Free seed starting tray calculator. Enter trays to find how many seeds to sow and how much seed starting soil you need.
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Enter the number of trays. This calculator models a standard 24 cell seed starting tray: one seed per cell, so 24 seeds per tray, and about 2 litres of seed starting mix per tray. Sow one seed per cell for most vegetables and flowers, water gently, and cover with a humidity dome until germination.
The seed starting tray formula is simple: seeds equal trays times cells per tray, and soil equals trays times litres per tray. For the 24 cell tray this calculator models, 2 trays need 48 seeds and 4 litres of seed starting mix, and 5 trays need 120 seeds and 10 litres. The same multiplication works for any tray size once you know its cell count, so count your cells before you buy seed.
For seeds with a high germination rate, one seed per cell is the standard advice across competitor guides, because two seedlings in one cell compete for light, water and nutrients and then need thinning. If your seeds are old or have a low germination rate, sow two seeds per cell instead, then thin to the strongest seedling once true leaves appear. Thinning early matters: tangled roots from two plants in one cell are hard to separate at transplant time.
Cell counts change what a tray can grow. A 24 cell tray gives each seedling a large cell and a long stay indoors, which suits tomatoes, peppers and other warm season crops. A 72 cell tray is the common middle ground for most vegetables, and a 128 cell tray packs many small seedlings into the same footprint but needs transplanting sooner. Deeper cells hold more soil and water, so seedlings in deep trays need watering less often than seedlings in shallow trays.
The standard 1020 seed tray measures slightly more than 10 inches by 20 inches, about 11 inches by 21 inches in practice, with a depth of 2 to 3 inches. That footprint is the industry default, so inserts, humidity domes and heat mats made for 1020 trays fit each other. Tray depth matters: at least 2 inches of soil gives roots room, while trays that are too deep stay wet and invite damping off.
Seed starting mix is usually a light, soilless blend that drains freely and holds moisture without packing down. A 72 cell tray takes about 4 to 4.5 litres of mix, which shows why volume, not weight, is the right way to shop for mix. Fill cells with pre-moistened mix, tap the tray to settle it, and top up any cells that sink after the first watering.
A humidity dome over a fresh tray holds moisture in while seeds germinate, and most guides say to remove it at the first sign of sprouting so mould cannot take hold. Water from below where possible by setting the tray in a shallow reservoir, which keeps the surface dry and discourages fungus gnats. Keep soil damp, never soaked, and give seedlings strong light as soon as they emerge.
Take 3 trays of the 24 cell type. Seeds are 3 times 24, which is 72 seeds, and soil is 3 times 2 litres, which is 6 litres of seed starting mix. If you switch to 72 cell trays, the same 3 trays would need 216 seeds, which is why cell count, not tray count alone, decides your seed order.
One seed per cell for seeds with a high germination rate. Sow two seeds per cell only for old or low germination seed, then thin to the strongest seedling.
It depends on the cell count: 24 seeds in a 24 cell tray, 72 in a 72 cell tray, and 128 in a 128 cell tray at one seed per cell.
About 2 litres per 24 cell tray in this calculator, and about 4 to 4.5 litres for a standard 72 cell tray. Always measure by volume, not weight.
Slightly more than 10 inches by 20 inches, usually about 11 by 21 inches, with a depth of 2 to 3 inches.
Yes for germination, then remove the dome as soon as seedlings emerge so excess moisture does not cause mould or damping off.
The germination rate is the share of seeds that sprout. A 95 percent rate on a 162 cell tray leaves only about 8 empty cells, while a low rate argues for sowing two seeds per cell.
Cover a seed with soil about one to two times the length of the seed, and check the seed packet for varieties that need light to germinate.
Thin as soon as two seedlings share a cell and true leaves appear, keeping the stronger plant so roots do not tangle before transplanting.