Johnny Seeds Planting Calculator: How to Use It

The Johnny Seeds planting calculator is a tool on the Johnny’s Selected Seeds website that helps you figure out how much seed to buy for a given area, or how much area a specific amount of seed will cover. You enter your bed dimensions (length and width) and the seed spacing or rate from the seed packet, and it returns the number of seeds or ounces/pounds needed. This article explains how the calculator works, how to use it correctly, and what to watch out for so you don’t end up with too little or too much seed.

Formula

The calculator uses the standard square-grid planting formula:

Seeds needed = (Length in inches ÷ Spacing in inches) × (Width in inches ÷ Spacing in inches)

Where:

• Length (l) = bed length in feet (convert to inches by multiplying by 12)
• Width (w) = bed width in feet (convert to inches by multiplying by 12)
• Spacing (s) = desired distance between plants in inches (from the seed packet or your plan)

The result is the number of seeds needed for a perfect grid. The calculator rounds up to the nearest whole seed because you can’t plant a fraction of a seed.

Worked Example

Using the default values from the calculator above:

• Length = 20 ft → 20 × 12 = 240 inches
• Width = 4 ft → 4 × 12 = 48 inches
• Spacing = 6 inches

Now apply the formula:

• Seeds along length = 240 ÷ 6 = 40 seeds
• Seeds along width = 48 ÷ 6 = 8 seeds
• Total seeds = 40 × 8 = 320 seeds

The calculator shows 320 seeds, which matches. The optional weight output (0.001 oz per seed) is just a rough estimate – actual seed weight varies by variety, so check your packet for the specific seed count per ounce.

Why Use a Planting Calculator?

Buying seeds is a balance. Too few seeds means gaps in your bed; too many means wasted money and leftover seed that may lose viability by next year. A calculator helps you:

• Order the right amount for a specific bed size
• Compare seed prices per area (e.g., a packet covers 50 ft² vs. 100 ft²)
• Plan succession planting – if you want three rounds of lettuce, multiply the result by 3
• Avoid overbuying when seed packets are sold by weight, not count

The Johnny Seeds calculator is especially useful for direct-sown crops like carrots, beans, and beets, where you don’t start transplants and need to know exactly how many seeds to put in the ground.

How to Use the Johnny Seeds Calculator

Here’s the typical workflow:

1. Go to the Johnny’s Selected Seeds website and find the planting calculator (usually under the “Resourcesâ€� or “Toolsâ€� menu).
2. Enter your bed length and width in feet (or inches, depending on the version).
3. Enter the seed spacing from the packet – for example, 2 inches for carrots, 12 inches for broccoli.
4. Click “Calculate� and read the number of seeds needed.
5. If the packet lists seeds per ounce or per gram, divide the number of seeds by that rate to get the weight to order.

Some versions also let you input the seed rate (e.g., seeds per foot) instead of spacing – use whichever your packet provides.

Common Mistakes and How to Avoid Them

Even a simple calculator can trip you up. Watch out for:

• Mixing units – If you enter bed length in feet but spacing in inches, the result is wildly wrong. Always convert to the same unit (the calculator above does it for you).
• Forgetting row spacing – The formula assumes equal spacing in both directions. If you plant in rows with 12 inches between rows but 2 inches within the row, you need a different calculation: (length ÷ in-row spacing) × (number of rows).
• Ignoring germination rate – Not every seed sprouts. If your packet says 80% germination, you might want to plant 20% extra to fill gaps. The calculator doesn’t account for this.
• Using the result for transplants – If you’re starting seeds in trays, you need one seed per cell, not per square foot. That’s a different calculation.

When the Calculator Doesn’t Apply

The grid formula works for crops that are planted at a uniform spacing in both directions – like lettuce, spinach, or onions. But it fails for:

• Broadcast sowing – For crops like arugula or cover crops, you scatter seed over the whole area. Use the packet’s rate (e.g., 1 oz per 100 ft²) instead.
• Intensive planting – Some gardeners use triangular spacing to fit more plants. That requires multiplying by a factor of about 1.15, but the Johnny calculator doesn’t offer that option.
• Perennial crops – Asparagus or rhubarb are planted once and harvested for years; you might plant fewer than the grid suggests to allow for growth.

In these cases, use the seed packet’s recommended rate per area, which is often listed in ounces or grams per 100 square feet.

Alternatives to the Johnny Seeds Calculator

If you don’t have access to Johnny’s specific tool, several other free options exist:

• Our own calculator above – it does the same math with no extra steps.
• Seed company websites – Most major seed companies (Burpee, Park Seed, etc.) have similar calculators. They all use the same basic formula.
• Spreadsheet formulas – You can make your own in Excel or Google Sheets: =(length*12/spacing)*(width*12/spacing).
• Paper and pencil – The math is simple enough to do by hand for small beds.

The key is to use the same spacing value consistently. If you’re mixing crops in one bed, calculate each crop separately and add the results.

Frequently Asked Questions

Does the Johnny Seeds calculator account for germination rate?

No, it assumes 100% germination. To compensate, multiply the result by 1.2 (for 80% germination) or 1.25 (for 75%). Check your seed packet for the germination percentage – it’s usually printed on the label.

Can I use the calculator for raised beds?

Yes, as long as you enter the inside dimensions of the bed. Raised beds often have deeper soil, but that doesn’t change the surface area calculation. Just measure the length and width of the soil surface, not the outer frame.

What if my bed isn’t a perfect rectangle?

Break it into rectangles and calculate each separately, then add the totals. For a triangular or circular bed, use the appropriate area formula (0.5 × base × height for triangles, π × radius² for circles) and then divide by the spacing squared.

How do I convert seeds needed to ounces or grams?

Divide the number of seeds by the seed count per ounce (or gram) listed on the packet. For example, if you need 500 seeds and the packet says 1,000 seeds per ounce, you need 0.5 ounces. If the packet doesn’t list this, you can weigh a known number of seeds yourself.

Is it better to buy more seeds than the calculator says?

It’s wise to add 10–20% extra for germination failures, pests, and thinning mistakes. Seeds are cheap compared to the cost of replanting later. However, don’t overbuy – most seeds lose viability within 2–3 years, so you’ll waste money if you buy far more than you can use.

Does the calculator work for flower seeds too?

Yes, the same grid formula applies to any plant that you space evenly. For flowers that are broadcast or densely planted (like wildflower mixes), use the packet’s rate per area instead.

Related

• Compost Calculator: How Many Yards Do You Need
• Trees Per Acre on 20×20 Spacing
• Lime per Acre: How Much to Apply

in l | Bed length | ft | 20 | 0 | 1000
in w | Bed width | ft | 4 | 0 | 1000
in s | Seed spacing | in | 6 | 0.1 | 120
out seeds | Seeds needed | seeds | ceil(round((l * 12 / s) * (w * 12 / s), 6)) | 0
out oz | Seed weight (oz) | oz | seeds * 0.001 | 2