Sowing Depth & By Seed Size & Soil
Sows beans
Enter seed diameter and soil type to get the right sowing depth — about 3–4 times the seed's diameter, deeper in sand and shallower in clay so seeds emerge strongly.
Find your sowing depth
Next: set your drill or dibber to sow at about 1.5 cm (15 mm) in this loam soil — deeper than the 15 mm rule on light soils, shallower on heavy ones.
The classic rule is to sow 2–4× the seed's diameter. Go shallower in cold, wet or crusting soils and deeper to chase moisture in dry, sandy ground.
Sowing depth — key facts
- Rule of thumb
- 3–4 × seed diameter
- Sandy soil
- sow a little deeper
- Heavy clay
- sow shallower to emerge
- Too deep
- seed exhausts before light
- Too shallow
- dries out, erratic stand
- Tiny seeds
- barely cover, keep moist
- Units
- millimetres and centimetres
- Privacy
- Runs in your browser; nothing uploaded
Deep enough for moisture, shallow enough to reach light
Get the sowing depth wrong and the best seed in the world won't make a crop. Sow too deep and the seed burns through its stored energy before the shoot reaches daylight, dying underground and leaving thin, patchy rows; sow too shallow and it sits in the layer that dries first, germinating unevenly or drying out after it sprouts. The reliable middle is a depth tuned to the seed's own size — about three to four times its diameter — then nudged for the soil it's going into.
This tool gives a base depth, a soil-adjusted depth, and both millimetres and centimetres from your seed diameter and soil type — deeper for moisture-hungry sand, shallower for crusting clay. Use it to set a drill or planter and to plan hand-sowing. Pair it with the Seed Rate, Dibbling and Germination Time tools for the full establishment picture.
Even stands
Right depth means uniform, strong emergence.
Stop deep losses
Avoid seed exhausting before it reaches light.
Match the soil
Deeper in sand, shallower in heavy clay.
Set the drill
Turn the rule into a real planter depth.
Frequently Asked Questions
How deep should I sow a seed?+
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.
Why 3–4 times the seed diameter?+
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.
How does soil type change the depth?+
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.
What happens if I sow too deep?+
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.
What happens if I sow too shallow?+
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.
Do tiny seeds need covering at all?+
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.
Does this work in millimetres and centimetres?+
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.
How do I measure seed diameter?+
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.
Does depth interact with temperature and moisture?+
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.
Are the depths exact?+
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.