Use the free tool below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
Why seed spacing matters
Every seed packet gives a spacing number, and it is easy to ignore it. You scatter seeds close together because an empty bed looks wasteful, and then the seedlings crowd each other. Crowded plants fight for light, water, and food in the soil. They grow thin stems, small leaves, and poor harvests. Good spacing looks empty at first, and that empty look is the point. Each plant gets room to grow to full size.
This calculator turns spacing into a shopping list. Enter the length of your row, the spacing between seeds, the width of your bed, and the spacing between rows. It tells you how many seeds fit in each row, how many rows fit in the bed, and how many seeds you need in total. It also adds ten percent spare, because not every seed sprouts.
How to measure your bed
Measure the row length where seeds will actually sit, not the full length of the bed boards. Leave a small margin at each end so plants at the edge still get light and air. If your bed is four feet wide and your rows sit twelve inches apart, you fit four rows. Measure in the same unit all the way through, or use the unit switch in the tool to change between inches and centimetres.
Seed spacing and row spacing are different numbers, and mixing them up is the most common mistake. Seed spacing is the gap between two seeds in the same row. Row spacing is the gap between two rows. Small seeds like carrots need only a few inches between seeds, but their rows still need room for weeding and watering. Check both numbers on the packet before you type anything.
A worked example
Take a row ten feet long, which is 120 inches. If the packet says to space seeds three inches apart, divide 120 by 3 and you get 40 seeds per row. If the bed is 48 inches wide and rows sit 12 inches apart, divide 48 by 12 and you get 4 rows. Multiply 40 by 4 and you need 160 seeds. With ten percent spare, buy 176 seeds, or two packets if packets hold 100 seeds each.
Not every row divides neatly. A row of 100 inches with four-inch spacing holds 25 seeds exactly, but a row of 102 inches holds 25 seeds with two inches left over. The calculator rounds down, because squeezing one extra seed into the leftover gap breaks the spacing for the whole row. Leftover space at the end of a row is normal, and it is better than a crowded row.
Thinning and gaps
Even with perfect spacing, some seeds fail. That is why the calculator adds spare seeds. Sow a few extra at the end of each row if you have room, and use them to fill gaps where nothing sprouted. Wait until seedlings have two or three true leaves before thinning, because the strongest seedlings are easier to spot then. Pull the weakest ones gently so you do not disturb the roots of the ones staying.
Thinning feels wasteful, but it is the opposite. Ten carrots with full spacing outgrow thirty crowded carrots. The thinnings of lettuce, spinach, and radish are edible too, so nothing is truly wasted. Eat the thinnings in salads and let the remaining plants grow big.
Common mistakes to avoid
The biggest mistake is sowing too thick and never thinning. A thick row looks lush for two weeks and then stalls, because every plant is starving. Another mistake is trusting memory instead of the packet. The same vegetable can need different spacing by variety: a small lettuce needs six inches, while a large head lettuce needs twelve. Always read the packet for the variety you actually bought.
People also forget paths and reach. A bed wider than your arm can reach from either side makes the middle rows hard to weed and harvest. Keep beds narrow enough to reach the centre without stepping on the soil, since stepped-on soil compacts and roots struggle in it.
Frequently asked questions
Can I sow closer and thin later? Yes, and many gardeners do, especially with tiny seeds that are hard to place one by one. Sow a little thick, then thin on time. The danger is only when thinning never happens.
Does spacing change in raised beds? Raised beds often use slightly closer spacing than open rows, because the soil is loose and rich. Even so, do not halve the packet spacing. Shave off a little, watch the plants, and adjust next season.
What about square foot gardening? Square foot spacing uses a grid instead of rows, and our raised bed calculator covers that layout. The idea is the same: each plant gets a fixed patch of soil and light.
Use the calculator above before you open a packet, buy seeds with spare included, and trust the packet over any tool when the two disagree. Garden results are general guidance estimates only. Your local climate, your soil, and the instructions on your seed packets govern what actually grows, and results vary with weather and conditions.