Calculator
Fertilizer Calculator
Most people do not need to know N-P-K by heart. Choose what you are growing. The calculator says which of the three bag numbers should be highest — or if they should be about equal. Amounts are a conservative top-dress for an established bed or container, not fertilizer to mix into new soil or potting mix. Too much fertilizer can burn plants. Compost-rich raised beds often need less than a bag chart implies.
Bag to look for
Mixed vegetables (whole bed)
Balanced5-5-5, 4-4-4, 10-10-10
- You are growing
- Mixed vegetables (whole bed)
- Look for
- Balanced (N, P, and K similar)5-5-5, 4-4-4, 10-10-10
- How to read the bag
- Three numbers close together
How much to apply
Start with about
2Tbsp
- In spoons
- 2 Tbsp2 Tbsp + 1 tsp · 1.2 oz by weight. Level spoons; granule size varies.
- Bed size
- 32 sq ftLight feeding
- This feeding
- LightDefault. Compost-rich bed, first feeding, or you would rather not overdo it
- Also supplies
- 0.007 lb P0.007 lb K
This is a conservative starting amount for one feeding. Do not repeat more often than the fertilizer label allows. If the bag lists a lower rate, use the bag.
How this calculation works
Bags print three numbers in order: nitrogen (N, leaves), phosphorus (P, early roots and flowers), potassium (K, fruit and overall toughness). Amount math is a conservative surface top-dress for one feeding on plants that are already growing. Light = 0.225, Typical = 0.5625, Stronger = 0.9 lb actual N per 1,000 sq ft. Compost-rich beds should start with Light. Beds use length × width. Pots use the circular top surface (πr² × number of pots). Bed and pot depth are not used, and the tool does not estimate a mix-in for new soil. Product (lb) = nitrogen needed ÷ (N% ÷ 100). Kitchen spoons assume 1 oz of typical granular fertilizer ≈ 2 Tbsp. Pellet size varies. The product label still wins, especially if it lists a lower rate.
Example: tomatoes in a mixed raised bed
Pick tomatoes. The tool points you to a higher-potassium bag — third number biggest, or at least matching nitrogen — such as a 4-6-8 or 5-5-10 style vegetable food. A 20-5-5 lawn fertilizer is the wrong aisle. Enter the bed or pot surface size, the three numbers from the bag, and Light / Typical / Stronger. The amount is a starting top-dress in Tbsp or tsp, not a mix-in for new soil. Follow the bag if it lists its own garden rate.
Frequently asked questions
What do the three numbers on the bag mean?
They are always in the same order: nitrogen, phosphorus, potassium. You do not enter them to get a recommendation. Pick your crop, then look for which number the result says should stand out.
Why teaspoons instead of ounces?
Most people measure a side-dress with a kitchen spoon, not a scale. One ounce of typical granular fertilizer is about 2 tablespoons. That is a scooping estimate — pellets and prills do not all weigh the same. If the bag prints cups or ounces, use the bag.
Do I need to know target nitrogen?
No. Enter the three numbers off the label and pick Light, Typical, or Stronger. Light is the default starting choice. If the bag prints its own rate for vegetable gardens, or a lower rate, use the bag instead.
Can I use this when filling a new pot or raised bed?
No. This estimate is for lightly top-dressing plants that are already growing. It does not calculate fertilizer to mix into new soil or potting mix, and it does not use bed or pot depth. For a mix-in rate, follow the product label.
Is a 10-10-10 always the safe choice?
It is a reasonable mixed-bed default. Leafy crops often want a higher first number. Fruiting crops often want the third number to keep up. Compost already in the bed can cover a light feeding. Repeated use of a balanced fertilizer also adds phosphorus and potassium.
Why is high phosphorus not the default for tomatoes?
Bloom-booster bags are easy to overuse. Many compost-filled raised beds already have enough phosphorus. This tool only points to a high-P bag for new transplants, or when you have a reason such as a soil test.
Will you recommend a specific brand?
No. This calculator points you to a bag type — higher nitrogen, phosphorus, potassium, or balanced. It does not recommend a brand.
Can this tell me if I will burn plants?
No calculator can promise that. These amounts are a conservative starting point for one feeding, and the default is Light. Do not repeat more often than the fertilizer label allows. Burn still depends on the product, wet vs dry granules, and whether fertilizer sits on stems. Keep it off the plant, water it in, and stop if leaf edges scorch. The label and a soil test override this tool.