Ask the seller for the area in square feet or square metres as recorded in the revenue record (the 7/12 extract, jamabandi, khatauni or your state's equivalent).
Treat the katha figure as a label, not a measurement, and compare plots per square foot.
Once you have the area in square feet, convert it to gaj (square yards) with the area unit converter.
Common questions
How many gaj in 1 katha?
There is no single answer: a katha differs by state and often by district. Ask for the area in square feet or square metres as recorded in the revenue record instead of converting a katha figure.
Which states use the katha?
Land records or local usage in Bihar, Jharkhand, West Bengal and Assam use the katha, each with its own local size.
Why does the site not convert katha?
Any single figure would be wrong somewhere. A rate or area in katha cannot be compared across a district boundary; one in square feet can.
Which units are the same everywhere?
Square feet, square metres, square yards (gaj), acres, hectares, kanal and marla, guntha, cents and decimals, grounds and ares have one fixed size, and convert the same way in every state.