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Agriculture & Farming

Fertilizer NPK calculator

Enter your recommended N, P and K dose for the area and get the exact urea, DAP and MOP to buy — with the nitrogen inside DAP counted first.

Last verified 29 October 2026 Sources 2 No sign-up, no storage Formula and sources shown
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Checked, not guessed.2 sources, 4 build tests and a changelog are on this page.
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The method

How it is calculated

The formula this page actually runs

Farmers buy fertilizer by the bag, but recommendations are written as kg of N, P₂O₅ and K₂O per acre. The conversion is a nutrient-balance sum, and the order matters: DAP brings nitrogen with it.

Formula DAP = P₂O₅ dose ÷ 0.46 · N from DAP = DAP × 0.18 · Urea = (N dose − N from DAP) ÷ 0.46 · MOP = K₂O dose ÷ 0.60
01

Scale the dose to the area

The default dose is per acre: 48 kg N, 24 kg P₂O₅ and 16 kg K₂O. For 2 acres, double all three totals first — 96 kg N, 48 kg P₂O₅ and 32 kg K₂O — then convert. The worked steps below stay on 1 acre so the arithmetic is easy to follow.

02

Meet phosphorus with DAP

DAP is 46% P₂O₅, so 24 kg of P₂O₅ needs 24 ÷ 0.46 = 52.2 kg of DAP. That DAP also carries 18% nitrogen — 9.4 kg of N here.

03

Top up N with urea, then add MOP

Only the remaining 48 − 9.4 = 38.6 kg N comes from urea: 38.6 ÷ 0.46 = 83.9 kg, or two 45 kg bags. Potash is 16 ÷ 0.60 = 26.7 kg of MOP.

The numbers

Fertilizer nutrient grades

Reference values, not hidden defaults
FertilizerN (%)P₂O₅ (%)K₂O (%)Used here for
Urea4600The main nitrogen top-up.
DAP (di-ammonium phosphate)18460Phosphorus, plus some nitrogen.
MOP (muriate of potash)0060Potassium.
SSP (single super phosphate)0160Phosphorus plus sulphur and calcium.
NPK 10-26-26102626Complex grade; supplies N, P and K together.
NPK 20-20-020200Complex grade with no potash.
NPK 19-19-19191919Balanced complex grade.
Nutrient percentages are the standard FCO specifications. Complex fertilizers supply more than one nutrient, so always subtract what the first product already contributes. Bag sizes vary by product and supplier. The N, P₂O₅ and K₂O dose inputs are per acre; 1 hectare = 2.471 acres.
Before you use it

What most pages get wrong

4 things worth knowing before you use the number
01 · The credit

DAP already contains nitrogen

DAP is 18% N. If you work out urea from the full nitrogen dose without subtracting that 18%, you over-apply nitrogen and waste money.

02 · The grade

Urea is not 100% nitrogen

Urea is 46% N, so 46 kg of actual nitrogen needs 100 kg of urea. A 45 kg bag carries about 20.7 kg of N.

03 · The phosphorus

DAP fixes the P dose, urea only tops up N

If your P dose is zero, no DAP is needed and all the nitrogen comes from urea. If the P dose is high, DAP can supply more N than you need and urea drops to zero.

04 · The source

Use a soil test, not a thumb rule

The default doses are only an example. Your state's soil-test-based recommendation accounts for the nutrients already in your field and should replace the defaults.

Questions

Frequently asked questions

The queries people actually search
How much urea do I need for 1 acre at 40 kg N?+
If no DAP or other nitrogen source is used, 40 ÷ 0.46 = 87 kg of urea, which is two 45 kg bags. If DAP is also applied, the nitrogen it supplies is subtracted first and the urea figure falls.
Why does urea go down when I use DAP?+
DAP is 18% nitrogen and 46% P₂O₅. The nitrogen in DAP counts toward your N dose, so the calculator subtracts it before working out how much urea is still needed.
How do I convert kg/acre to kg/hectare?+
Multiply by 2.471. A 48 kg N per acre dose is about 118.6 kg N per hectare. Divide by 2.471 to go the other way.
Can urea alone meet all three nutrients?+
No. Urea supplies only nitrogen. Phosphorus has to come from DAP, SSP or a complex fertilizer, and potash from MOP or an NPK complex. Applying only urea eventually unbalances the field.
What if the DAP supplies more nitrogen than my N dose?+
The urea requirement becomes zero — you cannot add negative urea. In that case you would need to rethink the plan, because you are already applying more N than intended through DAP.
Should I split the urea application?+
Usually yes. Agronomy guidance commonly splits nitrogen into a basal dose and one or two top dressings through the season rather than applying all of it at sowing. This calculator gives the total quantity; split timing is a crop decision.
Trust layer — how to check this page

Sources, dates and a changelog.

Every number this calculator uses is either an input you control or a value from the sources below. The formula is compiled and tested at build time, so the page cannot ship with a broken calculation.

Sources

This page

  • Last verified: 29 October 2026
  • Next review: 29 April 2027
  • Vertical: Agriculture & Farming
  • Build tests: 4 passed
  • Changelog: 1 entry

How we handle errors

  • Spotted a mistake? Report it — we correct and log it.
  • Corrections appear in the changelog below.
  • Read the methodology and the disclaimer.
Changelog
2026-10-29First published with FCO fertilizer grades and the DAP nitrogen credit.