Measuring the need of Capacity or scaling SQL server This would be more of General approach question rather than too technical:

I have one of my application reach out to me wandering if we need to scale up SQL server in terms of CPU memory disk or CPU as they are planning to increase the volume by double the current one.

From the current volume, i have captures which shows CPU/memory and disk running without issues.

But how can i measure with confidence what all will change if there is increased in volume which might need a bump in capacity at my end?

Need your expertise how can i be pro-active and what all can i measure to make up a decision , atleast close to what can be expected with the growth in volume?

Yes i do have SQL server monitoring in place which stores performance data for last 15 days. Most of that data comes from performance counters and DMV's.

Update- Adding more data as i get to update- Avg 800 transactions/sec i day and 400 batch req/sec in a day followed by steady 40% cpu usage over the day, includes peak day. Disk response are well under 5 ms for both read/write. PLE throughout the day is over 10K.

Any help on how to approach this is greatly appreciated.I know answer is broad to this question, but some great ideas can really help me not only for this server but something similar in future also comes up , thanks!
As others have said, there is no magic formula that will tell you how your database server will respond to a sudden "increase the volume by double the current one" (volume of what exactly, by the way).

The two things that can help you to get a general idea (neither of which you have, apparently):


Historical (over many months) data that show you the correlation between your server load, the volume of data, and the application workload (concurrent connections, user sessions, registered users, daily orders, or whatever makes sense in you case). "[P]erformance data for last 15 days" is completely useless.
Performance testing environment that's configured meaningfully similar to your production. In this day and age spinning up a virtualized test environment on one of the cloud providers and running it for a week costs peanuts, compared to the price you will pay for a production outage caused by unexpected performance problems. Performance burn-down tests should be a part of your release process. 


If you start preparing both of these tomorrow, in half a year you will be able to answer your own question with some degree of confidence.