I am writing this question for my husband because he just delays any movement on this issue. I want to move fast, if your answer says this is possible. We have most of our first floor in beautiful handscrapped hickory engineered hardwood. The planks are laid on a diagonal and were installed as a free floating floor. The planks are a bit over 3 inches wide - a hard size to find these days. The floor was installed about 5 years ago. I love this flooring.
So, we want to take out the carpet in the living area and put in matching floor. Amazingly, after much searching, we have found the flooring - same color, same brand, same size. It is at a warehouse that basically sells leftover or discontinued supplies.
So I want to take up the cut pieces of flooring that are at the line where the carpet meets the current floor, and replace these pieces with the new flooring and then continue the floor in the living room. Is this possible? How?
If this is possible, how do we estimate the amount of flooring needed in the living room, when it is at a diagonal?
Not sure if this is important, but we do have some extra unused flooring from the installation. Thank you for your time!

The size of the rooms will tell if you could make the flooring into one big floater. It is recommended that 20 feet width of flooring have an expansion joint since the flooring is not fastened in place. A T-molding will not be much of a distraction and allow both floors to have some movement where the floors will join. Can you find the right T-molding?
If the rooms are small and you want to weave-in from one room to another it will take a lot of work and luck to make them blend together. It depends on the glue used to join the T&G whether you can get enough residue cleaned out to slide new flooring into the old part. Just a little of the old glue remaining will be a big problem.
I add 10% more on the wood order when laying the floor diagonally.The waste is only the 45's cut off at the ends.

I think you are telling me what I don’t want to hear! The rooms are basically a combined dining and living room, with the living area a bit wider. So basically one room. The line where the current wood floor meets the carpet is 13’ 6”. I think putting T molding in the middle of the room would not be ideal. We want to get rid of this wood/carpet separation and distraction, and make this one unified floor type.
Adding the dining part to the living part the length of the room is about 24 feet.
So, we have access to the T molding, but want to avoid this.
It sounds like we will have to weave-in the two, but this is a lot of boards we will be working with. And almost impossible if the glue is an obstacle. Plus we will be going past the recommendation of 20 feet without an expansion joint.
My husband is a very patient, hardworking guy, but he chose the wrong, very impatient partner!
We will have to think on this awhile, but if we do decide to try this, at least you have given us an idea of how much flooring we need. It sounds like a risky investment if we buy the wood flooring, and end up with this being impossible to accomplish. We might be grumping at each other big time!
Thank you for your fast response.