Seed Quantity Calculator

This seed quantity calculator tells you how many seeds to buy and sow for the number of plants you want. Start from a plant count, or from a bed size and spacing, then set how many seeds you sow in each spot, the germination rate from the packet, and an allowance for losses. The calculator shows the seeds you need, how many spots are likely to come up empty, roughly how many extra seedlings you will thin, and how many packets to buy. It helps you avoid both bare patches in the bed and a drawer full of half-used packets.

How to use the seed quantity calculator

1. Choose your starting point. If you know you want 24 lettuce plants, start from the plant count. If you are filling a bed, choose bed size and spacing and the calculator works out the number of planting spots.
2. Set seeds per spot. Most gardeners sow 2 or 3 small seeds per spot and thin to one. Large, reliable seeds such as beans and peas are often sown one per spot.
3. Enter the germination rate. Seed packets in the US list a germination percentage. If yours does not, 80 to 85 percent is a reasonable assumption for fresh seed of most common vegetables.
4. Add extra for losses. Slugs, birds, damping-off and the occasional misplaced footstep all take a toll. Ten percent is a sensible default.
5. Enter seeds per packet. Packet counts vary enormously, so check the packet or the seller’s listing.

How the calculation works

The number of seeds to buy is the number of planting spots times the seeds per spot, plus your extra allowance:

Seeds = spots × seeds per spot × (1 + extra %)

The germination rate decides how reliable each spot is. If a seed has a probability g of germinating, the chance that every seed in a spot fails is (1 − g) raised to the number of seeds in that spot. Multiply that by the number of spots to estimate how many will come up empty.

Expected empty spots = spots × (1 − germination rate)^{seeds per spot}

This shows why sowing two seeds per spot is so common. At 80 percent germination, a single seed leaves a spot empty 20 percent of the time. Two seeds cut that to 4 percent, and three seeds to under 1 percent.

Worked example: 24 lettuce plants

You want 24 heads of lettuce, sowing 2 seeds per spot, with 85 percent germination and 10 percent extra. That is 24 × 2 × 1.1, rounded up to 53 seeds. With two seeds per spot, only about half a spot is expected to come up empty, so almost every position will have a seedling. You will thin about 17 extra seedlings.

Worked example: a bed of carrots

A 4-by-8-foot bed of carrots at 4-inch spacing has 288 spots. Sowing one seed per spot at 80 percent germination leaves about 58 spots empty. Carrots are usually sown more thickly and thinned for exactly this reason. At 2 seeds per spot, empty spots drop to around 12.

Germination rates and seed age

Germination rates printed on packets come from tests on that seed lot, and US federal rules require packets of vegetable seed to meet minimum standards or state the actual percentage. Fresh seed of most common vegetables germinates at 80 percent or more under good conditions.

Seed loses viability with age, and some kinds lose it much faster than others. Stored cool and dry, these are commonly cited ranges for how long seed stays reasonably viable:

These are rough guides. Storage matters more than the calendar. Seed kept in a sealed jar in a cool, dark place lasts far longer than seed left in a hot shed.

Test old seed before you rely on it

If you are unsure about older seed, run a quick germination test. Roll 10 seeds in a damp paper towel, put it in a plastic bag somewhere warm, and check after the typical germination time for that crop. If 8 sprout, your rate is about 80 percent. If only 4 sprout, enter 40 percent in the calculator and sow more thickly, or buy fresh seed.

How many seeds to sow per spot

• One per spot: large, reliable seeds such as beans, peas, corn and squash, when germination is high and the soil is warm. Some gardeners still sow two squash seeds and snip the weaker one.
• Two to three per spot: small seeds such as lettuce, chard, beets, herbs and brassicas, and anything with a lower germination rate.
• Sprinkled in a row and thinned: tiny seeds such as carrots and radishes, which are hard to place individually. Mixing them with fine sand helps spread them evenly.

Beet and chard “seeds” are actually clusters that often produce two or more seedlings each, so they almost always need thinning even when sown one per spot.

Seeds for transplants

When you start seedlings indoors to transplant later, plan for some losses between sowing and planting out. A common practice is to start about 20 to 30 percent more seedlings than you need, then plant the strongest and give away or compost the rest. For example, if you want 8 tomato plants, start 10 or 12. Enter the extra as your loss allowance in the calculator. The seed starting calculator helps with timing.

Succession sowing

Fast crops such as lettuce, radishes, bush beans and cilantro are best sown in small batches every two to three weeks, rather than all at once. This spreads the harvest out so you are not overwhelmed for one week and empty-handed the next. To plan seed for succession sowing, work out the seeds for one sowing and multiply by the number of sowings. For example, four sowings of 12 lettuce plants need four times the seeds of one sowing.

Seeds per packet and buying in bulk

Packet sizes vary hugely by crop and by company. A packet of tomato or pepper seed may hold a few dozen seeds, while a packet of lettuce or carrots can hold hundreds or thousands. Many seed companies list the approximate seed count on the packet or website. If they list weight instead, the seller’s catalog usually gives seeds per gram or ounce for that crop.

For crops you sow in quantity, such as carrots, beans and peas, larger packets or bulk seed are usually much cheaper per seed. For crops where you only need a few plants, such as tomatoes and peppers, one packet often lasts several years if stored well.

Getting better germination

• Sow at the right depth. Too deep is the most common cause of poor emergence. The plant database lists sowing depth for each crop.
• Keep the surface moist. Seeds need steady moisture until they sprout. A light cover of fine compost or a board over the row helps on sunny days, as long as you lift it as soon as seedlings appear.
• Wait for warm soil. Warm-season crops such as beans, corn and squash rot in cold soil. Cool-season crops germinate at lower temperatures.
• Firm the soil. Pressing the soil gently over the seed ensures good contact and steady moisture.

Common mistakes

• Sowing one seed per spot with low germination. It leaves gaps that are hard to fill later.
• Sowing everything at once. Fast crops come all at once and then stop.
• Not thinning. Extra seedlings compete and leave you with many small, weak plants.
• Storing seed badly. Heat and humidity shorten seed life sharply.

Sowing guide for common crops

Here is how gardeners typically sow some of the most popular vegetables, and what that means for how much seed you need.

• Tomatoes and peppers: started indoors, usually 2 seeds per cell, thinned to one. You rarely need more than a single packet, even for a large garden.
• Lettuce: 2 to 3 seeds per spot, or sown thinly in a row and thinned. Because lettuce is often sown in succession, plan for several sowings across the season.
• Carrots: sown thinly along a shallow furrow, then thinned to about 2 to 3 inches apart. Expect to sow several times as many seeds as the plants you keep.
• Beans and peas: one seed every 2 to 4 inches along the row, or 2 per spot in a grid. They germinate reliably in warm soil, so losses are usually low.
• Squash, cucumbers and melons: 2 seeds per spot, thinned to the stronger seedling, or started indoors in individual pots a few weeks before transplanting.
• Corn: 1 to 2 seeds every 8 to 12 inches. Plant in a block of several short rows rather than one long row, because corn is wind-pollinated.
• Beets and chard: each seed is a cluster that can produce several seedlings, so sow one cluster per spot and thin.
• Onions: grown from seed, sets or transplants. Seed is short-lived, so buy fresh seed each year rather than relying on leftovers.

Storing leftover seed

Most gardeners end the season with partly used packets. Stored well, many will be useful for several years, which makes larger packets better value. Seeds keep best when they are cool, dry and dark. Fold the packet closed, place packets in a sealed glass jar or airtight container, and keep it in a cool room or the refrigerator. A small packet of silica gel or a spoonful of dry rice wrapped in tissue helps absorb moisture. Avoid sheds, garages and greenhouses, where temperatures swing widely.

Label everything with the variety and the year you bought it. When you plan next season, check the year against the viability table above, run a quick germination test on anything older, and enter the real germination rate in the calculator.

If you save seed from your own plants, open-pollinated and heirloom varieties come true from seed, while hybrid varieties, often labeled F1, usually do not. Seed from a hybrid tomato will grow, but the plants can be quite different from the parent.

Frequently asked questions