Fertilizer Calculator Widget
Turn a soil-test recommendation into a shopping list. Readers enter the nutrient rate they need - nitrogen, phosphate or potash per hectare, acre or 1,000 square feet - and the three grade numbers on the bag, and see how much product to spread, what else it supplies and how many bags the area takes.
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<iframe src="https://a2z.tools/embed/w/fertilizer-calculator" title="Fertilizer Calculator by A2Z Tools" width="100%" height="720" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>
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<div data-a2z-widget="fertilizer-calculator" data-height="720"></div> <script async src="https://a2z.tools/embed.js"></script>
Adds a small script (what it does) that sizes the widget to fit its content, loads it lazily and keeps it isolated from your page's CSS.
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How it works
A fertilizer grade such as 20-10-10 states the percentage by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). To supply a target amount of one nutrient you divide that amount by its percentage written as a decimal. The University of Georgia Extension circular on fertilizer recommendations shows it plainly: 3.5 lb of N per 1,000 ft2 from an 8% N product needs 3.5 / 0.08 = 44 lb of fertilizer, and 60 lb of K2O from a 5% product needs 1,200 lb. The widget then multiplies the product amount by the other two grade numbers to show the phosphate and potash that come along whether you need them or not. With an area entered it gives the total product (lawn-and-garden rates per 1,000 ft2 are scaled by the square feet) and, with a bag size, the number of bags rounded up. Grades over 100% in total, or a zero for the chosen nutrient, are refused.
Calculation method
- Product per area = nutrient rate / (grade % of that nutrient / 100)
- Other nutrients supplied = product per area x (their grade % / 100)
- Total product = product per area x area (per-1,000 ft2 rates: x area / 1,000)
- Bags = total product / bag size, rounded up
Worked examples
UGA lawn example
Inputs: N; 3.5 lb per 1,000 ft2; grade 8-8-8; 5,000 ft2
Result: 43.75 lb per 1,000 ft2; 3.5 lb of P2O5 and K2O also supplied; 218.8 lb for the lawn
3.5 / 0.08 = 43.75 lb; x 5 = 218.75 lb.
Urea on a 2 ha field
Inputs: N; 120 kg/ha; grade 46-0-0; 2 ha; 50 kg bags
Result: 260.9 kg/ha; 521.7 kg in total; 11 bags
120 / 0.46 = 260.87 kg/ha.
Limitations
- Does not recommend a rate; that must come from a soil test and local guidance.
- Ignores nutrient availability, losses and timing - for example volatilisation of surface urea or slow release from organics.
- Local nutrient-management rules may cap rates near water or at certain times of year.
Where publishers use it
- An agricultural extension service's soil-test results page
- A farm-supply store's product listing for urea, DAP or compound fertilizers
- A lawn-care company blog on spring nitrogen per 1,000 square feet
- A kitchen-garden site explaining what the three numbers on a bag mean
- An agronomy course worksheet on nutrient budgeting
Questions
What do the three numbers on a fertilizer bag mean?
They are the percentages of nitrogen (N), phosphate (P2O5) and potash (K2O) by weight. A 50 kg bag of 20-10-10 contains 10 kg of N, 5 kg of P2O5 and 5 kg of K2O; the rest is other elements and filler.
How much urea supplies 120 kg of N per hectare?
Urea is 46-0-0, so 120 / 0.46 = 260.9 kg of urea per hectare. On 2 ha that is 521.7 kg, or 11 bags of 50 kg (10.43 rounded up).
Why does the widget show extra phosphate and potash?
Because a compound product delivers all three nutrients together. Supplying 3.5 lb of N from 8-8-8 also applies 3.5 lb each of P2O5 and K2O per 1,000 ft2, which may be more than the soil needs; a straight N source avoids that.
Is P2O5 the same as phosphorus?
No. Labels use the oxide forms. Elemental phosphorus is P2O5 x 0.436 and elemental potassium is K2O x 0.830, so 100 kg of P2O5 contains about 43.6 kg of P.
Can I use it for organic fertilizers?
Yes for the arithmetic - enter the analysis of the manure, compost or meal. Organic nitrogen is released slowly, so extension guides often credit only part of it in the first year; adjust the rate accordingly.
Which rate unit should I pick?
Use the unit of your recommendation: kg/ha or lb/acre for fields, g/m2 or lb per 1,000 ft2 for lawns and gardens. Then enter the area in that unit's area - hectares, acres, m2 or ft2.
Sources
- How to Convert an Inorganic Fertilizer Recommendation to an Organic One (Circular 853) - University of Georgia Cooperative Extension . Shows lb of fertilizer = lb of nutrient / nutrient fraction: 3.5 lb N / 0.08 = 44 lb per 1,000 sq ft; 60 lb K2O / 0.05 = 1,200 lb. Checked 2026-10-01.
Cite or recommend this tool
If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.
A2Z Tools Fertilizer Calculator https://a2z.tools/embed/fertilizer-calculator
<a href="https://a2z.tools/embed/fertilizer-calculator">A2Z Tools Fertilizer Calculator</a>
[A2Z Tools Fertilizer Calculator](https://a2z.tools/embed/fertilizer-calculator)
Fertilizer Calculator by A2Z Tools - https://a2z.tools/embed/fertilizer-calculator
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