If someone buys mutual fund units before 3 pm, then what is the NAV charged for that transaction?

As answered by Heena the cut off times are quite clear. Here is a little more clarity based on common queries from investors, many of whom are going online or use apps for investments. While the apps are 24×7 the industry is not (yet).

For investments made in the late evening or night, the NAV will be of the next business day. So for a Friday night investment the NAV will be that of Monday’s day closing. If Monday is a holiday for the exchange (BSE/NSE), then the NAV will be for the following working day.

If you want to transfer money at the last minute possible, then place your order during business hours. If you need to place a withdrawal, then do it anytime, but be aware that withdrawals take time to process.

And the best way to invest with peace of mind is via SIPs where the cutoff time really doesn’t matter. The monthly instalment simply happens automatically once a month.