What is the fastest way to calculate tonnage of AC when area is given in meters?

As a rule of thumb (HVAC Equations Data and Rules of Thumb2nd Ed)

you have to get the area by ft then divide it on the following

offices, commercial: sq. ft / ton

Banks court houses: sq. ft / ton

police station post office : sq. ft / ton

Restaurant: sq. ft / ton

Kitchen: sq. ft / ton

Hospitals: sq. ft / ton

Residential: sq. ft / ton

Apartment: sq. ft / ton

School: sq. ft / ton

library & museum: sq. ft / ton

malls: sq. ft / ton

sometime some thumb rule can b apply for example

150 - 170m2/ton for office and normal working area

100 - 120m2/Ton for public gathering area\\ food court

80 - 100m2 /Ton for mosque theater auditorium etc

but still depends on many factors

Multiply the area by the thickness to get the volume, then by the BRD to get the mass.