Fertilizer analysis and the amount of product
Every fertilizer bag or jug carries three numbers, always in the same order: the percentage by weight of nitrogen (N), available phosphate (P₂O₅) and soluble potash (K₂O). A bag labeled 21-0-0 is 21% nitrogen by weight, with no phosphate or potash; a 10-34-0 is 10% N and 34% P₂O₅. This is called the guaranteed analysis, and it's the same system whether the product is a dry granule or a liquid concentrate.
A nutrient recommendation, whether from a soil test, an agronomist or a lawn care program, is usually given as a rate of the actual nutrient, such as 1 lb of N per 1,000 sq ft or 60 lb of N per acre. Because the fertilizer bag is only part nutrient, you always need more weight of product than the nutrient target itself. Fertigation adds one more step for liquids: converting that weight into a volume using the product's density.
Check local rules first. Many states and irrigation districts regulate fertigation and require specific backflow prevention equipment. Where those rules apply, they take priority over anything calculated here.
When this calculator isn't the right tool
This calculator converts a nutrient rate and a fertilizer analysis into an amount of product. It's not built for:
- Setting your injector's pump speed. Use the injector rate calculator once you know the volume of product to inject.
- Checking the concentration in the irrigation water. Use the fertilizer ppm calculator for parts-per-million.
- Setting an injector's mixing ratio. Use the injector ratio calculator.
- Chlorine or acid dosing. Use the chlorine injection calculator or the acid injection calculator.
- Getting a nutrient rate in the first place. A soil test is the most reliable source; this tool assumes you already have a target.
More tools are listed in the fertigation calculators category.
How this fertigation calculator works
The calculator divides your target nutrient rate by the fertilizer's analysis to get a weight of product, scales that to your area if you enter one, and converts to a volume if you're using a liquid and enter its density.
1. Product needed per basis area
Amount of product
Product per basis area = Target nutrient rate ÷ (Fertilizer analysis ÷ 100)
"Analysis" is the guaranteed percentage for the nutrient you selected: the first N-P-K number for nitrogen, the second for phosphate (P₂O₅), the third for potash (K₂O).
2. Converting between 1,000 sq ft, acres, 100 m² and hectares
US units
Rate per acre = Rate per 1,000 sq ft × 43.56
1 acre = 43,560 sq ft = 43.56 × 1,000 sq ft.
Metric units
Rate per hectare = Rate per 100 m² × 100
1 hectare = 10,000 m² = 100 × 100 m².
3. Total for your area
Scaling to area
Total product = Product per basis area × (Area to treat ÷ basis size)
Basis size is 1,000 sq ft or 1 acre (US), 100 m² or 1 hectare (metric), matching the basis you selected.
4. Weight to volume (liquids)
Volume of product
Volume = Weight of product ÷ Density
Density (weight per gallon or per liter) is specific to each liquid fertilizer. It's usually on the product label or technical sheet.
Rounding policy
Every result here follows directly from the numbers you enter, so nothing is rounded to a "nice" figure the way a rough estimate might be. Amounts under 1,000 lb or gallons (kg or L) show two decimal places; amounts of 1,000 or more show a whole number with thousands separated by commas. Precise figures matter for chemical dosing, so the calculator never rounds away the last bit of accuracy.
Fertigation calculation examples
A hay field is being fertigated with 28% UAN liquid nitrogen (28-0-0), which weighs 10.65 lb per gallon. The target is 50 lb of actual nitrogen per acre, over a 40-acre field.
- Product per acre: 50 ÷ 0.28 = 178.57 lb of 28-0-0 per acre.
- Total product: 178.57 × 40 = 7,143 lb.
- Volume: 7,142.86 ÷ 10.65 = about 670.69 gallons for the field.
- Nitrogen actually applied: 50 × 40 = 2,000 lb N, confirming the rate.
The same calculation in metric
A 250 m² vegetable bed is being fertigated with a dry fertilizer analyzed at 15% N, targeting 1.5 kg of nitrogen per 100 m²:
- Product per 100 m²: 1.5 ÷ 0.15 = 10 kg of fertilizer per 100 m².
- Total product: 10 × (250 ÷ 100) = 25 kg.
- Nitrogen applied: 25 × 0.15 = 3.75 kg N, matching the 1.5 kg per 100 m² target over the 250 m² bed.
Both examples match what the calculator above shows for the same inputs.
Practical guidance
Reading the fertilizer label
Find the three-number analysis on the bag, jug or technical sheet. It's always nitrogen (N) first, available phosphate (P₂O₅) second and soluble potash (K₂O) third, regardless of whether the product is solid or liquid. Enter only the number that matches the nutrient you selected above; entering the wrong one of the three is the most common mistake with this kind of calculation.
Finding a liquid fertilizer's density
Density is usually printed on the product's technical data sheet, since it varies by formulation and even by manufacturer. If you can't find it, weighing a measured gallon (or liter) on a scale works, or ask your supplier. Common liquid nitrogen sources fall in a fairly narrow range, useful as a starting point if you're estimating before your product arrives:
| Product | Typical density |
|---|---|
| 28% UAN (28-0-0) | about 10.65 lb/gal (1.28 kg/L) |
| 30% UAN (30-0-0) | about 10.80 lb/gal (1.30 kg/L) |
| 32% UAN (32-0-0) | about 11.06 lb/gal (1.33 kg/L) |
| 10-34-0 starter solution | about 11.40 lb/gal (1.37 kg/L) |
| Water, for reference | 8.33 lb/gal (1.00 kg/L) |
Common mistakes
- Using the wrong N-P-K number. Match the number to the nutrient you selected, not just the first number out of habit.
- Assuming a liquid weighs the same as water. Most liquid fertilizers are denser than water (8.33 lb/gal), so using water's weight will under-estimate the product needed.
- Forgetting the other two nutrients. A single product supplies all three N-P-K numbers at once; hitting your nitrogen target may over- or under-apply phosphate or potash. Check all three against your recommendation.
- Running the full season's total in one event. If your program calls for split applications, use this calculator once per event with that event's rate, not the season total.
Fertigation safety
- Follow the product label for application rates, mixing order, re-entry intervals and any restrictions.
- Use backflow prevention equipment. A functioning check valve or chemigation valve on the injection point helps keep fertilizer from siphoning back into your well or water supply, and many states and local codes require it on any irrigation system used for fertigation.
- Wear the protective equipment the label specifies, typically including gloves and eye protection, when handling concentrated fertilizer.
- If you also inject acid to manage pH or prevent scale, always add acid to water, never water to acid, and set the injection rate from an actual titration test of your water, not from a pH reading alone.
What changes the result
The math is fixed, but a few practical details around it are easy to overlook.
| Factor | Effect | What to check |
|---|---|---|
| Multiple nutrients in one product | Hitting your target for one nutrient (say N) applies whatever P₂O₅ or K₂O the product also carries. | Run this calculator again with the other analysis numbers to see what else is being applied. |
| Split applications | Many programs spread a season's nutrient total across several smaller events. | Use the rate for one event, not the season total, unless you're applying it all at once. |
| Stock tank solubility | Dissolved or liquid fertilizers can only be mixed up to a maximum concentration before they precipitate. | Check the product's maximum solution concentration before assuming a full tank can hold the amount calculated. |
| Water quality | Hard or high-pH irrigation water can react with some fertilizers and precipitate in the lines. | Check your water with the irrigation water quality calculator. |
| Irrigation uniformity | Fertigated nutrient follows the water, so uneven coverage means uneven feeding. | Check distribution uniformity before relying on one blanket rate for a whole zone. |
| Salinity and leaching | Fertilizer adds to the total dissolved salts in irrigation water. | See the leaching requirement calculator if salt buildup is a concern. |
Frequently asked questions
What does the fertilizer analysis (N-P-K) number mean?
The three numbers on a fertilizer label, in order, are the percentage by weight of nitrogen (N), available phosphate (P2O5) and soluble potash (K2O) in the product. A 21-0-0 fertilizer is 21% nitrogen by weight, with no phosphate or potash. This calculator uses whichever of the three numbers matches the nutrient you select.
How do I calculate how much fertilizer to apply for a target nitrogen rate?
Divide your target nitrogen rate by the fertilizer's percent nitrogen, written as a decimal. For example, to apply 1 lb of nitrogen per 1,000 sq ft using a 21-0-0 fertilizer, divide 1 by 0.21, which gives about 4.76 lb of product per 1,000 sq ft. This calculator does that division and scales it to your area.
How do I convert the amount of fertilizer needed into gallons for a liquid injection system?
Divide the weight of product needed by the product's density, in pounds per gallon or kilograms per liter. Liquid fertilizer densities vary by formulation; common values include about 11.06 lb per gallon for 32% UAN and about 10.65 lb per gallon for 28% UAN. Check the product label or ask your supplier for the exact density of the fertilizer you're using.
Is this the same as an injector rate or a ppm concentration?
No. This calculator finds the total amount of fertilizer product a target nutrient rate requires; it doesn't set your injector's pump speed or the concentration in the irrigation water. For those, use the injector rate calculator or the fertilizer ppm calculator.
See the injector rate calculator or the fertilizer ppm calculator.
Can I use this for phosphate (P2O5) or potash (K2O) instead of nitrogen?
Yes. Select the nutrient you're targeting, and enter the matching number from the fertilizer's N-P-K analysis: the second number for phosphate (P2O5) or the third for potash (K2O). The calculation works the same way for any of the three.
What safety precautions does fertigation require?
Always follow the product label for rates and handling, and use backflow prevention equipment to keep fertilizer from siphoning back into your water supply, since many state and local regulations require it. Wear the protective equipment the label specifies, such as gloves and eye protection. If acid is also being injected to manage pH or prevent clogging, always add acid to water, never water to acid, and set the dose from a titration test rather than from pH alone.
Why does the amount of fertilizer change when I switch from per-1,000-sq-ft to per-acre?
The nutrient rate itself doesn't change, only the area it's spread over. An acre is 43,560 sq ft, or 43.56 times larger than 1,000 sq ft, so the same rate applied per acre requires 43.56 times more product than the same rate applied per 1,000 sq ft. This calculator converts between the two automatically.
Does this calculator include a nutrient recommendation for my crop or lawn?
No. It converts a nutrient rate you already have, from a soil test, agronomist or extension guideline, into an amount of product. If you don't have a target rate yet, a soil test from your local extension office or lab is the most reliable way to get one for your specific crop, turf and soil.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversions and both worked examples verified by hand against the fertilizer-analysis math shown above.
- Density values checked against the extension sources below.
- Follows the standard guaranteed-analysis calculation and university extension fertigation safety guidance.
Sources
- Iowa State University Extension and Outreach. How to Calculate Fertilizer Rates for the Lawn.
- Mississippi State University Extension. Fluid Fertilizers (Publication P1466).
- University of Georgia Extension. Chemigation in Georgia (Bulletin 1298).
- Oklahoma State University Extension. Using Water to Calibrate Sprayers for Fertilizers and Other Liquid Solutions.
- University of New Hampshire Cooperative Extension. Safe Use of Acid Checklist.