About SeedCalc

I built this calculator because my first 4x8 raised bed was a lettuce disaster.

Spring 2019. I bought a cedar kit from Home Depot, filled it with Mel's Mix (following the book by the letter), and planted a whole bed of Buttercrunch and Romaine lettuce at 6-inch spacing like the Bartholomew book said. What the book didn't tell me, or what I didn't read carefully enough, is that 6-inch spacing assumes you're thinning. I hadn't thinned anything, because nobody had explained what thinning was.

By early June I had about 120 lettuce plants jammed into a bed sized for 32. They were spindly, pale, and bolted to seed within two weeks. I pulled it all in late June and put in a round of bush beans out of spite.

That's when I got serious. I read the All New Square Foot Gardening book cover to cover (twice). I subscribed to Johnny's Selected Seeds' technical catalog. I started tracking every bed, every variety, every harvest in a notebook I still have. Six seasons later, my 2024 garden was four 4x8 beds, two 2x8 beds, a 30-foot row of pole beans, and a strawberry patch.

Varieties I actually grow

Tomatoes: Black Krim, Cherokee Purple, Brandywine, and Sungold cherries for the kids. Peppers: Jimmy Nardello for sweet, Jalapeno and Serrano for heat. Lettuce: Buttercrunch, Parris Island Cos (Romaine), Red Sails, and Deer Tongue when I can get it. Carrots: Scarlet Nantes and Paris Market. Radishes: French Breakfast, every single time, because they're pink on top and white on the bottom and never woody. Beans: Blue Lake bush and Kentucky Wonder pole.

Herbs: Genovese basil next to every tomato, flat-leaf Italian parsley, chives that have been in the same spot since 2020 and come back every year.

How the calculator formula was built

SFG densities come straight from Mel Bartholomew's All New Square Foot Gardening, 3rd edition, the table on pages 163-175. Row spacing numbers come from Johnny's Selected Seeds' technical cultural charts, which are the most rigorous commercial reference I know of. Germination days and packet averages are cross-checked against Baker Creek, Burpee, and High Mowing retail listings. The intensive multiplier (1.15) reflects the density bump Eliot Coleman documents in The New Organic Grower.

You can see the full formula and every assumption on the guide page. I believe in showing the work, not hiding it.

Who this is for

Anyone with a raised bed, a backyard row, or a bag of seeds and no clear idea how many of them to sow. Especially first- and second-year gardeners who want to stop guessing. If you're a market gardener pushing 100 beds, you've already got this math memorized and don't need me. If you're about to build your first 4x8 box and are staring at a dozen seed packets wondering what goes where, this is for you.

The 2022 drought, and what it taught me

Summer 2022 in Indiana was brutal. June through August saw three weeks of 95-plus degree highs with almost no rain. I was still hand-watering my beds, and I lost a full 4x8 of summer lettuce in two days because I went to a wedding and didn't arrange for the neighbor to water. That fall I installed a DripWorks drip system with a Rain Bird timer. Best $180 I've spent on the garden. Irrigation isn't optional in a hot zone. Build it in from the start.

What's next

I'm adding more calculators as I go. Seed starting calendar (sowing dates by ZIP code). Compost calculator. Drip-irrigation layout tool. If there's something you'd want, drop me a note.

About the ads

This site runs Google AdSense. The ads fund the hosting and let me keep the calculator free for anyone who needs it. If you want to support the site directly, sharing a link with a gardening friend helps more than you'd think.