Vegetable Seed Calculator
Pick a vegetable and a bed size to get the number of rows, plants and seeds you actually need to buy, adjusted for germination rate and a sensible spare allowance.
🌱 What is a Vegetable Seed Calculator?
A vegetable seed calculator turns a bed measurement and a spacing recommendation into the two numbers you actually need before ordering: how many plants the bed will hold, and how many seeds you have to buy to end up with that many plants. Those are not the same number, and the gap between them is where most first-time seed orders go wrong. Some seeds never germinate, some seedlings are lost in the first fortnight, some plants are thinned out deliberately, and a packet always seems to spill. The calculation therefore has two stages: a geometry stage that fits plants into the bed, and an allowance stage that inflates the plant count into a seed count.
Allotment holders use it at the start of the season to build a seed order across several beds without guessing. Market gardeners use it to cost a succession planting, where the same bed is resown four or five times and a small per-sowing error compounds. Community gardens and school growing projects use it to split a shared seed order fairly between plots. It also answers the reverse question that comes up constantly in seed catalogues, which is whether a five gram packet is generous or barely enough, since five grams of carrot seed is around 4000 seeds while five grams of bush bean is about fifteen.
The most common mistake is dividing rather than rounding down. A bed 1.2 m wide at 45 cm row spacing does not hold 2.67 rows, it holds two, because a partial row has nowhere to sit. The same applies along the length. That rounding is exactly why shaving a few centimetres off the row spacing sometimes adds an entire extra row and a large jump in the plant count, while at other times it changes nothing at all. The second frequent mistake is treating the germination percentage on the packet as a guarantee rather than a laboratory figure obtained under ideal conditions. Real field emergence outdoors, in cold or crusted soil, is routinely ten to twenty points lower, which is what the spare allowance is there to absorb.
This calculator carries sixteen vegetable presets with the standard in-row and between-row spacing for each, along with an approximate seeds-per-gram figure so it can also report the seed weight to order. Every preset is editable, so an intensive block planting or a wide-row method is a matter of overriding two fields. Underneath the results, a comparison table runs every crop through your exact bed dimensions at once, which makes it easy to see what the same bed would yield if sown to something else, and the chart shows how the total plant count responds as in-row spacing changes, including the stepped jumps caused by that rounding.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Lettuce in a Standard Raised Bed
6 m × 1.2 m bed, lettuce at 25 cm × 30 cm, 85 % germination, 10 % spare
Example 2 — Carrots Direct Sown With Poorer Germination
Same 6 m × 1.2 m bed, carrots at 5 cm × 30 cm, 75 % germination, 15 % spare
Example 3 — Tomatoes, Where Only One Row Fits
Same 6 m × 1.2 m bed, tomatoes at 60 cm × 90 cm, 90 % germination, 20 % spare
Example 4 — Bush Beans Sold by Weight
4 m × 1.2 m bed, bush beans at 10 cm × 45 cm, 80 % germination, 10 % spare
❓ Frequently Asked Questions
🔗 Related Calculators
How many vegetable seeds do I need for my garden?
Divide the bed length by the in-row spacing to get plants per row, divide the bed width by the row spacing to get the number of rows, and multiply. Then divide that plant count by the germination rate and add a spare allowance. A 6 m by 1.2 m lettuce bed at 25 cm by 30 cm spacing holds 96 plants and needs about 125 seeds at 85 percent germination.
What is the formula for seeds needed?
Seeds equals rows times plants per row, divided by the germination rate as a decimal, multiplied by one plus the spare allowance, then rounded up. Rows is the bed width divided by the row spacing rounded down, and plants per row is the bed length divided by the in-row spacing rounded down.
What is the difference between in-row and between-row spacing?
In-row spacing is the gap between neighbouring plants along a single row, and it sets how many plants fit in the length of the bed. Between-row spacing is the gap from one row to the next across the bed, and it sets how many rows fit in the width. Most seed packets quote both, in that order.
Why should I buy more seeds than plants?
Because not every seed germinates and not every seedling survives. Dividing by the germination rate accounts for seeds that never come up, and the spare allowance covers thinning, pest losses in the first fortnight, gaps you want to refill and a little spillage. Without both adjustments you will end up short.
What is a typical germination rate for vegetable seeds?
Fresh commercial seed is usually 85 to 95 percent, and many countries set a legal minimum for each species. Home-saved seed commonly runs 60 to 75 percent. Seed stored several years, or stored somewhere warm and humid, can fall well below 50 percent while looking completely normal.
How many carrot seeds are in a gram?
Roughly 800, which is why carrot is nearly always sold by weight rather than count. For comparison, lettuce is also about 800 per gram, beetroot about 50, sweet corn about 6, and bush bean only about 3. Those figures are approximate and vary by variety and seed grade.
Should I direct sow or start seeds indoors?
Root crops such as carrot, parsnip, radish and beetroot resent transplanting and should be direct sown. Tomatoes, peppers, brassicas and cucurbits transplant well and benefit from an indoor start. The plant count from this calculator is the same either way, but transplanted crops usually need a smaller spare allowance because you can select the strongest seedlings.
How do I test the germination rate of old seed?
Count out ten or twenty seeds, roll them in a damp paper towel, seal in a bag at room temperature, and check after the packet's stated germination period. The proportion that sprouts is your germination rate. Enter that figure here instead of the packet number and the seed quantity adjusts automatically.
Does this calculator work for square foot gardening?
Yes, if you enter equal in-row and between-row spacing. Square foot gardening uses a grid rather than rows, so setting both spacings to the same value reproduces it. For example, entering 15 cm for both gives the classic nine-plants-per-square layout for spinach and bush beans.
How much seed do I need per square metre?
It depends entirely on the crop. The results panel reports plants per square metre for your chosen spacing, which is the useful number. Lettuce at 25 cm by 30 cm gives about 13 plants per square metre, carrots at 5 cm by 30 cm give about 67, and tomatoes at 60 cm by 90 cm give under 2.
Why does the calculator round rows and plants down?
Because a partial row or a partial plant does not exist. If the width divides into 2.67 rows, only two full rows fit inside the bed. Rounding up would overstate the plant count and leave you with plants that have nowhere to go, so the calculator always rounds down to what actually fits.
How long do vegetable seeds stay viable?
Onion, parsnip and sweet corn are best used within one to two years. Carrot, pea and bean hold up for about three. Tomato, cucumber, squash, lettuce and most brassicas stay viable for four to six years when kept cool, dark and dry. Always retest anything older than its typical span before planning a bed around it.