We plant almost everything in plug flats -- 98 and 200 cell trays being the most common. Direct seeding will require larger quantities of seed because of the need to overseed and thin. We like to plan on a number of trays basis than feet of row or bed. But, as it works our, a 4-row bed 50 feet long with plants spaced one foot apart each way will require one 200 cell tray if all plugs yield transplants. Maps or plotplans don't lend themselves well to planning successive plantings.

Seed catalogs usually list number of seeds per package or seeds per ounce or gram from which number of seeds can be calculated. You must allow for the percentage of germination and a margin for bad luck and thinning.

We order much more seed that we use in a single planting or even a whole year. This is because pricing is much for favorable on a seed for seed basis when larger quantities are ordered. We keep our seeds in a refrigerator where many kinds will keep for decades. Each 10F of temperature below room temperature doubles seed life. For example, if a kind of seed keeps 2 years at 70F it will keep 4 years at 60F, 8 years at 50F and 16 years at 40F.

Good luck on your venture.

John Warner, near Fresno, CA
Hand-scale, no-tractor no-tillage growers since 1996

Here is a link that might be useful: Whole Systems Agriculture

If you don't already have a copy of Johnny's Selected Seeds. I would get one. Their catalog gives a lot of good information about planting and seed quantities per unit. How many seeds you need depend on how many plants you need per row. Here is an example. I want to make $2,000 on my cucumbers. I sell cucumbers for $20 a bushel so I need 100 bushel. MY cucumber rows are 150 feet long and I usually pick 50 bushel a row, so I will need at least 2 rows of cucumbers . In order to be on the safe side I will plant 3 rows which we will pick everyday during the season. I plant two seeds per Jiffy7 and set out plants at the rate of one jiffy7 per 12 inches of row. Therefore, I plant approximately 300 plants per 1500 foot row. So I make $2000 out of 900 cucumber seeds.
There really is no formula to follow when ordering seeds. I always check Johnny's catalog to see what they say an ounce of seed plants per foot. The size of the plants tells you how much a row is worth. A 150 foot row of beans can make you over $200 a row. Arow of Cabbage maybe only $75. Another bit of advice, don't make your rows longer than 150 feet.

Thanks to everyone that has posted advice

I figured out how much yield (normally in pounds) I want per crop per week/season. I'm not sure how to go from X pounds needed to how much seed needed.
In Johnny's they give how many oz. of seed per feet but they don't sell most seeds in oz. form. I'm still a bit stuck.

Giving an oz. of seed per Xfeet figure makes it easier to do the math than if they gave a figure in pounds, so even though they may not sell seed by the ounce, that's why they give that figure I'd bet.

Since you already know how much yield you want and how many feet you need to get it, simply use the oz. per Xfeet Johnny's gives you to figure it. For instance, if you know you need 200 feet of something and Johnny's says it takes 3 oz. seed to plant ten feet, then:
1. First figure out how many of the given unit (3 oz.) you'll need ~ 200 (number of feet you need) divided by 10 (number of feet the given unit will plant) = 20. You need 20 of the given unit to plant 200 feet of row.
2. So multiply 20 times the given unit (3 oz.) = 60. 60 oz. of seed is what you'd need.
3. To convert that answer into pounds, divide by 16 (number of oz. in a pound). 60 divided by 16 = 3.75 ~ that's how many pounds you need. Be sure to overbuy a bit to account for spotty germination, some plants not doing as well as expected, etc.

If they give a fraction of an ounce per Xfeet, like "1/8 oz. per ten feet of row", then before you do the above math, convert that into a "regular" number by dividing the top number by the bottom number. 1/8 = 1 divided by 8 = 0.125 Use 0.125 as the "given unit" to do the math above.

You may already know the following, so if you do, I apologize. Just disregard and let it stand for others' benefit...

When it comes time to make those rows and plant, keep in mind that most yield figures are for regular thin rows of single plants in a line. If you are planting wide rows, then when you actually get out there to plant them you won't have 200 feet of wide row planted when you run out of seed (for instance, if you're planting 30" wide beds, you'll probably be planting the equivalent of three thin single-plant rows squished together, so you'll only have about 67 feet of row done when you've planted all your seeds).

But for calculating on paper, these figures should line up and give you fairly close amounts of seed needed to get the pounds of veggies you want. Of course nothing will give you exact figures to make you end up with exactly how many pounds you need at the end of the season as there are too many variables when growing them out, but this will help you get close, again provided you buy and plant a little bit more to make sure. Experience will help you get closer to exact amounts needed, so keep a detailed garden journal (something I need to do myself *sigh*).

For lurkers' benefit (Hi, lurkers! ;), if you want to know how to find out yields per foot, go Googling for something like "vegetable crop yields per foot" or "vegetable row crop yields" and you'll find websites that will give you the average yield per foot (again, this is for thin single-plant rows). It's best if you get one tailored to your area, but here's one from Virginia's Coop Extension to give you an idea of what I'm talking about.

thanks, knittlin, for the link for yields. I have been looking for one. I had one and lost it somewhere.