Fertilizer Calculation: A Simple Formula with Worked Examples

Every fertilizer recommendation comes as kilos of nutrient per hectare, written as three numbers such as 90 30 30. That means 90 kg of nitrogen (N), 30 kg of phosphate (P2O5) and 30 kg of potash (K2O). The bags at the store carry grades instead. The math connects the two.

Your numbers should come from your own field. A soil test from the regional soils laboratory can give them. So can the Minus One Element Technique (MOET) kit, the Rice Crop Manager or your municipal agriculturist. PALAI, an AI helper for rice, now also suggests which fertilizer to use, how much per hectare and when.

The fertilizer calculation formula

There is only one formula to remember:

Kilos of product = kilos of nutrient needed, divided by the grade number over 100

The grade number is the percent of that nutrient in the bag. Urea is 46 0 0, so its nitrogen share is 0.46. Complete fertilizer is 14 14 14, so each share is 0.14. You can also work backward to see what a bag gives. Kilos of nutrient equal kilos of product times the share.

Worked example 1: fertilizer computation for one hectare of palay

Say your recommendation for one hectare of irrigated palay reads 90 30 30. This is an example to learn the math, not a rate for your field. For reference, the modified general recommendation for irrigated lowland is 97 kg of nitrogen per hectare in the wet season and 139 kg in the dry season.

Plan A: 14 14 14 fertilizer plus urea

Plan B: 16 20 0 fertilizer, muriate of potash and urea

Phosphate comes first again. Here 30 divided by 0.20 is 150 kg, or exactly 3 bags of 16 20 0 fertilizer. Those bags also carry 24 kg of nitrogen. Potash comes from muriate of potash (0 0 60): 30 divided by 0.60 is 50 kg, one bag. The nitrogen left is 90 minus 24, or 66 kg. So you need 66 divided by 0.46, or 143.5 kg of urea.

At the same September 2026 prices, 3 bags of ammophos cost about PHP 5,261 and one bag of potash PHP 1,668. The 143.5 kg of urea adds about PHP 6,659. The total is about PHP 13,589, some PHP 620 less than Plan A for the same 90 30 30. Prices differ by province, so run both plans with your own dealer's prices.

Then split the fertilizer application

Buying the right amount is only half the job. The general recommendation for irrigated lowland rice splits the season's fertilizer like this:

• All the phosphorus at the first application
• Nitrogen: 25 percent at 7 to 14 days after transplanting, 25 percent at mid tillering, and 50 percent at maximum tillering or early panicle initiation
• Potash: 70 percent at 7 to 14 days after transplanting, and 30 percent from 5 to 7 days before or after panicle initiation

Our guides to fertilizer for rice and fertilizer application methods explain how to put each split on.

Worked example 2: a small vegetable plot

Small plots use the same formula, only scaled down. Say you have a 200 square meter plot, 10 by 20 meters. The recommendation for your crop is 120 60 60 per hectare. Again, this is only an example.

First scale it: 200 divided by 10,000 is 0.02 hectare. So you need 120 times 0.02, or 2.4 kg of nitrogen. You also need 1.2 kg each of phosphate and potash.

Next, 1.2 kg of phosphate divided by 0.14 is 8.6 kg of 14 14 14 fertilizer. That also brings 1.2 kg of nitrogen. The remaining 1.2 kg of nitrogen divided by 0.46 is 2.6 kg of urea. If the plot holds 400 plants, each gets about 21 grams of complete at planting. Later, each gets about 6.5 grams of urea. Weigh one cupful on a kitchen scale once, so you know how many grams your cup holds.

Fertilizer urea price and cost per hectare

The FPA publishes dealer prices every week. Its report for September 14 to 18, 2026 put the fertilizer urea price at PHP 2,320.69 per bag. That is the national average for 50 kg of prilled urea, while granular averaged PHP 2,322.28. Regional averages for prilled urea ran from about PHP 2,156 in CAR to PHP 2,678 in Region X. So your local price matters.

To compare sources, divide the bag price by the kilos of nutrient inside. Urea works out to about PHP 101 per kg of nitrogen, and ammonium sulfate to about PHP 116. Fertilizer made up about 8 percent of rice farmers' total production cost in 2020. Fertilizer prices then rose sharply from 2021. So a few minutes of fertilizer computation before you buy pays for itself.

Common mistakes in fertilizer computation

• Treating the grade number as kilos. A bag of 14 14 14 holds 7 kg of each nutrient, not 14.
• Forgetting the nitrogen that comes with 14 14 14 or 16 20 0, then adding too much urea.
• Mixing up P with P2O5, or K with K2O.
• Guessing the lot size instead of measuring it.
• Rounding bags too early. Round only at the end, then buy whole bags.
• Putting everything on at once instead of splitting it by crop stage.

Let anee.io keep the numbers

Once you have the math, anee.io keeps it working all season. In the Protocol Builder you plan every fertilizer task, material and rate ahead of time. You can save versions and import the plan as a new cropping schedule. Farm maps measure each lot over satellite imagery, so the area in your formula is real. The inventory tracks every bag and its cost, and the expenses report adds up what the season's fertilizer cost. If you get stuck, ask Anee in Tagalog or English to walk through the computation with you.

Fertilizer calculation questions