Sowing Depth & By Seed Size & Soil

The rule of thumb is to sow at about 3–4 times the seed's diameter. A 2 mm seed goes roughly 6–8 mm deep; a 10 mm bean goes 30–40 mm. This puts the seed deep enough for stable moisture and anchorage, but shallow enough that the emerging shoot can reach the light before its stored energy runs out.

A seed has only the energy stored inside it to push a shoot up to the light. Larger seeds carry more reserves and can be sown deeper; tiny seeds carry little and must sit near the surface. Three to four diameters is the depth that reliably gives moisture and contact with soil while staying within reach of that stored energy.

Sandy soils dry out and warm fast, so sow a little deeper to reach steady moisture. Heavy clays are cold, wet and crust over, making it hard for shoots to push through, so sow shallower so the seedling can emerge. The calculator nudges the base depth up for sand and down for clay around the seed-size rule.

The seed exhausts its energy reserves before the shoot reaches the surface and the seedling dies underground — the classic cause of patchy, thin stands. Too-deep sowing also means slower, more uneven emergence because the seed sits in colder soil. When in doubt, err shallower than the rule rather than deeper.

Shallow seed sits in the layer that dries first, so germination is erratic and seedlings may dry out after sprouting. Very shallow seed is also exposed to birds and can be washed out or heaved by rain. The right depth balances reliable moisture against the shoot's ability to reach the light.

Very fine seeds (lettuce, many flowers) are sometimes pressed onto the surface and barely covered, or covered with a thin sprinkle, because they need light or have almost no reserves. Follow the 3–4× rule and you'll naturally land on a near-surface depth for them — just keep that thin layer constantly moist until they germinate.

Yes — enter the seed diameter and the tool returns both a base depth and a soil-adjusted depth in millimetres and centimetres. Seed sizes are easiest to judge in millimetres, while drill and planter settings are often in centimetres, so both are shown side by side.

Lay a few representative seeds against a ruler, or estimate the average width of the seed at its widest. For mixed or irregular seed, use the typical size rather than the largest. Precision isn't critical — the 3–4× rule is a band, not a single point, so a close estimate gives a sound depth.

Yes. In cold, wet conditions stay shallower so the seed warms and emerges; in warm, drying conditions go a touch deeper to chase moisture. The soil adjustment here captures the main texture effect — combine it with judgement about the current weather and seedbed moisture when you sow.

They're solid, field-tested guide depths. Real best depth varies with crop, seedbed firmness, moisture, temperature and drill type. Treat the result as a starting point, check your stand after emergence, and fine-tune the setting on the next sowing if establishment was thin or slow.