How Deep to Plant Seeds: A Simple Depth Formula

The Depth Rule in Five Facts

• The standard rule is to plant a seed 2 to 3 times as deep as its own diameter, not by a fixed inch measurement.
• Tiny seeds like lettuce and carrots often need barely any covering — sometimes just a light dusting of soil.
• Large seeds like beans and squash tolerate, and need, an inch or more of depth to anchor their root systems properly.
• Planting too shallow risks the seed drying out; planting too deep risks it running out of stored energy before reaching light.
• Soil type shifts the ideal depth slightly — sandy soil allows a bit deeper, heavy clay favors shallower placement.

Seed packets print a depth number, but few explain where it comes from. It isn't arbitrary — it's derived from the seed's own size, using the ratio above, which holds up across nearly every vegetable you'll plant. Once you understand the multiplier, you can estimate planting depth for a seed even without a packet in hand.

The 2-to-3x Diameter Rule

The formula is straightforward: planting depth equals 2 to 3 times the seed's diameter. A pea seed is about a third of an inch across, so it goes in at roughly three-quarters to one inch deep. A carrot seed is barely a sixteenth of an inch across, so its ideal depth is about an eighth to a quarter inch — essentially a light covering, not a buried seed. The multiplier stays constant; only the seed size changes the result.

Why the Ratio Exists

A germinating seed runs on stored energy in its own tissue until its first leaves reach sunlight and start photosynthesizing. A bigger seed stores more energy, so it can afford to push through more soil before running out. A tiny seed has almost no reserve, so even an inch of soil above it can be more than the seedling can push through before it exhausts its energy supply and dies underground. The 2-to-3x ratio balances "enough soil contact and moisture" against "not so much that germination fails."

Worked Examples by Seed Size

Bean seeds run about half an inch in diameter, giving a planting depth of 1 to 1.5 inches — which is why bean packets universally say plant 1 inch deep. Squash and cucumber seeds are similar in size, landing in the same 1-inch range. Lettuce seed is closer to a thirty-second of an inch, so 2 to 3 times that is barely an eighth of an inch — press it into the surface and add a whisper of soil, don't bury it. Tomato seeds sit in between at about a sixteenth of an inch, calling for a quarter-inch planting depth.

What Happens When You Get It Wrong

Plant too shallow and small seeds dry out fast, since the top eighth-inch of soil loses moisture within hours on a sunny day — germination rates can drop by 30 percent or more in hot, dry conditions if seeds aren't covered at all. Plant too deep and large seeds burn through their energy reserve before reaching sunlight; a bean seed pushed 3 inches down (triple the recommended maximum) often fails to emerge at all, because it runs out of stored starch about an inch short of the surface.

Adjusting for Soil Type

The 2-to-3x rule assumes average loam soil. In sandy soil, which dries faster but offers less resistance, lean toward the deeper end of the range (3x) to keep seeds in contact with moisture longer. In heavy clay, which resists root and shoot penetration, lean toward the shallow end (2x) so the seedling doesn't have to fight through dense, crusting soil on its way up.

Seasonal Adjustments

In early spring when soil is cool and slow to dry, planting at the shallow end of the range speeds germination since the seed is closer to warmer surface temperatures. In midsummer heat, adding a bit more depth (toward 3x) helps keep the seed in cooler, moister soil rather than the hot top layer, which can spike above what many seeds tolerate for germination.

You don't need to memorize a different depth number for every vegetable. Measure the seed, multiply by 2 to 3, and you have a defensible planting depth for almost anything in your seed box — a formula that travels better than any single memorized number ever could.