those are probably bulkier than you'd want on a restaurant chair but you can probably tuck a decent sized box under the seat and no one would notice/care. Anyway, I would say some sort of pressure sensor is the way to go.

Edit: you might even be able to use something a little more passive. If the chair was equipped with a small, low-power RFID tag that had enough logic to simply toggle its state [vacant|occupied] based on the pressure sensor, you could then position an RFID reader somewhere in the dining area that periodically polled the devices in the chairs. Then you don't have to worry about the chairs having enough power to run an active device.

Sounds the way to go. I am googling "passive rfid tag with pressure sensor". Some promising solutions...

Stick an RFID tag on every customer's posterior when they arrive at the restaurant.

Bingo, now you can track which rear end is sitting where.

Seriously though, there are some good options with RFID tags. Does it really have to be per chair, or could it be people per table?

You could put an RFID chip on each menu (though the menu would need to be left at the table for the duration of the customer's visit, and you would always need to provide 1 menu per customer).

If you didn't want to use menus then you could use something else instead, as long as you could RFID tag it.

When you go into clothes store changing room you often get given a tag with a number on it. That tag number defines that number of clothing items you have taken into the changing room with you and it reduces theft.

You could operate a similar system, but based on the customers sitting at the table. It won't tell you which seats are being sat on, but it will tell the occupancy per table.

there is usually a number of guests in the receipt, cheek the time at which your client paid and time he/she made an order

IMO this is the most realistic and cost effective solution to problem. Most restaurant POS systems that handles reservation, seating and billing should be able to provide needed info.

Any RFID or sensor base system will be complex, cumbersome and costly. Any passive sensor will require manual scanning. Any active sensor system with receiver will be expensive specially for restaurant with large seating area.

Another option may be an incentive based passive RFID sticker. A guest receives a sticker upon arrival and gets some sort of incentive/reward on scanning the sticker at exit. Then you can capture the time in the restaurant or on the table.

Maybe you could use a simple pressure sensor hooked upto bluetooth and check to see if there are variations over short periods of time to make sure theres a person on it.

I imagine you'd have to match up multiple measurements to get an accurate count. A busy happy hour will throw off your numbers because you'll have 20 people sitting but 50 more standing who won't be counted.

Are you interested in just the number of people in the restaurant? Or, more importantly, how many people are ordering food? Because you could have a table of four share a $10 appetizer and you'd count 4 people.

Ideally I would account for all of it. But I need to start somewhere, and I don't think looking for all-in-one solution is a good path. So just know how many people are sitting and don't care what they are ordering or who is standing up, is enough for now.

Why not just track the number of people who walk in and out?

How much added value does the sitting aspect add, considering that it would cost a lot of if you had to retrofit these onto your seats?

Knowing how many people walk in and walk out tells you how many customers you served but can't tell you how long any one of them stayed. Knowing how long a specific seat was occupied can.

probably a good solution too. I think it will depend on the kind of business. Thanks

I'd look at patents. There are several expired patents for seat occupancy sensors that rely on EM wave disturbance. They're in pretty much any "auto off" passenger airbag system.

Camera counting - every chain store these days has a camera at the front door counting people in and out - seems a well known, commercialised and direct solution.