Window Measurements?

Is there a sure fire way to measure an existing window in order to know the size to buy for a replacement? I have old aluminum track double hungs in desperate need of relief. I am pretty sure I could measure the window once it is out, but I would like to have a new one ready to replace it once it is out.
Please don't be afraid of insulting my intelligence. I am handy but this is something I have never done. Any help would be greatly appreciated.
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I just went thru this and if you measure your inside window opening and take that with you to the home store then get them to match up the correct window size. They usually have charts or pull one off the shelf and measure it...I found the best price for my window size at Home Depot. Note some windows are for replacement, some for new construction so be aware of that. also get low expansion foam to seal up the gaps, not that cheap high expansion stuff. Also some windows are made to accept vinyl sidind by having a channel all around the outside for the vinyl to go in. Not sure if that is what you need.
BUT due to cost,time,trim and siding removal to do the job I ended up repairing my aluminum windows. www.swisco.com had the parts to raise and lower them, they were nearly all broken and would not go up(channel balances). I found the weather strip had shrunk about 3 inches per window on the bottom of the sash and was letting in air. I replaced that with heavy duty weatherstrip. So for $100 I was able to get my windows fixed rather than $1200 plus many hours of work and other materials. I dont plan on staying here much longer so I did not want to put that much into windows. Enjoying a nice breeze thru the house now, I had not opened most of these windows in about 10 years! Now if you plan to stay in your home those vinyl windows sure look energy efficient.
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you need to measure the windows in 3 places top right middle left same for the sides. Also measure the outside if you have had any work done on your house. Don't know if that was the reason but we had siding put on our house then several years late had windows installed and they found the outside was smaller than the inside so the windows would not go all the way out causing me to lose part of the windowsill. To make matters worse a greenhouse window they made was way too big and they had to remake the window!
Wayne

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<< Is there a sure fire way to measure an existing window in order to know the size to buy for a replacement? >>
Yes there is. Remove the inside trim very carefully, (remember you need to put it back) and measure the rough opening, recording the dimensions of bottom to top at the corners, side to side at the corners and side to side at the middle. Finally, record the two corner to corner dimensions (that's how we know its square, OK?). Your new window size will then be 1/2 " smaller in all dimensions. Before deciding on repair or replace, find and study installation instructions for popular brands of windows. Note requirements for flashing, sealants, molding trim, etc.,and you'll have a good job. HTH
Joe
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