Some vendors are relying on CAPWAP or LWAPP sizing tools alone to determine AP placement. But I would say that there is not one tool alone or a single sizing guideline to use.
A general use of thumb is listed below. But be careful. This is not a substitute for a site survey. Why? Tools and guidelines cannot take into account RF properties of a building or area or the 802.11 clients. If your customer is firm about not purchasing a site survey, you may want to cover yourself by stipulating that any area that is not adequately covered or where issues arise will have to be investigated with a site survey tool and visual inspection to determine proper AP placement for throughput and coverage for the application. (Site survey after the fact). If voice is involved, look at the manufacturers transmitter and radio capabilities in the client itself. This may dictate much of the design if voice coverage is needed everywhere. Some voice vendors have a very weak transmitter in the voice (802.11) device and gets even weaker when the battery is at half battery power or lower. This means that while the APs can transmit adequately to the client, the client cannot transmit back in the coverage area. This can prove challenging without a site survey. Use the actual clients that your customer will use to test with and survey or use a site survey tool that allows you to adjust the transmit power.
If you must go in blind - (wouldn recommend it - but sometimes you must because of customer requirements)
1 AP per 2000 sq feet - EXTREMELY SAFE VOICE and DATA
1 AP per 3000 sq feet - VERY SAFE FOR DATA and VOICE
1 AP per 3500 sq feet - SAFE FOR DATA and SOME VOICE
1 AP per 4000 sq feet - AGGRESSIVE DATA ONLY
1 AP per 5000 sq feet - DATA ONLY
According to Cisco: