I can only answer for PA, but here 1 credit is 120-180 hours of a subject (i.e. Algebra would 1 CR, if she spent 180, it would be considered an AP course, but still only 1 CR). So, here, it's all kind of based on hours. My oldest, who is just finishing up 9th Grade did: Algebra 1 - 1 Cr World History - 1 Cr English 9 - 1 Cr (this consisted of a co-op literature class and writing class and additional work I assigned to make it over 120 hours) Biology - 1 Cr (she did this at a co-op also, the equivalent on this one is 180, so it's considered AP) Faith In Action - 1/2 Credit (service project course) Entrepreneur Class - 1/2 Credit (sort of an extra math, but her evaluator is putting it as an elective) Phys Ed - 1 Cr - she did 60 of Karate & 60 hours of volleyball (give or take) Art/Drama - 1 Cr - at a coop...I'm waiting to see what the drama really counts as...I'm not sure. Bible/Logic - 1 Cr - not sure, I'm thinking this will be an elective for her So, she's earned 8 CRs this year...PA State law is 21 Cr to graduate - they do require that some of them are in certain subjects (ie. English must do 4 cr over 4 years, Math 3 cr over 3 years, Science, etc). We do have some diploma programs we can go through to get a diploma issued by a "recognized" entity. (all it really helps with is PA financial aid). Some of them require more credits than the state requires. Hope this helps. Oh...you might want to check into YOUR state's Department of Education Graduation Requirements, that helped me determine what was considered a "credit" and where to feed it.
What Kbabe said about the hours is pretty standard. If you cover at least 80% of a textbook you should give 1 credit. I went to our local high school's website and found their graduation requirements and printed it out. They require 24 credits to graduate and it lists what credits they should have. That is what I am going by. Several local college admissions officers spoke at our local homeschool group recently. They said they look at courses taken (not specific curriculum used), gpa, and ACT or SAT results. We are in a homeschool friendly state and none of them had a problem with a "Mom" transcript. We just finished 9th grade and here is what it looks like. Algebra 1 - 1 credit Government - 1/2 credit (1 semester) Economics - 1/2 credit (1 semester) Spanish 1 - 1 credit Literature - 1 credit Language arts - 1 credit (comp, grammar, vocab) Art - 1/2 credit Music - 1/2 credit (weekly guitar lessons) Computer - 1/2 credit (did not like curriculum, so did not finish) Biology - 1 credit So, 7 1/2 credits for 9th grade. By 12th grade they will only have about 5 credits left. I am making my own transcript using an Excel spreadsheet. Don't stress, you will figure it out!
A standard Carnegie Credit is equal to 120 hrs of instruction. In NYS we only need a 108 hrs for a unit credit but I always aim towards the 120 since many colleges go by Carnegie Credits. So the way it normally works is 1 year high school course equals 1 credit. If you take a college course a 1 semester college course counts as a full year of a high school course.
Also in Ohio it's roughly .5 per semester. Rachael took classes her Senior year at ECOT (on-line school), and they did a quicker program, where you covered the same amount in half the time. This allowed you to get 1 credit for a semester's course, and a .5 course only took half a semester. Generally, I assign a credit to any year-long course, regardless of how long it takes. For example, Faythe spent a year and a half in Alg. 1, but she only got 1 credit.