Can I solve for different variables?

Yes! This calculator supports reverse solving. You can solve for: Target nitrogen rate (lb N / 1000 ft²), Bag label nitrogen (%), Bag label phosphate (%), Bag label potash (%), Field area, Fertilizer required (lb / 1000 ft²), Total fertilizer (lb), Bag weight (lb), Bags needed, Phosphate applied (lb P₂O₅ / 1000 ft²), Potash applied (lb K₂O / 1000 ft²).

What units can I use?

This calculator supports multiple unit systems. Select your preferred units from the dropdown menus next to each input field.

Fertilizer Calculator

Determine how much N-P-K fertilizer is required to hit agronomic nitrogen goals and report the corresponding P and K applications.

Formula shown6 inputs definedUpdated Nov 2025By CalcuTotal TeamFree, no sign-up

Quick Summary

Use this when you need:

Turn agronomic N recommendations into actual bags to buy
See how much unintended P or K accompanies a nitrogen application
Adjust bag weight or analysis to stay within a fertilizer budget

You’ll need:Target nitrogen rate (lb N / 1000 ft²), Bag label nitrogen (%), Bag label phosphate (%), Bag label potash (%), and 2 more

You’ll get:Fertilizer required (lb / 1000 ft²), Total fertilizer (lb), Bags needed, Phosphate applied (lb P₂O₅ / 1000 ft²), Potash applied (lb K₂O / 1000 ft²)

How this is calculated

Formula:

fertilizer_rate = target_N / (bag_N%/100); fertilizer_weight = fertilizer_rate * field_area/1000; P_rate = bag_P%/100 * fertilizer_rate

In plain language:

Dividing the desired nitrogen rate by the bag's nitrogen fraction yields pounds of product per 1000 ft², which scales with area for total product and automatically produces accompanying P₂O₅ and K₂O applications via their label percentages.

The formula is always visible so you can verify the math and explain your numbers to anyone who asks.

What these terms mean

Target nitrogen rate (lb N / 1000 ft²)

Target nitrogen rate (lb N / 1000 ft²) used in the calculation.

Bag label nitrogen (%)

Bag label nitrogen (%) used in the calculation.

Bag label phosphate (%)

Bag label phosphate (%) used in the calculation.

Bag label potash (%)

Bag label potash (%) used in the calculation.

Field area

Field area used in the calculation.

Fertilizer required (lb / 1000 ft²)

Fertilizer required (lb / 1000 ft²) used in the calculation.

Total fertilizer (lb)

Total fertilizer (lb) used in the calculation.

Bag weight (lb)

Bag weight (lb) used in the calculation.

Bags needed

Bags needed used in the calculation.

Phosphate applied (lb P₂O₅ / 1000 ft²)

Phosphate applied (lb P₂O₅ / 1000 ft²) used in the calculation.

Potash applied (lb K₂O / 1000 ft²)

Potash applied (lb K₂O / 1000 ft²) used in the calculation.

When to use this calculator

Turn agronomic N recommendations into actual bags to buy
See how much unintended P or K accompanies a nitrogen application
Adjust bag weight or analysis to stay within a fertilizer budget

Last updated: November 19, 2025

Fertilizer calculator explained

Agronomy recommendations are usually given as pounds of nitrogen per 1,000 square feet or per acre, but you buy fertilizer as an N-P-K analysis on a bag. This calculator divides your nitrogen target by the bag's N percentage to determine pounds of product needed, scales for the treatment area, and reports how much phosphate (P2O5) and potash (K2O) will go down at the same time.

How the conversion works

If you need $N_t$ pounds of nitrogen per 1,000 ft² and the fertilizer label shows $N\\%$ , then pounds of product per 1,000 ft² ( $F_{1k}$ ) equal

F_{1k} = \\frac{N_t}{N\\% / 100}

Total product mass is $F = F_{1k} \\times (\\text{area} / 1000)$ . Bag counts divide by bag weight, and accompanying P2O5/K2O rates follow from label percentages $P\\%$ and $K\\%$ :

P_{1k} = \\frac{P\\%}{100} \\times F_{1k},\\quad K_{1k} = \\frac{K\\%}{100} \\times F_{1k}

Units and conversions

Quantity	Units	Notes
Target $N_t$	lb N/1000 ft²	Enter agronomic recommendation.
Label percentages	%	Typical turf products range from 15-30% N.
Field area	ft², acres, m²	The calculator converts everything to thousand-square-foot units.
Bag weight	lb	Default 50 lb; change for mini-bags.
Output $F$	lb product	Multiply by 0.4536 for kilograms.

Worked examples

Spring turf feeding
Target 1 lb N/1000 ft² with a 24-0-12 fertilizer on 10,000 ft².

F_{1k} = \\frac{1}{0.24} = 4.17\\ \\text{lb}\\,/1000\\ \\text{ft}^2

Total product $= 4.17 \\times 10 = 41.7$ lb, so $41.7 / 50 = 0.83$ bags are needed. Potash applied equals $0.12 \\times 4.17 = 0.50$ lb K2O/1000 ft².

Hectare conversion
Apply 1.5 lb N/1000 ft² on 0.4 ha (43,560 ft² per acre, 2.471 acres per ha) using a 30-0-0 product.

Area in thousand-square-foot units $= 0.4 \\times 2.471 \\times 43.56 = 43.0$ .
Product $= (1.5 / 0.30) \\times 43.0 = 215$ lb, or 4.3 fifty-pound bags.

Tips and pitfalls

Always base calculations on actual turf or field area; subtract driveways or beds to avoid over-application.
Check state phosphorus restrictions; many regions prohibit P when soil tests already show adequate levels.
Use the bag count output to round up to whole bags and record leftovers for future treatments.
Balance multiple nutrient goals by repeating the calculation for different analyses to see which blend best fits the agronomic plan.

References and further reading

Frequently Asked Questions

Was this calculator helpful?

Your feedback helps us improve calculators for everyone.

Found an error?

Report incorrect formulas, unit conversions, or worked examples.

Report an issue

Calculator info

Version:: v1.0.0
Last reviewed:: Nov 15, 2025
Sensitivity:: low
References:: 1

Last updated: November 19, 2025

Fertilizer calculator explained

How the conversion works

If you need $N_t$ pounds of nitrogen per 1,000 ft² and the fertilizer label shows $N\\%$ , then pounds of product per 1,000 ft² ( $F_{1k}$ ) equal

F_{1k} = \\frac{N_t}{N\\% / 100}

Total product mass is $F = F_{1k} \\times (\\text{area} / 1000)$ . Bag counts divide by bag weight, and accompanying P2O5/K2O rates follow from label percentages $P\\%$ and $K\\%$ :

P_{1k} = \\frac{P\\%}{100} \\times F_{1k},\\quad K_{1k} = \\frac{K\\%}{100} \\times F_{1k}

Units and conversions

Quantity	Units	Notes
Target $N_t$	lb N/1000 ft²	Enter agronomic recommendation.
Label percentages	%	Typical turf products range from 15-30% N.
Field area	ft², acres, m²	The calculator converts everything to thousand-square-foot units.
Bag weight	lb	Default 50 lb; change for mini-bags.
Output $F$	lb product	Multiply by 0.4536 for kilograms.

Worked examples

Spring turf feeding
Target 1 lb N/1000 ft² with a 24-0-12 fertilizer on 10,000 ft².

F_{1k} = \\frac{1}{0.24} = 4.17\\ \\text{lb}\\,/1000\\ \\text{ft}^2

Total product $= 4.17 \\times 10 = 41.7$ lb, so $41.7 / 50 = 0.83$ bags are needed. Potash applied equals $0.12 \\times 4.17 = 0.50$ lb K2O/1000 ft².

Hectare conversion
Apply 1.5 lb N/1000 ft² on 0.4 ha (43,560 ft² per acre, 2.471 acres per ha) using a 30-0-0 product.

Area in thousand-square-foot units $= 0.4 \\times 2.471 \\times 43.56 = 43.0$ .
Product $= (1.5 / 0.30) \\times 43.0 = 215$ lb, or 4.3 fifty-pound bags.

Tips and pitfalls

Always base calculations on actual turf or field area; subtract driveways or beds to avoid over-application.
Check state phosphorus restrictions; many regions prohibit P when soil tests already show adequate levels.
Use the bag count output to round up to whole bags and record leftovers for future treatments.
Balance multiple nutrient goals by repeating the calculation for different analyses to see which blend best fits the agronomic plan.

Quick Summary

Calculator Tool

How this is calculated

What these terms mean

Target nitrogen rate (lb N / 1000 ft²)

Bag label nitrogen (%)

Bag label phosphate (%)

Bag label potash (%)

Field area

Fertilizer required (lb / 1000 ft²)

Total fertilizer (lb)

Bag weight (lb)

Bags needed

Phosphate applied (lb P₂O₅ / 1000 ft²)

Potash applied (lb K₂O / 1000 ft²)

When to use this calculator

Fertilizer calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Plant Population Calculator

Acres Per Hour Calculator

Animal Mortality Rate Calculator

Log Reduction Calculator

qPCR Efficiency Calculator

Benadryl Dosage Calculator for Dogs

Was this calculator helpful?

Found an error?

Calculator info

Fertilizer calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Plant Population Calculator

Acres Per Hour Calculator

Animal Mortality Rate Calculator

Log Reduction Calculator

qPCR Efficiency Calculator

Benadryl Dosage Calculator for Dogs

Was this calculator helpful?

Found an error?

Calculator info