So i just got a new front 650 rim and i'm going to get a new fork in 650 to use brakes.

How do i find out what offset or rake i need?

I believe stock is 28 or 30. How do i find out? (Stock is a 700c fork)

Most comon is around 45, but to get a real number you need at least to have the crown height for the fork you will need for the frame you have, and the headset you will run, and the head tube angle. Also, since it doesn't hurt anything to have more info, you may as well know the actual inflated diameter of the wheel and tyre you will run, though that starts to get picky unless a few similar small numbers get added up.

Wait a second, are you running the 650 on a 700 frame. Otherwise why aren't you just measuring the offset on your current fork. What kind of brakes are you moving to.

Your question isn't really a framebuilding one, unless you are planing to make a 650 fork that will fit properly in the place of the 700c.

I am, Im going to use a 650c fork with a 650cc rim in place of a 700c fork and 700cc rim.

Im just using my Apex brakes. If it doesnt fit i'll just get deeper brakes. But i need to find out how i would measure my offset? How would i know what is correct.

Im assuming 30mm. But thats just a guess. im prolbally going to get the 30mm anyways because it is a Oem factory in china with primer on top of carbon composite and im getting it shipped in from my uncle for free. He works for a cross sea boating company. But its not getting boated its fedex and no logistics check so it wont take 9 months.

But 30mm will do i think if not ill just spend another 30 to get a new one accomidating the offset before. I don't really care about toe bite. I just need my geo to be a little better.

Moved from Framebuilding

Rake is the difference between the axis of steering and the center line of the axle in the wheel.

If you are putting a 650C sized fork in a 700C sized frame you are also changing the head angle, increasing it,

by using a shorter fork and a smaller diameter wheel 571 . rather than 622 .

so you are changing the trail , that is the measurement on the ground ,
where the line thru the head angle crosses the vertical line thru the axle.

more rake/offset reduces the trail if the head angle axis stays the same

change the head angle too, and you throw in another variable ..

650B on the other hand is a less drastic change, 584 and a wider tire gets closer to 700C with a skinny tire.

and many people have converted 27'' and 700C bikes to use 650B wheels and add a longer reach brake caliper.

the fork remains unchanged , so the whole bike sets slightly lower ..

Oh and CC is .. cubic centi meters a measure of volume , not distance .

the common diameter number used is what the tire and rim manufacturers arrived on
to see to it their products fit each other ..

that is tire Bead Seat diameter , you will find that on the tire sidewall ... like 32-622 etc.

Actually, cc = cubic centimetres. This is common measurement for displacement in engines or litres. There are actually 1000 cubic-centimetres in a litre or 10 cc per cubic centi-litre. So one cc = 0.1 cubic centi-litre.

Point taken , but its still a measurement unit of volume not distance.

any how the original poster is changing more than just the rake, so needs to plot out

what happens when frame geometry is changed,

Smaller wheel radius drops the BB and makes grounding a pedal more likely,
for one thing.

Sutherlands bike mech handbook shows several different 650 wheel sizes ..